abeam
A relative location approximately at right angles to the longitudinal axis of the aircraft. An object beside an aircraft is said to be abeam of it.

accelerated stall
A stall induced by turns, pull-ups, or other maneuvers that increase the load factor (G-load) on an airplane. The loads imposed by such maneuvers typically increase the speed at which a wing stalls, but they do not affect the angle of attack at which the stall occurs. That angle, the critical angle of attack, is always the same.

ace
A military pilot who has destroyed at least five enemy aircraft. The term originated in France during World War I, where it was given only to top pilots who had downed at least 10 aircraft. Later, "ace" was commonly used for any pilot who claimed at least five aircraft. German aces in World War I were called Kanone ("cannons").

active runway
The runway currently in use for takeoffs and landings. Many large airports have more than one runway, usually arranged in a pattern to take advantage of prevailing winds. Some airports have parallel runways to accommodate more takeoffs and landings. Under specific conditions at airports with operating air traffic control towers, aircraft may also use intersecting runways for takeoffs and landings to expedite the traffic flow.

advection
The horizontal movement of air or atmospheric properties. In meteorology, this process is sometimes referred to as the horizontal component of convection.

advection fog
Fog resulting from the movement of warm, humid air over cooler ground or water. Advection fog is most common along coastal areas, although it often develops deep in continental areas. At sea, it is called "sea fog." Advection fog deepens as wind speed increases up to about 15 knots. Stronger winds lift the fog into a layer of low stratus or stratocumulus clouds.

advisory circulars (ACs)
In the United States, nonregulatory information and procedures published by the FAA. Advisory circulars provide background information and more detail on subjects not completely outlined in the FARs or AIM. Advisory circulars are published by the Government Printing Office; many publishers also reprint important ACs as references for pilots. ACs are arranged in series, designated by a two- or three-digit number. Those in series 00 cover general subjects. ACs in the 10 series deal with procedural rules. ACs in the 20 series deal with aircraft; those in the 60 series cover pilots. ACs in the 70s series deal with airspace, while 90-series ACs cover air traffic and general operating rules. Issues related to air carriers (airlines) are covered in 120-series ACs.

aerobatics
Precision maneuvers, such as barrel rolls, loops, hammerhead stalls, spins, and Cuban eights. Often performed at airshows and competitions, many of these maneuvers are also part of a military pilot's training and can be used in aerial combat. In fact, many basic aerobatic maneuvers evolved from air-combat tactics invented during World War I.
"Aerobatics" is also defined in FAR 91.303, which describes restrictions on aerobatic flight. In that section, "aerobatic flight" means "any intentional maneuver involving an abrupt change in an aircraft's attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight."

aerodrome forecast (TAF)
A concise statement of the expected meteorological conditions at an airport during a specified period (usually 24 hours). TAFs use the same codes employed in METAR weather reports. TAFs are scheduled four times daily for 24-hour periods, beginning at 0000Z, 0600Z, 1200Z, and 1800Z. They contain the following information: type of report, location, issuance time, valid time, and forecast.

aerodynamics
The study of air in motion, in particular, the interactions between air and surfaces, such as an aircraft wing. Aerodynamicists--scientists and engineers who specialize in aerodynamics--use wind tunnels, computer models, and other tools to design and build aircraft and airfoils.

aeronaut
The pilot or navigator of an airship or balloon. Famous aeronauts include the Montgolfier brothers, inventors of the hot-air balloon; Henri Giffard, builder and pilot of the first powered dirigible; and August Piccard, who rode a balloon into the stratosphere.

Aeronautical Information Manual (AIM)
An official source of information about navigational aids, airport operations, air traffic control procedures, and other subjects of importance to pilots and air traffic controllers. Published by the FAA, the AIM is available from many publishers in printed and electronic formats.

aeronautical sectional chart
A 1:500,000-scale aeronautical map that includes topographical and navigational information for pilots to use during VFR flight.

aeronautics
The study or science of flight.

aerospace
The study of the science and technology of travel in the space above the earth. Aerospace includes travel within the atmosphere as well as in space beyond the earth’s atmosphere.

Aérospatiale
French state-controlled aircraft-manufacturing company formed by the merger of Sud Aviation and Nord Aviation. Based in Toulouse, Aérospatiale has produced a wide range of helicopters and was the manufacturer of the supersonic Concorde airliner.

aerostat
A lighter-than-air craft, such as a hot-air balloon or dirigible, that gets lift principally from buoyancy instead of generating lift with airfoils.

afterburner
Device that injects fuel into the exhaust stream of a turbojet engine. The combustion of the unburned oxygen from the exhaust and the jet fuel provides an extra boost at takeoff or during combat maneuvers. Also known as "reheat," especially in Great Britain.

AGL
Abbreviation for "above ground level." The height of clouds in airport weather observations and forecasts is usually reported in AGL.

ailerons
Movable control surfaces, usually located near the wing tips, that control the rolling motion of an aircraft. The pilot deflects the ailerons by moving the control yoke or stick left and right. The ailerons move simultaneously in opposite directions. For example, when the pilot moves the yoke or stick left, the aileron on the left wing moves up, decreasing the lift on the left wing. At the same time, the right aileron moves down, increasing the lift on the right wing. The word derives from the French word aile, meaning "wing."

air mass
In meteorology, an extensive body of air within which the conditions of temperature and moisture in a horizontal plane are essentially uniform.

air traffic control (ATC)
A network of control towers, approach and departure controls, and Air Route Traffic Control Centers (ARTCCs) designed to ensure the safe and efficient flow of air traffic. ATC's primary responsibility is to maintain separation between aircraft operating under Instrument Flight Rules (IFR), but ATC also provides services to aircraft operating under Visual Flight Rules (VFR).

ATC's functions are divided into several segments. Ground control supervises aircraft taxiing to and from runways. The tower, or "local control," handles aircraft in the vicinity of an airport, clearing them for takeoff or landing. Departure and approach controls manage the airspace surrounding one or more airports, and en route centers control traffic between airports.

Airbus Industrie
Aircraft manufacturing consortium that includes France's Aérospatiale, Germany's Deutsche Airbus, British Aerospace, and CASA of Spain. Based in Toulouse, France, Airbus is the world's second-largest manufacturer of commercial airplanes (after Boeing). Its major products are the A310, A320, A321, A330, and A340 airliners.

aircraft
A flying machine. In the United States, the FAA divides aircraft into classes and categories. With respect to the certification of aircraft, classes are broad groupings, such as airplanes, rotorcraft, gliders, balloons, landplanes, and seaplanes. Categories define aircraft based on their intended use or operating limitations; for example, transport, normal, utility, acrobatic, limited, experimental, restricted, and provisional. With respect to the certification of pilots, class means a classification of aircraft with similar operating characteristics; for example, single engine, multiengine, land, water, and helicopter. Category means a broad classification of aircraft, such as airplane, rotorcraft, and glider.

Aircraft Owners and Pilots Association
The world's largest aviation organization, dedicated to making flying safer, more productive, more affordable, and more fun. AOPA is headquartered in Frederick, Maryland.

airfoil
A device that interacts with a moving stream of air to produce lift or thrust. Wings, propellers, tail surfaces, helicopter rotor blades, ailerons and other control surfaces, and turbine blades are all airfoils.

airframe
From 14 CFR Part 1: "The fuselage, booms, nacelles, cowlings, fairings, airfoil surfaces (including rotors but excluding propellers and rotating airfoils of engines), and landing gear of an aircraft and their accessories and controls."

airline transport pilot certificate (ATP)
In the United States, the pilot certificate required to act as pilot in command of an aircraft for an air carrier (airline) and for certain other operations. The ATP is often referred to as "the Ph.D. of aviation." To be eligible for an ATP certificate, a pilot must be at least 23 years old and hold a current first-class medical certificate. In general, an ATP applicant must also have at least 1,500 hours of flight time, including minimum amounts of time on cross-country flights, flights at night, and instrument flight time.

airship
An engine-driven, lighter-than-air aircraft that can be steered.

airspace
The part of the atmosphere that lies above the surface and is under the jurisdiction of a nation or controlling authority. In the United States, there are two categories of airspace: regulatory and nonregulatory. Within those two categories, there are four types of airspace: controlled, uncontrolled, special use, and other. Airspace is assigned to one of the types according to the density and complexity of the air traffic using the airspace, the types of operations conducted in that airspace, and other factors.

airspeed
The rate at which an aircraft moves through the surrounding air. Pilots use several types of airspeed during flight. For example, indicated airspeed (IAS) is the speed shown on the airspeed indicator (usually in knots). Pilots use IAS to control an aircraft and manage its performance. Calibrated airspeed (CAS) is IAS corrected for instrument and installation error. True airspeed (TAS) is IAS corrected for changes in atmospheric temperature and pressure. Pilots use TAS to solve navigation problems.

The aircraft's speed over the ground—ground speed—is TAS corrected for the effect of a headwind or tailwind.

airspeed indicator
The instrument that displays an aircraft's speed relative to the air in which it is moving. Most modern aircraft have airspeed indicators calibrated in knots or in Mach number.

altimeter
The indicator that displays an aircraft’s present altitude. It is usually calibrated to give mean sea level (MSL) altitude. Most altimeters are called "pressure altimeters" because they measure the decrease in atmospheric pressure as the aircraft climbs. Because of this, the altimeter must be calibrated to the local atmospheric pressure to compensate for regional variations in pressure that would make the readings inaccurate.

altimeter setting
The value to which the scale of a pressure altimeter is set so it reads true altitude at field elevation. The setting is usually given in inches of mercury (Hg) or millibars.

altitude
Height of the aircraft above a reference level. Altitude above ground level (AGL) is the absolute height above the earth. Altitude above mean sea level (MSL) is the height above the average level of the earth’s oceans.

altocumulus
White or gray layers or patches of middle clouds, often with a waved appearance. Altocumulus clouds appear as rounded masses or rolls. They are composed mostly of liquid water droplets which may be supercooled. At subfreezing temperatures, altocumulus clouds may contain ice crystals.

altocumulus castellanus
A type of middle cloud that includes some vertically developed, cumuliform protuberances (some of which are taller than they are wide, as castles). These cumuliform sections give the cloud a crenelated or turreted appearance that is especially evident when seen from the side. This cloud indicates instability and turbulence at the altitudes where it appears.

altostratus
Stratiform—flat, layered—clouds that form in the middle altitudes. The height of their bases ranges from 6,500 to 23,000 feet (1,980 to 7,000 meters) in middle latitudes.

anemometer
An instrument for measuring wind speed.

aneroid barometer
A barometer that operates on the principle of changing atmospheric pressure bending a metallic surface, which, in turn, moves a pointer across a scale graduated in units of pressure.

angle of attack
The angle between the wing and the oncoming airflow—the relative wind. It's important to understand that the angle of attack is related to the direction in which an aircraft is moving, not to the angle the wing makes with the horizon. In general, as angle of attack increases, so does the amount of lift a wing produces. However, at a specific point, called the "critical angle of attack," the air flowing over a wing can no longer follow the airfoil's contour, and it becomes turbulent. The sudden loss of lift at this point is called a "stall."

angle of incidence
The angle at which a wing or propeller blade is mounted to the aircraft fuselage or to the propeller hub. The pilot cannot control the angle of incidence.

anticyclone
An area of high atmospheric pressure with a closed anticyclonic circulation. Viewed from above, the circulation is clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere, and undefined at the equator.

antitorque pedals
Pedals in a helicopter that control the tail rotor speed to compensate for the torque of the main rotor. Antitorque pedals are used to maintain coordinated flight.

anvil cloud
Popular name given to the top portion of a cumulonimbus (thunderstorm) cloud that has an anvil-like appearance.

approach
The phase of flight just prior to touchdown during landing. It is important that airspeed and attitude be stabilized during approach.

approach attitude
The aircraft’s longitudinal axis angle with respect to the horizon when making a landing approach.

approach lighting system (ALS)
Color-coded or sequenced flashing lights that clearly define the approaches to a runway. The ALS helps pilots make the transition to visual references at the end of an instrument approach. It can also aid pilots operating at night under visual flight rules (VFR).

area forecast (FA)
In the United States, a forecast of general weather conditions over an area the size of several states. It is used to forecast en route weather and to interpolate conditions at airports that do not issue terminal forecasts. FAs are issued three times a day by the National Aviation Weather Advisory Unit (NAWAU) in Kansas City, Missouri, for each of six areas in the contiguous 48 United States. Other offices issue reports for Alaska, Hawaii, and the Gulf of Mexico.

An area forecast contains a 12-hour specific forecast, followed by a 6-hour (18-hour in Alaska) categorical outlook, giving a total forecast period of 18 hours. The information in an FA includes several parts, including a synopsis, VFR clouds and weather, and other items of significance to VFR flight, including thunderstorms and strong winds, if forecast.

area rule
An aeronautical engineering principle that helps designers reduce drag at speeds near the speed of sound. Pinching in the fuselage where it meets the canopy, tail, or wings—creating the so-called Coke-bottle effect—cuts drag dramatically, reducing the amount of power an aircraft needs to reach supersonic speeds.

Aresti notation
A system of "aerocryptographics" for pilots and judges at aerobatic competitions and airshows. Named for José Louis Aresti, a Spanish aerobatic pilot who devised the notation system and built a dictionary of maneuvers and sequences.

artificial horizon
See attitude indicator.

aspect ratio
The ratio between the length (span) and width (chord) of a wing. In general, a wing with a high aspect ratio is more efficient than a wing with a smaller aspect ratio.

atmospheric pressure
The pressure exerted by the air on the earth and everything on it. This is measured in inches (or millibars) of mercury on an instrument called a barometer. Thus, the term "barometric pressure" is frequently interchanged with atmospheric pressure. Typically, the pressure measures between 28 and 32 inches of mercury at sea level, decreasing at higher elevations. See also barometer.

attitude
The aircraft’s position around its axis.

attitude flying
Flying based on an aircraft’s attitude (orientation to the world around it).

attitude indicator (artificial horizon)
The instrument that shows the aircraft's pitch and bank attitudes with respect to the ground. Pilots use the attitude indicator, sometimes called the "artificial horizon," when the true horizon isn't visible.

autocoordination
In Flight Simulator, an option that automatically synchronizes the actions of the ailerons and rudder to maintain coordinated, that is, balanced, flight, especially during turns. You can turn autorudder on or off by choosing the Realism Settings command from the Aircraft menu.

autogyro
An engine-powered aircraft that uses a propeller for forward motion and a large, free-rotating, horizontal rotor for lift. Also spelled "autogiro."

automatic direction finder (ADF)
A radio navigation instrument that receives signals from nondirectional radio beacons (NDBs) or AM radio stations. The needle on the ADF indicator always points toward the selected radio signal. A pilot can determine the magnetic bearing to the station by using the formula, Relative Bearing + Magnetic Heading = Magnetic Bearing.

automatic pilot (autopilot)
A device that automatically controls an aircraft. Similar in concept to the cruise control feature on an automobile, simple autopilots keep an airplane's wings level. More sophisticated autopilots can fly an airplane from immediately after takeoff all the way to its destination and then make an automatic landing. Autopilots use gyros and other sensors to keep track of the airplane's altitude, speed, and position, and then send signals to the appropriate controls to keep the airplane on course and altitude.

automatic terminal information service (ATIS)
A continuous-loop recording played over a specified frequency that gives pilots the current weather, runway or runways in use, and other airport information. ATIS is available at many airports with an operating control tower. The tape is updated hourly or whenever there is a significant change in weather or airport information. Each update is labeled with a letter of the alphabet, which is pronounced according to the ICAO standard. For example, the broadcast at 11:00 a.m. might be designated "Information Delta"; the next update would be designated "Information Echo." Pilots are expected to listen to the ATIS and indicate that they have received the current information before they make initial contact with air traffic controllers.

autorotation
The descent of a helicopter without power being applied to its rotor. Aerodynamic forces cause the rotor to spin.

autothrottle
In addition to an autopilot, Flight Simulator jets are equipped with an auothrottle that can control airspeed automatically . The autothrottle works independently from the autopilot, although most autothrottle controls are presented on the mode control panel (MCP) along with the autopilot controls.

avgas
Grades of gasoline approved for use in aircraft.

avionics
Derived from "aviation electronics," this term most commonly refers to the electronic communication, navigation, and flight-control equipment on board an aircraft.

axial flow
Flow of air in a path parallel to the center. In an axial-flow turbine engine, air enters the front and follows a straight path through a series of turbine blades that compress the air before it enters the combustion chamber.

axis indicator
In Flight Simulator, a display that shows the current axis of the aircraft and gives a good indication of where the aircraft's center is pointing. You can turn the axis indicator on and off or change its shape by choosing View Options from the Views menu, and then choosing an Axis Indicator option.

azimuth
Angular measurement made in a horizontal plane and in a clockwise direction from a fixed reference direction to an object. Two points on adjacent legs that both extend from the same right angle are said to be 90 degrees in azimuth from one another.

---

balloon (verb)
To increase the pitch attitude and angle of attack too rapidly. Ballooning can lead to a low-altitude stall, a hard landing, or porpoising if a tricycle-gear airplane lands nose-wheel first.

bank
The angle of an airplane's wings with respect to horizon; rotation about an aircraft's longitudinal axis. Pilots control bank using the ailerons. Airplanes turn principally because banking the wings creates a horizontal component of lift. Pilots measure bank in degrees.

barnstormer
A pilot who tours the countryside, performing at exhibitions and taking passengers on sightseeing flights.

barometer
An instrument for measuring the pressure of the atmosphere. The two most common types are mercurial and aneroid.

barometric altimeter
A barometer that measures altitude by registering changes in atmospheric pressure as an aircraft climbs or descends. Large aircraft often are also equipped with a radio altimeter that measures the height of an aircraft by sending a radio beam to the ground and converting the time it takes to return into height above the surface. Radio altimeters are particularly useful during instrument approaches.

barometric pressure
See atmospheric pressure.

base leg
The leg flown in a standard left or right traffic pattern that is at right angles to both the downwind and final approach legs and is parallel to the threshold of the runway.

bearing
The horizontal direction to one object from another. An example would be the horizontal direction to a navigation transmitter from an aircraft.

Beech, Walter
1891–1950. American aircraft manufacturer. In 1932, Beech and his wife, Olive Ann, founded Beech Aircraft, which became a leading producer of light airplanes. Earlier in his aviation career, Beech was a salesman and test pilot for the Swallow Airplane Company. Failing to persuade the company to adopt metal airframes, he quit and teamed up with Clyde Cessna and Lloyd Stearman to form the Travel Air Manufacturing Company, of which Beech was president.

Bell, Alexander Graham
1847–1922. Scottish-born American inventor. Bell's early experience in teaching deaf students led to an interest in communications. By 1875, he was an expert in electric wave transmission, and the following year, he invented the telephone. In 1892, Bell became interested in aviation. He experimented with kites, photographed Samuel Langley's successful launch of the model Aerodrome, and later formed the Aerial Experiment Association with a group of young aviators that included Glenn Curtiss.

Bernoulli, Daniel
1700–1782. Swiss scientist. His most important discovery, known as "Bernoulli's principle," states that the total energy of fluid in motion remains constant: If its speed increases, its pressure decreases; if its speed decreases, its pressure increases. This principle was important to early scientists studying airflow, and their applications led to the design of wings capable of lifting heavier-than-air craft off the ground.

Bernoulli's principle
The physical law stating that the total energy of fluid in motion remains constant: if its speed increases, its pressure drops; if its speed decreases, its pressure increases. Wings are designed to exploit this relationship by accelerating air that passes over their curved upper surfaces, thereby decreasing its pressure. The difference between the lower pressure above a moving airfoil and the relatively higher pressure below creates lift. This principle was first established by the Swiss scientist Daniel Bernoulli (1700–1782). Although aeronautical engineers continue to debate the theory of how lift is created, Bernoulli's principle is still regarded as a fundamental explanation of how airfoils produce lift.

biplane
An airplane equipped with two pairs of wings, usually one above and one below the fuselage. Early-model biplanes had their wings connected to each other with struts and wires. Modern biplanes are most often used for aerobatic flying.

bleed off
To decrease airspeed or altitude in a slow, carefully controlled manner.

blimp
A nonrigid airship generally shaped like a cigar. Internal gas pressure maintains both the blimp's buoyancy and its shape. There are many theories as to the term's origin—one of the most popular states that "blimp" is the sound made if you plunk the envelope with your finger.

Boeing
American aircraft company, founded by William Boeing in 1916, and today, the world's largest manufacturer of commercial airplanes. Boeing also produces military aircraft and spacecraft, but it is best known for its "7" series of passenger jetliners—the 707, 727, 737, 747, 757, 767, and 777.

booster
See performance booster.

buffet
Vibration, usually affecting the horizontal stabilizer and elevator, caused by turbulent airflow. Buffet occurs as the wing approaches its critical angle of attack and the smooth flow of air over the top surface of the wing becomes turbulent, like water flowing over rocks. This turbulent air strikes the airplane's tail surfaces and induces a mild vibration that the pilot feels through the flight controls. Pilots use the onset of buffeting as one indication of an impending stall.

Buys Ballot's law
The law that states that if an observer in the Northern Hemisphere stands with his or her back to the wind, lower pressure is to the left.

---

cache
See disk cache.

calibrated airspeed (CAS)
Indicated airspeed (IAS) corrected for instrument and installation error. The position of the pitot tube and static ports, flap setting, and pitch attitude of an aircraft can affect the accuracy of the airspeed indicator—especially at the low end of an airplane's speed range. Aircraft operating handbooks and placards on the instrument panel often include a table to help pilots determine CAS.

call sign
The identification that ATC and a pilot use for a particular flight or aircraft. Call signs are generally a combination of the aircraft type or manufacturer and the aircraft registration for civilian planes, a combination of the airline and flight number for airline flights, and a combination of branch of service and flight number for military flights. Call signs should always be included in any communication with ATC to avoid confusion about who’s talking.

canard
A horizontal surface, mounted in front of an aircraft's main wing, that serves as a stabilizer to control pitch. Canards also reduce drag by cutting down the force generated by a conventional tail. Canards were first used in the pioneering days of aviation, and today, their principle advantage in modern design is in preventing deep stalls. The canard is designed to stall before the main wing, lowering the angle of attack of the main wing before that wing can stall. The term "canard" is also used to describe any aircraft that flies tail first.

canopy
A transparent enclosure that covers an aircraft cockpit. Modern canopies are formed from a single piece of transparent plastic.

canted gyroscope
A gyroscope within a flight instrument, usually the turn coordinator, with its rotational axis tilted, or canted, with respect to the aircraft's longitudinal axis. The tilted axis causes the gyro to respond to a banking or yawing motion.

cantilever wing
A wing attached to the fuselage without external struts or wire bracing.

carburetor icing
Because of the effect of vaporizing fuel and the decrease in pressure in the carburetor’s venturi, ice can form in the carburetor throat with the presence of moisture in the air. This can occur even on warm days with temperatures as high as 100 F (38 C), but is more likely when temperatures are below 70 F (21 C) and the relative humidity is above 80 percent.

carburetor
The part of a piston engine that mixes fuel and air, creating a combustible mixture that is ignited and burned in the cylinders. Filtered air enters the carburetor through a venturi, a narrow throat. As the air flows through the venturi, its velocity increases. The air pressure drops, creating a partial vacuum that draws in fuel through a needle valve. The fuel, atomized into tiny drops, mixes with the air, and the mixture flows into the intake manifold, a tube with branches that delivers the mixture to each cylinder.

category (aircraft)
In the United States, with respect to the certification of aircraft, a grouping based on intended use or operating limitations. In the United States, all aircraft are certified in a specific category. Examples include: transport, normal, utility, acrobatic, limited, restricted, and provisional. With respect to pilot certification, category refers to a broad classification of aircraft, such as airplane, rotorcraft, and glider.

ceiling
The height above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as "broken" or "overcast." The height of the ceiling is one of the factors that determines whether flight under visual flight rules (VFR) is possible. In the United States, the ceiling generally must be at least 1,000 feet (305 meters) for operations under VFR in controlled airspace.

ceilometer
A cloud-height measuring system. It projects light on the cloud, detects the reflection by a photoelectric cell, and determines height by triangulation.

cell
Another name for a thunderstorm or cumulonimbus cloud. Pilots, air traffic controllers, and meteorologists often use this term to report areas of intense precipitation, lightning, or turbulence associated with a thunderstorm.

center of gravity (CG)
The point at which an airplane would balance if it were suspended by a cable. The CG is also the point at which the three axes—longitudinal, lateral, and vertical—of an airplane intersect and the point at which the four fundamental forces of flight—lift, weight, thrust, and drag—are assumed to act.
Pilots must ensure that the CG of a loaded aircraft falls within a specified range, called the "CG envelope." If the CG is outside the envelope, the aircraft may be difficult or impossible to control. To determine the position of the CG, divide total aircraft movement by total weight.

centerline
A painted line running along the center of a runway (or taxiway) that divides it into two sections.

centrifugal flow
Flow of air outward from the center of rotation. Early turbine engines used the principle of centrifugal flow to compress air before it entered the combustion chamber.

centrifugal force
The force that tends to impel an object outward from the center of rotation.

Certified Flight Instructor
An individual certified by a country’s aviation authority to provide flight instruction.

certified flight instructor certificate
In the United States, a certificate that allows a person to give instruction to applicants for other pilot certificates. To be eligible for a certified flight instructor (CFI) certificate, a person must be at least 18 years old and hold a current second-class medical certificate and a commercial pilot certificate. The applicant must demonstrate proficiency in teaching and in demonstrating the maneuvers required for various pilot certificates. A CFI certificate must be renewed every 24 months.

Cessna Aircraft Company
American manufacturer of aircraft founded in 1927 by Clyde Cessna. Production of the Cessna "A" series began in 1928, but the Great Depression brought business to a near standstill. However, the success of civil and military versions of the T-50 "Bamboo Bomber" created a wartime boom for Cessna that helped the company later produce more aircraft than any other manufacturer.

Cessna, Clyde
1880–1954. American aviation pioneer and aircraft manufacturer. Inspired by the Moisant International Aviators' air circus, Cessna built his own monoplane and became a barnstormer. Later, he teamed up with Walter Beech and Lloyd Stearman in the Travel Air Manufacturing Company before forming the Cessna Aircraft Company in 1927. Cessna retired in 1935, handing the reins to his nephew, Dwane Wallace.

checklist
A procedure list used by pilots to systematically check and set an aircraft’s systems prior to takeoff and landing and during emergencies.

checkride
A flight administered by an examiner from a country’s aviation authority to test an individual before issuing a pilot certificate to that individual.

chord
Also called the "chord line," an imaginary line drawn in cross-section from the leading edge to the trailing edge of a wing. The chord line is the reference used to determine the angle of attack and to draw lift, weight, and other vectors when analyzing a wing that is producing lift.

cirriform
Clouds composed mostly or entirely of small ice crystals, usually transparent and white, often producing halo phenomena not observed with other cloud forms. Cirriform clouds include cirrus, cirrocumulus, and cirrostratus clouds. The average height of cirriform clouds ranges upward from about 20,000 feet (6,096 meters) in the middle latitudes.

cirrocumulus
A cirriform cloud appearing as a thin sheet of small white puffs resembling flakes or patches of cotton without shadows; sometimes confused with altocumulus.

cirrostratus
A cirriform cloud appearing as a whitish veil, usually fibrous, sometimes smooth. Cirrostratus clouds often produce halo phenomena and may totally cover the sky.

cirrus clouds
Feathery, high-altitude clouds composed of ice crystals. Cirrus clouds typically form in shallow, scattered-to-broken layers on the equatorial side of the jet stream when high-level moisture is available. They indicate strong upper winds. Cirrus clouds do not in themselves constitute a hazard to aircraft, but they should alert pilots to the possibility of high-altitude turbulence and the approach of developing or intense weather systems.

class
With respect to the certification of aircraft, this is a broad grouping of aircraft with similar characteristics of propulsion, flight, or landing. In the United States, the FAA recognizes the following classes of aircraft: airplanes, rotorcraft, gliders, balloons, landplanes, and seaplanes.

With respect to the certification of pilots, this is a grouping of aircraft with similar operating characteristics; for example, single engine, multiengine, land, water, and helicopter.

Class A airspace
In the United States, the airspace extending from 18,000 feet (5,486 meters) above mean sea level up to and including Flight Level 600 (approximately 60,000 feet [18,288 meters]). Formerly known as "positive control airspace (PCA)." All operations in Class A airspace are conducted according to instrument flight rules (IFR) under direct air traffic control. Class A airspace is not marked on aeronautical charts.

Class B airspace
In the United States, the airspace around the busiest major terminals. Formerly called a "terminal control area (TCA)," Class B airspace is typically arranged in rings centered on the primary airport. The innermost ring usually extends from the surface up to 10,000 feet (3,050 meters). The next rings each begin at different intermediate altitudes to allow nonairline traffic to transit the area beneath the Class B airspace and operate at satellite airports. This stair-step arrangement, drawn in cross-section, makes Class B airspace look like an inverted wedding cake.

All aircraft operating in Class B airspace must receive a clearance from air traffic control, regardless of current weather conditions. Aircraft must also have a transponder with automatic altitude-reporting capability. The boundaries of Class B airspace are marked with solid blue lines on United States aeronautical charts.

Class C airspace
In the United States, the airspace around airports that have an operating air traffic control tower, radar approach control, and a high level of IFR operations or airline traffic. Class C airspace is typically designated around second-tier airports that are not as busy as major terminals. Many military airports are also surrounded by Class C airspace.

Class C airspace is typically arranged in two rings centered around the primary airport. The innermost ring, with a typical diameter of 5 nautical miles (nm), generally extends from the surface up to 4,000 feet (1,220 meters). The next ring begins at 5 nm from the center and extends to 10 nm. It begins at 1,200 feet (365 meters) above the airport elevation and extends to 4,000 feet (1,220 meters). Class C airspace also includes an outer area, which extends to 20 nm from the primary airport

All aircraft operating in Class C airspace must establish communications with air traffic control, regardless of current weather conditions. Aircraft must also have a transponder with automatic altitude-reporting capability (Mode C). The boundaries of Class C airspace are marked with solid red lines on United States aeronautical charts.

Class D airspace
In the United States, the airspace around an airport that has an operating air traffic control tower. Class D airspace is typically a circle centered on the airport with a diameter of about 5 cm. It typically extends from the surface to 2,500 feet (762 meters) above the airport elevation. Class D airspace often includes extensions to ensure that the controlled airspace encloses instrument approaches.

All aircraft operating in Class D airspace must establish communications with the air traffic control tower, regardless of current weather conditions. The boundaries of Class D airspace are marked with dashed blue lines on United States aeronautical charts.

Class E airspace
Generally, in the United States, controlled airspace not designated as Class A, B, C, or D. Class E airspace includes low-altitude airways, extensions to Class D airspace, transition areas, and other airspace where air traffic control clearances are required when the ceiling and visibility do not meet the minimums for operations under visual flight rules (VFR).

Class E airspace typically begins at either 700 feet (213 meters) or 1,200 feet (365 meters) above the surface. It extends upward to the base of any overlying controlled airspace. When designated as an extension to Class D airspace, Class E airspace begins at the surface. Aircraft operating in Class E airspace must establish communications with the air traffic control tower only when operating under instrument flight rules (IFR). The boundaries of Class E airspace are marked with dashed red lines or with shaded red or blue lines on United States aeronautical charts.

Class G airspace
In the United States, airspace not designated as Class A, Class B, Class C, Class D, or Class E airspace. Class G airspace typically begins at the surface and extends upward to either 700 feet (213 meters) or 1,200 feet (365 meters) above ground level (AGL). The boundaries of Class G airspace are not explicitly marked on United States aeronautical charts.

clear ice
Relatively transparent ice with a homogeneous structure and few, small air spaces. Clear ice is usually associated with large, supercooled water drops typically found in cumuliform clouds. Clear ice forms when the liquid portion of a drop flows out over the aircraft surface before it freezes. Clear ice is hard, heavy, and difficult to remove.

clear-air turbulence (CAT)
Turbulence in an area where no clouds are present; in particular, turbulence at high altitudes. CAT is also sometimes used to describe turbulence associated with wispy cirrus clouds. Many phenomena can generate CAT, but it is often associated with the jet stream, especially in winter, when temperature contrast is greatest between cold and warm air. CAT also occurs in wind shears associated with sharply curved contours of strong lows, troughs, and ridges aloft, mountain waves, and areas of strong cold or warm air advection.

climbout
That portion of a flight between takeoff and the initial cruising altitude.

cloud layer
In Flight Simulator, a user-specified weather option that includes the type of cloud, the heights of the bases and tops of the clouds, visibility, turbulence, and whether icing conditions exist. You select these options in the Advanced Weather dialog box. You can create two cloud layers in each weather area.

col
In meteorology, the neutral area between two highs and two lows or the intersection of a trough and ridge. The col on a pressure surface is analogous to a mountain pass on a topographic surface.

cold front
Any non-occluded front that moves in such a way that colder air replaces warmer air.

collective
(Short for collective pitch control.) A helicopter's primary altitude and power control. It varies the lift produced by the main rotor system by increasing or decreasing the pitch of all the main rotor blades simultaneously (that is, collectively, hence the name of the control).

COM
Abbreviation for "communication"; usually refers to radio communication. Aircraft communications radios are typically labeled "COM 1," "COM2," and so forth.

commercial pilot certificate
In the United States, a pilot certificate that allows a person to act as pilot in command of an aircraft for compensation or hire. To be eligible for a commercial pilot certificate, a person must be at least 18 years old and hold a current second-class medical certificate and a private pilot certificate. The applicant must have at least 250 hours of flight time, including at least 10 hours of instruction in an aircraft with a controllable pitch propeller, flaps, and retractable landing gear. The applicant also must have at least 50 hours of cross-country flight time and meet other experience and knowledge standards.

compass rose
A graduated circle painted on a ramp or taxiway to be used by pilots to verify and compensate a magnetic compass in their aircraft.

composites
Materials consisting of glass or carbon fibers set in a matrix of plastic or epoxy resin. Composites are increasingly used in the construction of modern aircraft because they are stronger and lighter than metals. Many new kit-built and experimental aircraft are built primarily out of composites.

condensation
The change of water vapor to liquid water.

condensation trail
A cloud-like streamer that frequently forms behind aircraft flying in clear, cold, humid air. Condensation trails are also called "contrails" and "vapor trails."

contrarotating propellers
A pair of propellers, mounted one behind the other on the same engine, that rotate in opposite directions. Contrarotating propellers make more efficient use of the power from the engine and counteract the gyroscopic and aerodynamic forces generated by a single large propeller turning in one direction. The principle was first applied to an Italian Macchi-Castoldi racing plane in 1933.

control yoke
The steering-wheel-like control connected to the ailerons and elevator. A pilot turns the yoke to move the ailerons and bank the wings. The pilot moves the yoke forward and back to move the elevator, which lowers and raises the nose. Some airplanes have a stick or "joystick" instead of a control yoke.

controlled airport
An airport with an operating control tower. You must obtain a clearance to take off or land at a controlled airport and follow the tower controller's instructions while operating on or around a controlled airport. Controlled airports are usually located within Class B, Class C, or Class D airspace. The basic rules for operating at an airport with a control tower are contained in FARs 91.131, 91.130, and 91.129 and in the Aeronautical Information Manual (AIM).

controlled airspace
In the United States, airspace in which operations may require a clearance from air traffic control (ATC), especially when a low ceiling or restricted visibility requires aircraft to operate under instrument flight rules (IFR). Controlled airspace is divided into several classes (A, B, C, D, E, and G) according to standards established by ICAO. Each class has different operating rules, including requirements for communications with ATC, specific clearances from ATC, pilot qualifications, aircraft equipment, maximum operating speeds, and minimum ceiling and visibility for operations under visual flight rules (VFR).

convection
In meteorology, atmospheric motions that are predominantly vertical, resulting in vertical transport and mixing of atmospheric properties.

convective clouds
Clouds exhibiting vertical development; cumuliform clouds. Convective clouds form in air that is moving primarily upward rather than horizontally.

coordinated flight
Flight, especially during turns, in which the horizontal and vertical forces at work on the airplane are in balance. The inclinometer, part of the turn coordinator or "needle and ball," shows the pilot whether the aircraft is in coordinated flight. When the ball moves to the inside of a turn, the airplane is "slipping"—that is, the angle of bank is too steep for the rate of turn. If the ball moves to the outside of a turn, the airplane is "skidding"—that is, the rate of turn is too great for the angle of bank. Uncoordinated flight can also occur during flight at low airspeed if the pilot does not apply enough rudder to compensate for the yaw force created by the propeller and engine at high power settings.

Coordinated Universal Time
See Greenwich Mean Time.

coordinates
The intersection of lines of reference, expressed in degrees, minutes, and seconds of latitude and longitude, used to determine a position or location.

Coriolis force
A deflective force resulting from the earth's rotation. In the Northern Hemisphere, the Coriolis force deflects moving air to the right. In the Southern Hemisphere, the force deflects moving air to the left. The force acts at a right angle to wind direction and is directly proportional to wind speed. That is, as wind speed increases, Coriolis force increases. At a given latitude, double the wind speed, and you double the Coriolis force. Coriolis force also varies with latitude, from zero at the equator, to a maximum at the poles. It influences wind direction everywhere except immediately at the equator; but its effects are most pronounced in middle and high latitudes.

course deviation indicator
A vertical needle on the omnibearing indicator (OBI) that shows your deviation from the very high frequency omnidirectional range (VOR) radial set by the course selector. If the needle is to the right of center, the radial lies to the right of your current position.

course selector
The knob or other control a pilot uses to select a VOR radial. Also known as the "omnibearing selector (OBS)."

cowling
That removable portion of the airframe of an aircraft that covers the engine.

crab angle
The angle between an aircraft's heading and ground track. This angle is determined by the crosswind component and the airspeed of the aircraft; the stronger the crosswind and the lower the airspeed, the larger the aircraft's crab angle.

critical angle of attack
The angle of attack at which a wing stalls. The critical angle of attack is determined by the airfoil design. A wing always stalls when it reaches its critical angle of attack, regardless of the aircraft's airspeed or attitude. Most general aviation aircraft have wings with a critical angle of attack of 18–20 degrees.

cross-country flight
A flight from one airport to another covering a distance great enough to require the use of some form of navigation.

crosswind
A wind that is blowing at an angle to the flight path of an aircraft. Pilots must correct for crosswinds by pointing the nose of the aircraft at some angle into the wind in order to maintain the desired course across the ground.

cruise speed
The average speed of an aircraft during straight-and-level flight at normal power settings.

cumuliform
A descriptive term applied to all convective clouds that develop vertically rather than into horizontally extended stratiform types. Cumuliform clouds develop in unstable air.

cumulonimbus clouds
Dense, vertical cloud formations that usually produce heavy rain, thunderstorms, or hailstorms. "Cumulus" is from the Latin for "heap"; "nimbus," for "rain cloud." Like all cumuliform clouds, cumulonimbus form in unstable air. A cumulonimbus cloud is heavy and dense. It can have massive towers, often with tops in the shape of an anvil or plume. Cumulonimbus clouds are often associated with virga, precipitation, and low ragged clouds (scud), lightning, thunder, and sometimes hail. They occasionally produce a tornado or a waterspout. Cumulonimbus clouds can extend well into the stratosphere.

cumulonimbus mamma
A cumulonimbus cloud having hanging protuberances, such as pouches, festoons, or udders, on the underside of the cloud. It is usually indicative of severe turbulence.

cumulus clouds
Fluffy, flat-based clouds formed by rising, unstable air. "Cumulus" is from the Latin for "heap." Cumulus clouds are usually dense and well defined. They develop vertically in the form of rising mounds; the bulging upper part often resembles cauliflower. Sunlit parts of these clouds are mostly brilliant white; their bases are relatively dark and nearly horizontal.

cyclic
Cyclic controls a helicopter's pitch-and-bank attitude (serving the same function as the yoke or stick that controls the elevator and ailerons in an airplane). It is the primary airspeed control in flight. Applying forward cyclic causes airspeed to increase. Aft cyclic reduces airspeed.

cyclone (low)
An area of low atmospheric pressure with a closed, cyclonic circulation. Viewed from above, the circulation is counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere, and undefined at the equator. Because cyclonic circulation and relatively low atmospheric pressure usually coexist, in common practice, the terms "cyclone" and "low" are used interchangeably. Also, because cyclones often are accompanied by inclement (sometimes destructive) weather, they are frequently referred to simply as storms. "Cyclone"" is frequently misused to denote a tornado.

---

datum
An imaginary vertical plane or line used as a reference in weight-and-balance calculations; specifically, the line from which all "arm" measurements are taken. The center of gravity (CG) is often referenced from the datum. The aircraft manufacturer sets the datum for a particular airplane. Most manufacturers set the datum at the position of the firewall separating the engine from the passenger compartment or at the tip of the propeller spinner.

da Vinci, Leonardo
1452–1519. Italian artist and scientist; a man of towering intellect and a leader in the transition from the medieval to the modern European consciousness. Da Vinci's scientific interests included anatomy, botany, geology, optics, and mechanics. His designs for human-powered flying machines verged on the fanciful, but his sketches for a hypothetical parachute were scientifically sound.

dead reckoning (de’d reckoning)
The navigation of an airplane solely by computations based on airspeed, course, heading, wind direction and speed, ground speed, and elapsed time. The term derives from "deduced" reckoning. Also known as "ded reckoning."

deadstick
To execute a landing without engine power.

decision height (DH)
The altitude at which, during an ILS or other precision landing approach, a pilot must decide whether to land or execute a missed approach. A typical ILS approach has a DH of 200 feet (60 meters) above ground level.

density altitude
Pressure altitude corrected for variations from standard temperature. Density altitude measures the actual density of the air and, therefore, is a critical factor in calculating aircraft performance. If the temperature is warmer than standard, density altitude is higher than pressure altitude, and vice versa. For example, assume an airport is 3,000 feet (914 meters) above sea level and the altimeter setting is 29.92 (1013.2 millibars). If the temperature is 90 F (32 C), the density altitude is 5,592 feet (1,704 meters). An airplane taking off from this airport performs as if it's at nearly 5,600 feet (1,705 meters). The engine produces less power, the propeller is less efficient, and the wings create less lift. The airplane uses more runway to reach takeoff speed and climbs at a lower rate.

design maneuvering speed
See maneuvering speed.

detonation
The sudden, explosive burning of the air/fuel mixture in the cylinders of a piston engine. Detonation usually occurs when the air/fuel mixture is too lean—when there is not enough fuel for the current weight of air entering the cylinder. Detonation imposes excessive loads on the pistons and other engine components. If not corrected by enriching the mixture, detonation causes engine damage and may lead to sudden engine failure.

dew
Water condensed onto grass and other objects near the ground when the temperature of those objects has fallen below the initial dew point of the surface air but is still above freezing.

dew point
The temperature to which air must be cooled to reach saturation, that is, 100 percent relative humidity. Weather reports usually include the air temperature and dew point temperature. A narrow "temperature/dew point spread" (less than 2.8 C or 5 F) indicates a strong likelihood of fog, clouds, or precipitation.

dihedral
The angle at which an aircraft's wings tilt upward from the fuselage forming a slight "V" shape as seen from head-on. Dihedral increases stability about the longitudinal, or roll, axis of an airplane. It tends to level the wings after an airplane is established in a shallow bank. Anhedral, tilting the wings downward, produces the same effect, but it is much less common.

directional gyro
See heading indicator.

direct user access terminal (DUAT)
Automated weather service in the United States allowing pilots to receive weather briefings and file flight plans using personal computers.

dirigibles
A general term for airships or lighter-than-air powered aircraft. An engine and the ability to steer distinguishes airships from balloons. From the Latin, dirigere, "to direct."

disk cache
An information storage area. Flight Simulator stores scenery information in a cache directory on your hard drive for quick access as you fly.

distance-measuring equipment (DME)
Avionics that determine and display distance in nautical miles. DME equipment, which operates in the UHF band, is usually installed at a VOR station. A complementary unit installed in the aircraft transmits timed pulses to the ground station, which replies. The unit in the aircraft converts the time between the pulses and the replies into distance and also derives ground speed and time to reach the station. The distance displayed by DME equipment is the "slant range" from the station, that is, the length of the hypotenuse of a right triangle formed by the aircraft's altitude and its distance from the station as measured along the ground. The difference between the slant range and the aircraft's actual distance from the station is negligible when the aircraft is more than about 10 nm from the station and at a moderate altitude.

downburst
A strong downdraft that results in damaging winds at or near the ground. The size of a downburst can vary from less than one-half mile to more than 10 miles.

downdraft
A relative small-scale downward current of air often observed on the lee side of large objects that restrict the smooth flow of the air, or in precipitation in or near cumuliform clouds.

downwind
The leg of a standard left or right traffic pattern that is parallel to the runway but prior to the turn to base and final.

drag
The resistance of an object to movement through a fluid. With respect to aircraft, drag is one of the four fundamental forces in flight. It opposes thrust. There are two basic types of drag. Parasite drag is caused by friction. The airplane surface, antennas, landing gear, and other appendages all cause parasite drag, which increases in proportion to the square of the aircraft's velocity. Induced drag is a by-product of lift. At the tip of a wing, air moves from the high-pressure area below the wing to the low-pressure area above. The energy used to create these vortices manifests itself as induced drag, which increases as airspeed drops.

drift
The displacement of an aircraft from its intended course caused by wind.

drizzle
A form of precipitation composed of small water droplets that appear to float with the air currents while falling in an irregular path. Drizzle differs from rain, which falls in a comparatively straight path, and fog droplets, which remain suspended in the air. In weather reports and forecasts, drizzle is indicated by the abbreviation DZ. On weather charts in the United States, drizzle is indicated by a comma (,).

dry adiabatic lapse rate
The rate of decrease of temperature with height when unsaturated air is lifted adiabatically (due to expansion as it is lifted to lower pressure).

dual
In aviation, this term refers to either the number of sets of controls in the plane, or the fact that a pilot is flying under the supervision of an instructor.

dissymmetry of lift
A condition in which the main rotor of a helicopter does not produce lift equally across the entire rotor disk. This occurs only when in forward flight or hovering in a wind and is most apparent in a retreating blade stall.

---

Earhart, Amelia
1897–1937. American aviator and the first woman to fly solo nonstop across the Atlantic. Earhart was also the first woman to cross the United States nonstop (1932) and the first pilot to fly solo from Hawaii to California (1935). As an aeronautics adviser at Purdue University and an active proponent of women's aviation, she decided to attempt the first around-the-world flight made by a woman. Her second try in 1937 ended when she and navigator Fred Noonan disappeared over the Pacific.

EFAS (Flight Watch)
Shorthand for En-route Flight Advisory Service, a weather service provided in the United States by Flight Service Stations. EFAS disseminates weather reports and forecasts, particularly en route weather, and receives and distributes pilot reports (PIREPS). Flight Watch is available throughout the United States on frequency 122.0 from 5,000 to 17,500 feet (1,500 to 5,000 meters) and on a variety of discrete frequencies for high-altitude traffic.

effective translational lift
When air moves horizontally across a helicopter blade, the rotor produces more lift at a given power setting. This occurs in forward flight or when hovering in a light wind. It might take 90 percent of a helicopter's available power to fly at 20 knots and take only 80 percent to fly at 45 knots.

electronic flight instrument system (EFIS)
Computer-driven cockpit instruments and displays that replace electromechanical instruments in many modern flight decks. The EFIS can display basic flight information, such as engine status, moving maps, checklists, and so on.

elevator
A movable control surface located on the horizontal stabilizer of an aircraft's empennage, or tail. Although its name implies that the elevator makes the airplane climb or descend, it actually controls only the aircraft's pitch attitude, that is, the angle of the nose above or below the horizon. The pilot moves the elevator by applying forward pressure on the control yoke or stick to decrease pitch attitude and by applying back pressure to increase the pitch attitude. On some aircraft, the entire horizontal stabilizer moves. This arrangement is often called a "stabilator" or "flying tail."

elevator trim
See trim.

empennage
The tail assembly of an airplane. The empennage usually includes the fin (vertical stabilizer), rudder, horizontal stabilizer, and elevator.

Experimental Aircraft Association (EAA)
One of the largest aviation organizations in the world, the EAA was founded in 1953 by Paul Poberezny and several other aviation enthusiasts as a club for people who build their own airplanes. The first fly-in convention that same year attracted about 40 people. Today, the EAA numbers more than 150,000 members around the world and brings more than 800,000 people each year to its Oshkosh Fly-In, including homebuilders, restorers of classic and antique aircraft, and warbird enthusiasts.

Extended Range Twin-Engine Operations (ETOPS)
An FAA certification that allows airlines to operate twin-engine aircraft for extended distances over water.

---

fast file
A feature of the Flight Service Station telephone system that allows a pilot to file IFR flight plans by recording them via voice mail. After the pilot hangs up, a briefer processes the flight plan.

feather
The action of turning the propeller blades until they are parallel to the aircraft fuselage. After an engine failure, a windmilling propeller can cause enough drag to seriously degrade aircraft controllability. Feathering reduces that drag by reducing aerodynamic forces on the prop, allowing it to stop windmilling. Feathering the prop is accomplished by moving the propeller levers in the cockpit into the feather position.

Federal Aviation Administration (FAA)
The federal government agency responsible for the regulation and promotion of aviation in the United States. The FAA regulates and certifies pilots, aircraft, airports, and airspace. It also manages the air traffic control system and supervises and inspects airlines, flight schools, flight instructors, maintenance technicians, and facilities. The FAA was created in 1958 to supersede the Civil Aeronautics Authority (CAA). It operated under the name Federal Aviation Agency until 1966, when it became the Federal Aviation Administration, an organization within the Department of Transportation.

Federal Aviation Regulations (FARs)
In the United States, the parts of the Code of Federal Regulations that pertain to the certification and regulation of pilots, flight schools and instructors, maintenance technicians and facilities, aircraft, air navigation, airspace, air traffic control, and other activities related to aviation and air commerce. Pilots must be familiar with and adhere to a variety of FARs. The most important parts of the FARs for day-to-day operations are: Part 1, Definitions and Abbreviations; Part 61, Certification: Pilots and Flight Instructors; and Part 91, General Operating and Flight Rules. Other parts pertain to commercial and airline operations, certificated flight schools, transportation of hazardous materials, certification standards for aircraft, and procedures for reporting accidents.

final
The leg of a standard traffic pattern that is aligned with the runway prior to touchdown. The final approach leg can be short for small planes in light air traffic or extend out a few miles from the threshold for large planes or in heavy air traffic.

fix
A reference point in space usually defined by a signal from one or more navigational aids and used by pilots for navigation.

fixed gear
Landing gear that cannot be retracted.

fixed-base operator
At an airport, a person or an organization that sells fuel, sells or rents aircraft, and possibly provides flight instruction.

flaps
A hinged portion of an airplane's wing, generally on the trailing edge, that can be lowered during takeoff and landing to increase the wings' lift and drag. When partially extended, a flap adds lift by increasing the camber, or curvature, of the wing. Because flaps extend into the oncoming air, they also increase drag, helping an aircraft descend steeply without building up speed. Modern aircraft use several types of flaps; the most common designs are plain, split, and Fowler. Often confused with ailerons, flaps are not the primary control surfaces of an airplane.

flare
To level off and establish the correct landing attitude just above the runway prior to landing. A pilot flares by applying back pressure to the control yoke or stick, which raises the nose of the aircraft. When done properly, the flare is a smooth, continuous transition from a nose-low, descending flight path to a nose-high attitude that almost stops the aircraft's descent.

flight level
The altitude measurement used by aircraft flying above 18,000 ft. Flight levels are expressed in three digits representing the pressure altitude in hundreds of feet. An aircraft flying at 35,000 feet is at FL350.

flight path
The track over the earth’s surface defined by an airborne aircraft.

flight plan
Specific information about a flight that is filed either orally or in writing with air traffic control.

flight school
A facility that offers aviation training.

Flight Service Station (FSS)
A government facility that provides a variety of services to pilots. FSS personnel take weather observations, brief pilots, coordinate flight plans, and assist aircraft in distress. In the United States, the FSS network includes one or more stations in each state. Each FSS typically has responsibility for a large geographic area. FSS specialists communicate with pilots over a network of remote transmitters and receivers. They also have direct telephone and computer connections to air traffic control facilities and search-and-rescue organizations.

fly-by-wire controls
Electronic flight control system in which there are no direct mechanical links between the controls in the cockpit and the aircraft's control surfaces. A computer detects movements of the flight controls, interprets them, and sends signals to move the rudder, ailerons, and elevators. Fly-by-wire systems were first used in fighter aircraft to make aircraft more maneuverable and ensure that the pilot could not exceed the design limitations of the aircraft. Today, large transports, such as the Boeing 777 and several Airbus models, include fly-by-wire systems.

fractus
Clouds in the form of irregular shreds. They have a torn, clearly ragged appearance. The term applies only to stratus and cumulus cloud types, such as cumulus fractus and stratus fractus.

frame rate
The speed at which successive frames of a computer image display on the screen, similar to the frames of a movie passing through the light source of a movie projector.

freezing level
A level in the atmosphere at which the temperature is 0 C (32 F).

front
The boundary between two different air masses. More specifically, a surface, interface, or transition zone of discontinuity between two adjacent air masses of different densities.

frontal zone
A front or zone with a marked increase of density gradient. The term is used to describe an area of rapid transition of meteorological elements.

frontogenesis
The initial formation of a front or frontal zone.

frontolysis
The dissipation of a front.

fuel injection system
A set of controls, pumps, nozzles, and other components used in many piston engines to deliver fuel to the cylinders. The system squirts fuel directly into the cylinders or just ahead of the intake valve, where it mixes with air. Because a fuel injection system requires high-pressure pumps, an air/fuel control unit, fuel distributor, and discharge nozzles for each cylinder, it's generally more expensive than a carburetor. However, a fuel-injected engine is more efficient than a carburetor, and fuel injection is used on most large piston engines.

fully articulated
One of three types of main rotor systems used in modern helicopters. A fully articulated system utilizes three or more rotor blades that flap independently to compensate for dissymmetry of lift. Fully articulated rotor systems are more expensive than semirigid systems, but are less susceptible to low G conditions and mast bumping. However, they are more affected by ground resonance.

fuselage
The body of an airplane that holds the crew and passengers or cargo. From the French, fuselé, for "spindle-shaped."

---

G force
See Gs.

game pad
A device used as a game controller that consists of a handheld body and several buttons. Unlike a joystick, the game pad uses a D switch, which is operated by the user’s thumb, for directional control.

general aviation
In the United States, a term applied to all aviation not related to the military or scheduled airline service. It includes flight training, charter flights, flying for pleasure, and business-related aviation. The term is also used to describe a broad class of aircraft not used by the military or scheduled airlines. General aviation is sometimes abbreviated as GA.

glass cockpit
Refers to the replacement of conventional cockpit gauges with computerized cathode ray tubes (CRTs) or liquid crystal displays (LCDs). A number of gauges are combined into the displays, and the pilot can often flip to different "pages" to view navigation or aircraft system information.

glide ratio
Ratio of horizontal distance traveled per unit of descent. For example, a sailplane with a 60:1 glide ratio travels 60 meters forward for every 1 meter it descends. A typical single-engine aircraft has a glide ratio of about 10:1.

glide path
The vertical path defined by an aircraft in a controlled descent.

glider
See sailplane.

glide slope
The electronic approach path projected as part of an instrument landing system (ILS). Glide slope transmitters, located near the end of a runway, send out radio signals to form the proper descent path to the runway. The angle of the glide slope is usually set about 3 degrees to the horizontal.

Global Positioning System (GPS)
A constellation of satellites that sends signals to a ground-based, seaborne, or airborne receiver. By interpreting three or more signals, the receiver can give incredibly precise information about the location of the receiver on the face of the earth, usually within 50 meters or so.

go-around
An air traffic control command for a pilot to abandon his approach to landing.

Greenwich Mean Time
The local time at the Greenwich Observatory, which lies on the prime meridian, or 0 degrees longitude. This time, now known officially as Coordinated Universal Time (UTC), is used in air navigation to provide a standard for proposed departure and arrival times, weather observations and forecasts, and air traffic control functions. Also known as Zulu time, UTC is expressed in a 24-hour format; for example, 6 P.M. is 1800 hours.

ground clearance
In aviation, this usually refers to the distance between the tips of an aircraft’s propeller blades and the ground surface.

ground effect
A decrease in induced drag as an aircraft flies near the surface. The effect is caused when the ground interferes with the normal flow of air from below the wing to the low-pressure area above the wing. Ground effect is most apparent when an aircraft's height is about one-half wingspan or less above the surface. In practical terms, an aircraft flying in ground effect stalls at lower airspeed than normal. Pilots can use ground effect to lift off from a soft or short field with a minimum ground roll, but they must accelerate to normal flying speed before climbing away from the surface.

ground fog
In the United States, a fog that conceals less than 0.6 percent of the sky and that is not contiguous with the base of clouds.

ground loop
An aggravated, uncontrolled, tight turn on the ground, usually during rollout after landing or while taxiing. A ground loop often involves a turn of more than 90 degrees and frequently results in one wing touching the ground. Aircraft with tailwheels (often called taildraggers) are most susceptible to this loss of control because the center of gravity in such aircraft lies behind the main landing gear.

ground school
The portion of pilot training that is conducted in a classroom on the ground.

ground speed
An aircraft's speed relative to the ground; an aircraft's true airspeed corrected for the effects of a headwind or tailwind. For example, if an aircraft is flying level at 120 knots with a 15-knot headwind, its ground speed is 105 knots.

Gs
A measurement of the load factor, or apparent gravity, experienced by an aircraft during flight. One G represents the force of gravity exerted on a body at rest. When an aircraft climbs, turns, or accelerates, positive G forces act upon it. When it descends or decelerates, negative G forces act upon it.

gust
A sudden, brief increase in wind. In weather reports and forecasts, gusts are indicated by "G," followed by the 2- or 3-digit maximum speed, and units, usually knots (KT). For example, "G25KT" indicates wind gusting to a maximum of 25 knots.

gyro
Short for gyroscope, an instrument based on a free-spinning wheel mounted within a ring. Gyros are used in instruments such as the attitude indicator, heading indicator, and turn coordinator. Because a spinning gyro maintains its orientation even when an airplane banks, climbs, or dives, it provides a stable reference to help pilots control an aircraft while flying in clouds or poor visibility.

gyroplane
A type of rotorcraft that relies on aerodynamic forces to spin the main rotor during normal flight.

gyroscopic precession
The reaction of a gyro when a force is applied to the spinning wheel. When force is applied to a gyro, it reacts as if the force had been applied at a point 90 degrees from the point of actual application, in the direction of rotation. Precession affects propellers, which act like gyros, and gyro instruments. Its principal effect is on the heading indicator, which tends to drift over time.

---

hail
Frozen precipitation often associated with thunderstorms. Hail forms when supercooled water droplets begin to freeze. After a drop has frozen, other drops latch on and freeze to it. The hailstone grows—sometimes into a huge iceball. As hailstones fall through warming air, they begin to melt, and precipitation may reach the ground as either hail or rain. Rain at the surface does not mean the absence of hail aloft. You should anticipate possible hail with any thunderstorm, especially beneath the anvil of a large cumulonimbus. In weather reports and forecasts, hail is denoted by the abbreviation "GR," from the French, grêle.

heading
The direction in which the aircraft is pointed, usually in reference to magnetic north. Because wind pushes an airplane in flight, heading does not necessarily correspond to the aircraft's path over the ground, that is, its track. For example, if you want to fly due east with respect to the ground and the wind is blowing from the north, you must turn the aircraft slightly into the wind to correct for drift.

heading indicator
A gyro instrument that accurately and quickly shows changes in aircraft heading, sometimes called the "directional gyro," or "DG." Because the heading indicator is driven by a gyro, it provides smooth, precise indication of heading or turns. The compass, which is subject to acceleration, deceleration, dip, and other errors, often oscillates or leads or lags a turn. However, because gyros are affected by precession, the pilot must periodically set the heading indicator to correspond to the compass (unless the heading indicator is "slaved" electronically to the compass).

headwind
A wind that blows in opposition to the intended course of flight.

high
An area of high barometric pressure. Also called an anticyclone or high-pressure system.

high-performance airplane
As defined in FAR 61.31(e), an airplane that has more than 200 horsepower or that has retractable landing gear, flaps, and a controllable propeller. A private or commercial pilot may not act as pilot in command of a high-performance airplane unless a flight instructor has certified that the pilot is competent to fly such aircraft.

holding pattern
An ovoid-shaped pattern flown in reference to a fixed (nav radio) reference point. Holding patterns are assigned by ATC to provide traffic separation during peak load times.

hood
A view-limiting device worn by pilots who are learning or practicing flight by instruments. The hood covers the upper portion of the pilot's field of vision. A safety pilot or instructor must always accompany a pilot who is using a hood.

horizontal situation indicator (HSI)
An instrument that combines the functions of the heading indicator and the VOR indicator in one display.

horizontal stabilizer
The horizontal surface of the tail, or empennage. The horizontal stabilizer is an airfoil that creates a downward force on the tail to balance the upward force generated by the wing. It also incorporates the elevator, the control surface used to adjust the aircraft's pitch attitude. On some airplanes, the entire horizontal stabilizer acts as an elevator.

horsepower
A unit of power equal to 745.7 watts or 33,000 foot-pounds per minute. The power exerted by one horse pulling.

hypersonic
Speeds at or above Mach 5 (that is, five times the speed of sound).

hypoxia
A condition in which an insufficient amount of oxygen reaches the tissues of the human body. The complete lack of oxygen, called anoxia, is fatal. Hypoxia is a serious hazard at high altitudes, particularly because its warning symptoms are varied and sometimes difficult to detect. Cabin pressurization or oxygen equipment is generally necessary for flying at altitudes at or above 10,000 feet (3,048 meters).

---

IFR en route charts
Navigational charts that depict aids to navigation, airways, and restricted airspace. Used for navigation by pilots flying instrument flights.

Immelmann
An aerobatic maneuver said to have been invented by World War I ace Max Immelmann in which an airplane reverses its direction of flight while gaining altitude. The maneuver begins with a half loop. At the top of the loop, the pilot rolls the plane upright. In modern aerobatic competitions, an Immelmann is called a "half loop, half roll."

Immelmann, Max
1890–1916. One of Germany's first great fighter pilots, credited (probably erroneously) as the inventor of the "Immelmann" maneuver. Nicknamed "the Eagle of Lille," Immelmann learned the basics of air combat from Oswald Boelcke, with whom he often flew missions. Among the numerous decorations Immelmann earned before being killed in action was the coveted Blue Max.

inclinometer
An instrument that displays the inclination to the horizontal of an axis. In most aircraft, there is an inclinometer at the bottom of the turn coordinator. It indicates when the aircraft is yawing to the left or right.

indefinite ceiling
A ceiling classification that describes vertical visibility into a surface-based obscuration.

indicated airspeed (IAS)
The speed shown on the airspeed indicator uncorrected for variations in atmospheric density, installation error, or instrument error. Except at sea level under standard atmospheric conditions, IAS does not correspond to the aircraft's actual speed through the surrounding air (that is, its true airspeed, or TAS).

indicated altitude
Altitude read directly from the altimeter after it is set to the local barometric pressure corrected to sea level. Indicated altitude is not corrected for temperature. Pilots use indicated altitude to control their aircraft.

induced drag
The portion of total drag created by lift. Induced drag is created when high-pressure air below a wing swirls around the wing tip to the low-pressure area above. This motion creates vortices, which in effect siphon off the aircraft's energy. This lost energy is induced drag. Induced drag increases as airspeed decreases.

infrared
Electromagnetic radiation with wavelengths beyond the red end of the visible color spectrum. Infrared photography, used in aerial surveillance, can penetrate haze and clouds to capture images of objects that are not visible to the naked eye.

initial climb
An aircraft’s climb away from the runway after liftoff.

inner marker
Marker beacons are selective signal transmitters used in conjunction with instrument landing systems. The inner marker is located between the middle marker and the runway threshold. Used during Category II instrument approaches, it is the point at which an aircraft will be decision height on the glide slope.

instrument approach chart
Also known as the "approach plate." Approach charts depict, both horizontally and in elevation, the procedure to be flown during an instrument approach at a specific airport.

instrument approach procedure (IAP)
An official procedure designed to guide aircraft to a runway when a visual descent isn't possible. IAPs describe the route and altitude aircraft are to fly as they transition from en route flight to landing. There are two basic types of instrument approach procedures: nonprecision and precision. Nonprecision approaches do not have an electronic glide slope to provide vertical guidance to landing aircraft. Examples of nonprecision approaches include VOR, NDB, localizer, and GPS approaches. Precision approaches have electronic glide slopes that provide precise vertical guidance. The most common precision approach is the ILS.

instrument flight rules (IFR)
In the United States, regulations that apply to pilots, aircraft, and aircraft operations when weather conditions do not meet the criteria for visual flight, when aircraft are operated in Class A airspace—that is, at altitudes at and above 18,000 feet (5,486 meters), or when pilots choose to operate in controlled airspace under those rules regardless of the prevailing weather. The rules establish minimum fuel reserves, equipment requirements and checks, and other operational standards for instrument flight.

The abbreviation "IFR" is sometimes used to describe weather that doesn't meet the minimums established for flight under visual flight rules (VFR). For example, if an air traffic controller alerts a pilot to nearby traffic and the pilot is flying in the clouds, the pilot may say, "I'm IFR" and ask for a change in heading or altitude to avoid a conflict. The proper abbreviation for describing those conditions is "IMC," for "instrument meteorological conditions."

instrument flight time
Flight time during which the pilot operates an aircraft solely by reference to instruments under actual or simulated instrument flight conditions. Instrument flight time is not necessarily the total time during which an aircraft operates under instrument flight rules (IFR).

instrument landing system (ILS)
A system of navigation aids and approach lights that provide both horizontal and vertical guidance to aircraft approaching a runway. The ILS is the primary precision-approach system in use today around the world. A typical ILS includes a localizer and a glide slope, as well as outer, middle, and inner marker beacons. The localizer transmits a directional signal that provides left/right guidance. The glide slope is an electronic glide path that defines the proper descent angle to the runway. Marker beacons indicate distance from the runway.

instrument meteorological conditions
Weather conditions that require flight under instrument flight rules (IFR). In controlled airspace in the United States, IMC conditions generally mean that the ceiling is less than 1,000 feet (305 meters) and flight visibility is less than 3 miles (5 kilometers).

instrument rating
A rating added to a pilot certificate that allows a pilot to act as pilot in command of an aircraft solely by reference to flight instruments. An instrument rating is required for operations in clouds or when the ceiling and visibility are less than required for flight under visual flight rules (VFR). A pilot also must hold an instrument rating to act as pilot in command of an aircraft in class A airspace. In the United States, class A airspace begins at 18,000 feet (5,486 meters) above mean sea level.

instrument scan
A methodical scan of the primary flight instruments during a flight in instrument flight conditions (flight by reference to the instruments).

instrument
From 14 CFR Part 1: "A device using an internal mechanism to show visually or aurally the attitude, altitude, or operation of an aircraft or aircraft part. It includes electronic devices for automatically controlling an aircraft in flight."

intercooler
A mechanical device between the turbocharger and the carburetor. When the turbocharger compresses air, the air becomes too hot to use. The air is cooled by being passed through the intercooler before it enters the induction system.

International Civil Aviation Organization (ICAO)
Based in Montreal, this body sets international standards for airports, pilots, communications, and other matters related to air transportation.

International Standard Atmosphere (ISA)
An arbitrary standard established as a baseline for calculations used in meteorology, aviation, and aerodynamics. A set of standard conditions exists for each altitude. At sea level, standard conditions are defined as 29.92 inches of mercury (1,013 millibars) and 59 F (15 C).

intersection
In aviation, a point in space defined by the intersection of signals from two or more navigation transmitters.

interstage turbine temperature (ITT)
Temperature of gases in a turbine engine as measured between the high-pressure and low-pressure turbine wheels. ITT is a limiting factor of the amount of power an engine can produce.

inversion
An increase in temperature with height. An inversion is the reverse of the normal decrease of temperature with height in the troposphere.

inverted
Upside down.

isobar
A line of equal or constant barometric pressure, as shown on a weather chart.

isogonic line
A line on a chart connecting points of equal magnetic variation.

isotherm
A line of equal or constant temperature, as shown on a weather chart.

---

Jet-airways
Airways delineated by navigation radio signals for high routes (18,000 to 45,000 feet MSL). Sometimes referred to as "J" airways, or highways in the sky.

jet blast
The high-velocity stream of air exiting a jet engine’s exhaust nozzle.

jet stream
A quasihorizontal stream of winds of 50 knots or more concentrated within a narrow band. The term usually applies to such winds embedded in the westerlies in the high troposphere.

Johnson, Clarence "Kelly"
1910–1990. American airplane designer and head of Lockheed's Advanced Project Development Group for 30 years. Johnson, known as the "King of the Skunk Works" at Lockheed, made major contributions to the development of more than 40 airplane models, including the Electra, the Constellation, the U-2 spyplane, and the SR-71 Blackbird.

joystick
Name sometimes given to the stick used to control the ailerons and elevators in some aircraft. The joystick is said to have been invented by French aviator Robert Esnault-Pelterie in 1907.
In Flight Simulator, "joystick" refers to an input device connected to a computer's game port and used to control the ailerons and elevator. A computer joystick may also incorporate buttons and switches to control the throttle, landing gear, flaps, and other functions.

---

katabatic wind
A wind blowing downslope.

kneeboard
A clipboard used in the cockpit to hold charts and other items the pilot needs to have at hand. Kneeboards are sometimes, but not always, strapped to the pilot’s knee.

knots
Nautical miles per hour. Abbreviation: kt, kts, or KTS. One nautical mile (nm or NM) measures 6,076 feet (1,852 meters). This distance is based on the length of one minute of arc of a great circle—an arc representing the shortest distance between two points on a globe. One knot equals about 1.15 statute miles per hour; therefore, 100 knots equals about 115 mph (185 kilometers per hour), 150 knots equals about 172 mph (278 kilometers per hour), and 200 knots equals about 230 mph (370 kilometers per hour). All speeds filed on flight plans and for air traffic control purposes are in knots. In the United States, light aircraft manufactured in 1976 and later have airspeed indicators marked in knots. Earlier models had airspeed indicators marked in statute miles per hour.
Note that "knots" by definition assumes "per hour." You should never state speed as "knots per hour.

---

landing gear
The wheels, struts, and other equipment that an aircraft uses to land or maneuver on the ground. Also known as the "undercarriage." The two most common types of landing gear are "taildragger" and ""tricycle" arrangements. On a taildragger, the front of the aircraft is supported on two wheels, while the tail rests on the ground on a skid or a tailwheel. With tricycle landing gear, the aircraft sits level on the ground with one nosewheel and two wheels farther back on the aircraft. The main landing gear are those nearest the airplane's center of gravity. Main landing gear almost always come in pairs and are designed to withstand a greater landing shock than the more-fragile nosewheel or tailwheel.

landing roll
The distance from the point where an aircraft touches down on the runway to the point where the aircraft comes to a stop or can exit the runway.

landplane
An airplane with wheels that can land on land, as opposed to a floatplane or a skiplane.

lapse rate
The average decrease of temperature with altitude. In the lower level of the atmosphere, the standard lapse rate is 2 C (3.6 F) per 1,000 feet (305 meters). Weather forecasters and pilots use the actual lapse rate to estimate the altitude at which the temperature and dew point will merge and lead to the formation of clouds or precipitation. An increase in temperature with altitude is an inversion.

large aircraft
In the United States, an aircraft with a maximum certificated takeoff weight of more than 12,500 pounds (5,670 kilograms). To act as pilot in command of a large aircraft, a pilot must have a type rating for that aircraft.

laser
A device that produces an intense, focused beam of energy in the form of light rays.

lateral axis
One of the three axes of an aircraft, the lateral axis is defined by an imaginary line running from wing tip to wing tip. Movement about the lateral axis is called "pitch" and is controlled by the elevator.

leading-edge slats
Openings near the forward edge of a wing designed to allow more air to flow over the top of a wing at high angles of attack, thus delaying the onset of a stall. Leading-edge slats and flaps are often known as high-lift devices.

Lear, William Powell
1902–1978. American designer and manufacturer of business jets. Despite the fact that his education stopped at the eighth grade, Lear built a successful electronics business. He sold it to finance a new aircraft company, which he started up in 1962 to build business jets based partly on fighters he had helped design for Swiss American Aviation Corporation.

left-turning tendency
The result of four forces that together cause a propeller-driven aircraft to yaw to the left. This tendency is most pronounced when the airplane is flying at a low airspeed and a high angle of attack. The pilot compensates by applying and holding right rudder pressure.

The four forces are: the reactive force, the spiraling slipstream, gyroscopic precession, and "P factor." The reactive force is the opposite and equal force generated by the rotation of the propeller. This force induces a rolling motion about the airplane's longitudinal axis. The spiraling slipstream is the rotating column of air produced by the propeller. It swirls around the fuselage and strikes the left side of the vertical stabilizer, producing a left yaw. Gyroscopic precession occurs when the nose of the airplane rises or falls. This change in pitch attitude applies a force to the spinning mass of the propeller, which is 90 degrees ahead of the point where the force was applied. "P factor," or asymmetric propeller loading, induces a left yawing motion because the downward-moving propeller blade has a higher angle of attack and produces more thrust than the upward-moving blade.

lenticular cloud (lenticularis)
A species of cloud whose elements have the form of more or less isolated, generally smooth lenses, or almonds. These clouds appear most often in formations of orographic origin—the result of lee waves—in which case, they remain nearly stationary with respect to the terrain (standing cloud), but they also occur in regions without marked orography.

lift
The upward force produced by an airfoil, such as a wing interacting with the air. Lift acts at right angles to the relative wind or the aircraft's flight path. Lift, one of the four fundamental forces in flight, is opposed by weight.

liftoff
The moment at which an aircraft leaves the ground during takeoff.

light aircraft
Generally speaking, small single- and twin-engine aircraft. More precisely, in the United States, aircraft with a maximum-certified takeoff weight of 12,500 pounds(5,760 kilograms) or less.

light-gun signals
Colored lights that control tower operators use to issue instructions to aircraft without radios or to aircraft that have a radio failure. The red, green, and white lights convey such messages as "cleared to land," "stop," and "exercise extreme caution." The meaning of all the signals is defined in FAR 91.125.

light icing
Ice, regardless of type, that accumulates slowly. Anti-icing or deicing equipment prevents accumulations. Light icing may become hazardous to aircraft without anti-icing or deicing equipment if conditions persist for an hour or more.

light turbulence
In aviation weather reports and pilot reports, turbulence that momentarily causes slight, erratic changes in altitude, attitude, or both. When light turbulence produces rhythmic bumpiness without appreciable changes in altitude or attitude, it is called "chop." In light turbulence, occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. On large aircraft, food service may be conducted and walking is possible with little or no difficulty.

lightning
Short, intense electrical discharges generated by thunderstorms. Lightning is rarely a great hazard to most aircraft, but it can damage electronic equipment. Lightning can also temporarily blind a pilot.

Lilienthal, Otto
1848–1896. German aeronautical engineer and hang glider pilot whose experiments with stability, lift, and control were highly influential to such later aviators as the Wright brothers. Lilienthal applied his studies of bird flight and aerodynamic theory to the design, construction, and experimental flight of 18 gliders. The first man to be photographed in a glider, Lilienthal was killed when one of his craft crashed in 1896.

Lindbergh, Charles A.
1902–1974. American aviator who was the first pilot to make a solo, nonstop flight across the Atlantic, for which he won the Orteig Prize and the Medal of Honor. Lindbergh started his career as a barnstormer and airmail pilot. After his historic flight, he and his wife Anne mapped air routes for Pan Am. Lindbergh eventually moved to Hawaii, where he wrote several books, including The Spirit of St. Louis, which won a Pulitzer Prize in 1954.

load factor
The ratio between the total load supported by an aircraft's structure and the actual weight of the aircraft and its contents; also known as "Gs." In steady-state flight, the load factor is 1. When an aircraft turns or pulls up out of a dive, load factor increases. For example, an airplane in a level turn at a bank angle of 60 degrees experiences a load factor of 2. In such a turn, the aircraft's structure must support twice the airplane's weight and the pilot must increase the aircraft's pitch attitude to produce more lift.

local airport advisory
Pertinent, known field conditions and local weather provided to pilots by a Flight Service Station or the military for airports without an operating control tower.

local weather area
In Flight Simulator, a user-defined region with similar weather characteristics. You can create two local weather areas, each with a different type of weather. Unless you specifically create a local weather area, all weather characteristics are assigned to the global weather area.

localizer
The component of the instrument landing system (ILS) that provides left-right guidance to a pilot approaching the runway. The localizer is a highly directional radio signal transmitted on one of 40 channels between 108.10–111.95 MHz. The beam is funnel-shaped. It is typically 10 degrees wide 18 nautical miles from the runway, narrowing to just 700 feet (213 meters) wide at the threshold.

logbook
An FAA-required record of events to be kept by pilots listing all flight activity. Also required by the FAA are logs on engines, airframes, propellers, and rotors showing the amount of time in service and any maintenance performed on each part.

Lomcevàk
A tumbling aerobatic maneuver. First conceived of by Ladislav Bezàk, a former world-champion aerobatic pilot, the maneuver has at least five variations—all initiated from a near-vertical attitude and flown under negative G forces. Although Lomcovàk is commonly believed to be the Czech or Polish word for "headache," it apparently comes from a Slovak slang term for a large, stiff drink.

longitudinal axis
An imaginary line running from the nose to the tail of an aircraft; one of the three axes of an aircraft. Rotation about the longitudinal axis is called "roll" and is controlled using the ailerons.

longitudinal separation
A minimum distance expressed in minutes or miles between aircraft at the same altitude.

loop
An aerobatic maneuver in which an aircraft flies in a complete vertical circle. An outside loop, begun at the top of the circle, is considerably more difficult to perform, because the pilot encounters negative G-forces throughout the maneuver.

low
An area of low barometric pressure, including the attendant system of winds. Also called a barometric depression, or cyclone.

low G
A condition in which a helicopter's rotor blades have a load of less than 1 G (the weight of the helicopter) exerted upon them. This can occur due to abrupt cyclical control movements, flying in turbulence, or when pushing over from a steep climb. When pushing over from a steep climb, a low G condition will cause the nose to drop and the aircraft to roll to the right. The main rotor may hit the tail boom, and helicopters with semirigid rotor systems may experience mast bumping. Either effect could cause the loss of the main and tail rotors.
Recovering from a low G condition before losing control entails gently appling aft cyclic to raise the nose and load the main rotor. Apply left cyclic to counteract the right-turning tendency.

---

Mach, Ernst
1838–1916. Austrian physicist and member of the Austrian Parliament. Mach's writings on empirical methodology and his theories on sensation and perception established the study of the philosophy of sciences. His work in ballistics and on measuring the speed of sound contributed greatly to the branch of aerodynamics concerned with supersonic flight.

Mach number
The ratio of an aircraft's speed to the speed of sound. The speed of sound varies with the density of the medium carrying the sound waves. For example, sound travels faster through iron or water than through air. Because the density of air decreases with altitude, the speed of sound also decreases. Sound travels at approximately 1,226 kilometers per hour (762 mph) at sea level, 1,138 kilometers per hour (707 mph) at 20,000 ft (6096 meters), and 664 mph (1,068 kilometers per hour) at 35,000 feet (10,668 meters). Named for Ernst Mach, Austrian physicist.

Mach 1
The speed of sound. Mach 1 varies according to altitude and temperature: At sea level, it is about 762 mph (1,226 kilometers per hour); at the bottom of the troposphere, Mach 1 is about 660 mph (1,062 kilometers per hour). Above the tropopause, at about 36,000 feet (11,000 meters), the speed of sound remains constant.

magnetic compass
A device for determining direction relative to the earth's magnetic field.

magnetic course
A line drawn on a chart between two points with its direction referenced to the earth's magnetic North Pole.

magnetic declination
See magnetic variation.

magnetic heading
The direction in which an aircraft is pointed, measured relative to the magnetic north pole. The magnetic heading is displayed on the compass. A pilot determines the magnetic heading to fly by compensating for the difference between true north and magnetic north and then adjusting the resulting magnetic course to compensate for wind. If the wind is blowing directly along the intended course, the magnetic heading equals the magnetic course. If a crosswind component is present, however, a pilot must turn the aircraft slightly into the wind to compensate. Under these conditions, the airplane's track over the ground is slightly different from the direction in which the nose is pointing.

magnetic variation
The angle between "true north" and "magnetic north"; that is, the angle between the geographic North Pole and the magnetic North Pole as measured from a point on the earth. To determine the magnetic course between two points on the earth's surface, a pilot must determine the true course—the angle formed between the line drawn on a chart and lines of longitude—and then add or subtract the variation at points along that course.

magneto
A device that creates an electric current by rotating a magnet. In aircraft engines, the crankshaft turns the magnetos, which provide the electrical energy to fire the spark plugs. This arrangement ensures that the spark plugs fire even if the aircraft's battery and electrical system fail. Certified aircraft engines typically have two sets of magnetos for additional redundancy.

maneuvering speed (Va)
The maximum speed at which the pilot can use full, abrupt control movement without creating excessive G forces that could damage the aircraft; the maximum speed at which you can safely stall an aircraft. Pilots also use maneuvering speed when flying through turbulent air.

Abbreviated "Va," maneuvering speed is not marked on the airspeed indicator.

manifold pressure gauge
An instrument that measures the air pressure in the intake manifold of a piston engine. Usually calibrated in inches of mercury, this instrument (really a barometer) is used in combination with the tachometer to set engine power. Most small training aircraft have only a tachometer. Aircraft with larger engines and aircraft with constant-speed propellers usually have manifold pressure gauges.

manually coordinated flight
See uncoordinated flight.

marker beacons
Low-power radio beacons that identify specific positions along an instrument approach, usually an ILS. A typical ILS has at least two marker beacons. The outer marker (OM) normally indicates the point at which an aircraft intercepts the electronic glide slope. It transmits three dashes in Morse code. On the instrument panel, the OM is indicated by a blue light. The middle marker (MM) indicates a position about 3,500 feet (1,067 meters) from the runway threshold. It also marks the point where an aircraft is about 200 feet (61 meters) above the elevation of the runway touchdown zone. The middle marker transmits a "dot-dash-dot-dash" code and is indicated in the cockpit by an amber light.

Some ILSs also have an inner marker (IM), which indicates the point at which an airplane flying along the proper glideslope reaches the decision height. The inner marker transmits a rapid "dot-dot-dot-dot" code and is indicated in the cockpit by a white light.

maximum L/D
Maximum lift-to-drag ratio. This is the speed at which the aircraft will travel the farthest distance for a given altitude at a given weight. Also known as Best Glide.

minimum sink
The speed at which the aircraft will lose the least amount of altitude over time at a given weight.

mayday
The international call for help. The term comes from the French phrase, m’aidez (help me), pronounced "mayday." This is the voice-transmission equivalent of the letters SOS used in code transmissions.

mean sea level (MSL)
The average level of the earth's oceans; used as reference for true altitude—the vertical distance of an aircraft above sea level. Airport, terrain, and obstacle elevations shown on aeronautical charts are expressed as true altitude.

measured ceiling
A ceiling classification applied when the ceiling value has been determined by instruments or the known heights of unobscured portions of objects other than natural landmarks.

medical certificate
In the United States, a certificate showing that a pilot, required flight crewmember, or air traffic controller meets a set of physical and mental standards established for the safe operation of an aircraft or the performance of other duties. The FAA issues third-, second-, and first-class medical certificates. The certification standards are more restrictive with each class. Student, recreational, and private pilots are required to hold at least a third-class medical certificate, which is valid for either 24 or 36 months. A second-class medical is required for certain commercial operations. It is valid for 12 months for those operations and for 24 months for noncommercial operations. Airline pilots require a first-class medical, which is valid for 6 months for air carrier operations, 24 months for certain commercial operations, and 24 months for noncommercial operations.

METAR
From the French term for Aviation Routine Weather Report, the worldwide standard report for hourly weather observations taken at airports. The United States recently adopted the METAR to replace SA reports. A METAR includes the following information: type of report, station designator, time of report, wind, visibility, weather and obstructions to visibility, sky condition, temperature and dew point, altimeter setting, and any remarks.

microburst
Violent, localized winds often associated with thunderstorms. In a microburst, strong vertical downdrafts spill out of a thunderstorm and then spread along the ground, like water poured from a bucket. These downdrafts can exceed 6,000 feet per minute (1,829 meters per second) and create strong wind shears—areas where the speed and direction of the wind change abruptly. An airplane flying through a microburst and the associated wind shear experiences rapid changes in airspeed and can even sink to the ground if caught in a strong downdraft.

middle marker
A marker beacon located approximately 3,500 feet from the runway threshold. This is the point at which an aircraft will be approximately 200 feet AGL on the glide slope.

millibar (mb)
An international unit of pressure equal to 1,000 dynes per square centimeter. It is convenient for reporting atmospheric pressure.

mixed ice
A combination of clear and rime ice that can form rapidly. Mixed ice forms when water drops vary in size or when liquid drops are intermingled with snow or ice particles. Ice particles become imbedded in clear ice, building a rough accumulation, sometimes in a mushroom shape, on the leading edges of an aircraft's surfaces.

mixture control
A device for controlling the ratio between fuel and air entering an engine's carburetor or fuel injection system. In most aircraft, the mixture control is a push-pull knob or lever marked in red, usually located to the right of the throttle.

Because aircraft engines operate over a wide range of altitudes, the pilot must adjust the mixture to produce the most efficient fuel/air mixture as an airplane climbs into less dense air or descends into more dense air. A mixture that is too rich contains too much fuel for the existing conditions and causes the engine to run rough and lose power. A mixture that is too lean can cause an engine to overheat or can cause detonation—the sudden, explosive combustion of fuel within the cylinders.

mmo
Abbreviation for Mach maximum operating speed; the maximum airspeed, indicated in Mach number, at which an aircraft can be operated safely. The actual value of mmo varies with atmospheric pressure, temperature, and other factors.

moderate icing
Ice, regardless of type, that accumulates at a rate that requires the use of anti-icing or deicing equipment. Aircraft without such equipment must divert immediately.

moderate turbulence
In aviation weather reports and pilot reports, turbulence that causes changes in altitude or attitude and small variations in airspeed. The aircraft remains in positive control at all times, Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. On large aircraft, food service and walking are difficult.

monoplane
An airplane having only one main pair of wings. Monoplanes create less drag than biplanes, but early aircraft relied on biplane designs because of the ease of constructing a sufficiently strong plane this way. Once engineering advanced to the stage where strong monoplanes were possible, the biplane design became obsolete.

Mooney, Al
1906–1986. American airplane designer and manufacturer. Mooney began designing light airplanes in 1922. He worked for a number of aircraft companies around the United States, including Bellanca and Culver Aircraft, before joining Dart Manufacturing to design the Dart monoplane. Mooney designed his M-18 Mite in 1946; that same year, he and his brother Art founded Mooney Aircraft.

mountain waves
A standing wave or lee wave to the lee of a mountain barrier; waves created by massive amounts of air pushed over mountain ranges, such as the Rockies and the Sierra Nevada in the United States. Intense mountain waves are typically produced when a strong, turbulent jet stream flows directly across a mountain ridge. Sailplane pilots often use mountain waves to soar to high altitudes—50,000 feet (15,240 meters) or more.

---

N1
The rotational speed of the low-pressure compressor in a dual-spool gas turbine engine.

N2
The rotational speed of the high-pressure compressor in a dual-spool gas turbine engine.

National Aeronautics and Space Administration (NASA)
A civilian agency of the United States government created in 1958 with the responsibility for all nonmilitary developments in aeronautics and space flight. NASA replaced the National Advisory Committee for Aeronautics.

National Flight Data Center
A facility in Washington, D.C. established by the FAA for the dissemination of aeronautical information essential to flight safety.

National Transportation Safety Board (NTSB)
In the United States, the government agency that investigates aircraft accidents and determines their probable cause. The NTSB is independent of the FAA, which regulates aviation. The FAA participates in accident investigations and responds to recommendations made by the NTSB when the board determines that new regulations, procedures, or equipment would help prevent future accidents.

nautical mile (nm)
A distance of about 6,076 feet (1852 meters). The nautical mile is based on the length of one minute of arc of a great circle. In aviation, distances and speeds are measured in nautical miles (nm) and nautical miles per hour (knots).

NAV
Abbreviation for navigational; usually refers to a navigational radio, as in "NAV 1" or "NAV 2."

NAV/COM
A radio that combines the functions of communication and navigational radios.

navigation lights
The basic anticollision light system required on all aircraft certified to fly at night. The system includes a red light on the left wing tip, a green light on the right wing tip, and a white light on the tail. These lights tell other aircraft which direction an aircraft is flying when only the lights can be seen. Navigation lights must be turned on between sunset and sunrise.

navigational aid
Any visual or electronic device, either airborne or ground-based, established to provide point-to-point guidance information or position data to aircraft in flight.

nimbostratus
A principal cloud type, gray colored, often dark. Its diffuse appearance is caused by more or less continuously falling rain or snow, which in most cases reaches the ground. A nimbostratus cloud is thick enough throughout to blot out the sun.

Ninety-Nines (99s)
Association of women fliers formed in 1929 with 99 charter members, including Amelia Earhart, Jackie Cochran, Louise Thaden, and Amy Johnson. The Ninety-Nines were responsible for the acceptance and respect accorded women contestants in the air races of the 1930s. Today, the Ninety-Nines is a worldwide organization with thousands of members.

nondirectional radio beacon (NDB)
A radio beacon that transmits nondirectional signals in the low- or medium-frequency band (190–535 kHz); today, used primarily for NDB nonprecision approaches and in conjunction with the outer marker component of an ILS. An automatic direction finder (ADF) points to these beacons.

nonprecision approach
Nonprecision approaches do not have an electronic glide slope to provide vertical guidance to landing aircraft. Examples of nonprecision approaches include VOR, NDB, localizer, and GPS approaches.

nose gear
In an aircraft equipped with tricycle landing gear, the wheel located under the forward end of the fuselage.

nose over
To rapidly lower the nose relative to the horizon; to decrease pitch. On the ground, a nose over—an aircraft tipping forward or doing a somersault—can result when the nosewheel digs into a soft surface or when a tailwheel-equipped airplane decelerates too rapidly.

Notice to Airmen (NOTAM)
A notice containing information not known far enough in advance to publish, provided for persons concerned with flight operations. NOTAMs generally deal with changes in facilities, services, procedures, or hazards in the National Airspace System.

---

obscuration
In weather reports, denotes a sky hidden by surface-based phenomena and restricted vertical visibility overhead.

occluded front (occlusion)
A composite of two fronts as a cold front overtakes a warm front, or a quasistationary front.

oleo-pneumatic
A hydraulic-air system usually incorporated into aircraft landing-gear systems. Oleo-pneumatic shock absorbers dissipate the shock of landing by forcing fluid through a restrictor valve from the upper chamber of the strut to the lower chamber.

omnibearing indicator (OBI)
The instrument that displays information about an aircraft's position relative to a VOR station. The OBI includes a needle, or course deviation indicator (CDI), to show the aircraft's position relative to a selected course or "radial" and a TO-FROM-OFF indicator that shows the aircraft's position relative to the VOR station. A glideslope needle is also included in some OBI. Each OBI is typically connected to a navigation receiver (NAV 1 or NAV 2).

ornithopter
Hypothetical human-powered flying machine based on flapping wings. Leonardo da Vinci was perhaps the first to attempt a scientific design of ornithopters, but the idea reaches back at least to the Greek legend of Daedalus and Icarus. No actual ornithopter has ever been built.

orographic
Of, pertaining to, or caused by mountains, as in orographic clouds, orographic lift, or orographic precipitation.

outer marker
A marker beacon that usually indicates a point at which an aircraft at the appropriate altitude on a localizer course will intercept the glideslope on an ILS approach.

oversquare operation
Running with the manifold pressure at a higher setting than the rpm. For instance, "square" would be 24 inches of manifold pressure (MP) and 2400 rpm. "Oversquare" would be 26 inches MP and 2400 rpm. Pilots used to be warned not to run the engines oversquare, but now, oversquare is considered acceptable and can be more efficient.

ozone
An unstable form of oxygen. The heaviest concentrations of ozone are in the stratosphere. Ozone is corrosive to some metals and it absorbs most ultraviolet solar radiation.

---

panel
Equivalent to an automobile’s dashboard, the panel is the surface in which the aircraft’s instruments and radios are installed. Larger aircraft often have multiple panels, sometimes on the sides or ceiling of the cockpit.

Pan-Pan
Indicates uncertainty or alert when transmitted three times successively followed by the nature of the urgency.

parasite drag
Resistance to motion through the air composed of form drag (due to landing gear, radio antennas, shape of the wings, and so on), skin friction, and airflow interference between aircraft components (such as the junction of the wings and fuselage, or fuselage and tail). Parasite drag increases as the square of velocity. It is one component of total drag, the force that opposes thrust. The other component is induced drag, a by-product of lift.

partial obscuration
A designation of sky cover when part of the sky is hidden by surface-based obscuring phenomena.

performance booster
Software to enhance performance of Flight Simulator or scenery add-ons. The performance booster monitors your aircraft's position and automatically loads scenery from the CD-ROM drive to a disk cache on your hard drive.

phonetic alphabet
A spoken alphabet endorsed by the International Civil Aviation Organization (ICAO) and used by pilots and air traffic controllers to avoid confusion during radio communications. Instead of saying "a," "b," "c," pilots say "alpha," "bravo," "charlie," and so on.

Pilcher, Percy
1866–1899. Scottish marine engineer whose experiments with gliders between 1895 and 1899 produced a number of design advances. Pilcher was developing a light engine for use with one of his gliders but did not complete the project. Like his mentor, Otto Lilienthal, Pilcher died when one of his gliders crashed.

pilot certificate
In the United States, a pilot license. The FAA issues several types of pilot certificates, including student, recreational, private, commercial, certified flight instructor, and airline transport pilot certificates. Pilot certificates define broad privileges and limitations. Ratings on those certificates (single-engine land, instrument, multiengine land, helicopter, and so on) further specify the classes of aircraft that a pilot may fly and whether the pilot is allowed to act as pilot in command when the weather does not meet the minimums specified for flight under visual flight rules (VFR).

pilot in command
The pilot responsible for the operation and safety of an aircraft during flight time. For purposes of logging flight time, a pilot holding a recreational, private, or commercial pilot certificate may log as pilot-in-command time (PIC time) only that flight time during which that pilot is the sole manipulator of the controls. The pilot must hold an appropriate pilot certificate and rating for the aircraft. An airline transport pilot may log as PIC time all of the flight time during which that pilot acts as PIC. A certified flight instructor may log as PIC time all flight during which that pilot acts as PIC.

pilot report (PIREP)
A PIREP describes actual in-flight conditions, such as the height of clouds, visibility, precipitation, turbulence, and icing. On weather reports in the United States, pilot reports are preceded by the letters "UA." A pilot report includes the following items: position relative to a weather reporting station or navigation aid, time (UTC), altitude, aircraft type, cloud types and altitudes, visibility, outside air temperature, wind, turbulence, and any remarks. Some of the information may be omitted.

pilotage
Flying cross-country from one visible landmark to another using only a chart.

Piper, Bill
1881–1970. American aircraft manufacturer who founded Piper Aircraft Corporation in 1937. Piper left his earlier career in oil exploration to invest in Taylor Brothers, builder of the E-2 Cub. He took over the company in 1932 and changed its name five years later. Piper Aircraft was, for many years, the world's leading producer of light aircraft.

piston engine
One of the common terms for an internal-combustion reciprocating engine.

pitch
Movement of an aircraft about its lateral axis (nose up or nose down), or the angle of an airplane's nose above or below the horizon. The pilot moves the control stick or yoke forward and back to move the elevator, located on the horizontal stabilizer. Forward pressure lowers the nose; back pressure raises the nose.

pitch/power rule
The pitch/power rule states that for normal flight, unless the throttle is fully open or fully closed, power changes should be used to change airspeed, and pitch changes should be used to change altitude.

pitot tube
A small metal probe, usually attached to a wing or the nose of an aircraft, that measures ram air pressure as the aircraft moves. The pitot tube is part of the pitot-static system. It is connected directly to the airspeed indicator, which displays the ram air pressure on a scale, usually calibrated in knots. The pitot tube usually has a heater to prevent ice from blocking the device. The probe is named after Henri Pitot (1695–1771), a French scientist who invented devices to measure the flow of water in rivers and canals.

pitot-static system
The sensors, connecting lines, and flight instruments that measure, transmit, and display information about an aircraft's airspeed, altitude, and rate of climb or descent. The pitot-static system includes the pitot tube, static ports, and tubing that connects the sensors and instruments in the cockpit. The airspeed indicator is connected to both the pitot tube and the static ports. The altimeter and vertical speed indicator are connected only to the static ports.

pivot point
The center of gravity on an aircraft.

Poberezny, Paul
American fighter pilot and flight instructor in World War II and the Korean War, and founder and chairman of the Experimental Aircraft Association (EAA). Poberezny organized the EAA and its first fly-in in 1953, from which he became a leading figure in representing the interests of sport-flying enthusiasts and amateur aircraft builders worldwide. Involved in aviation for most of his life, Poberezny has designed more than 15 airplanes and has served as editor for a number of aviation publications. His son, Tom, is the current president of the EAA.

positive control
From 14 CFR Part 1: "Control of all air traffic, within designated airspace, by air traffic control."

power glide
A long, shallow approach in which engine power is used to maintain the glide. Power glides should be avoided when they are not required to maintain instrument flight rule (IFR) approach angles, because an engine failure can cause an aircraft to land short of the runway.

power plant
An aircraft engine, propeller, and all components necessary for their proper functioning.

practical test
In the United States, an oral and flight test required for a pilot certificate or rating. Practical tests are given by government inspectors or designated examiners. The required knowledge and performance standards for each pilot certificate or rating are specified in a series of Practical Test Standards (PTS) available from the Government Printing Office or from many publishers of aviation-related books and study guides.

precipitation
Any or all forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the surface. Precipitation is distinguished from cloud and virga in that it must reach the surface.

precipitation-induced fog
Fog formed when relatively warm rain or drizzle falls through cool air. Evaporation from the precipitation saturates the cool air and forms fog. Precipitation-induced fog can become quite dense and persistent. It often extends over large areas, completely suspending flight operations. It is most commonly associated with warm fronts, but it can occur with slow-moving cold fronts and with stationary fronts. Precipitation-induced fog is especially critical because it occurs in precipitation and related hazards such as icing, turbulence, and thunderstorms.

preflight briefing
The acquisition of weather conditions and forecasts for a route of flight prior to takeoff.

pressure altimeter
An aneroid barometer with a scale graduated in altitude instead of pressure. Using standard atmospheric pressure-height relationships, a pressure altimeter shows indicated altitude (not necessarily true altitude). It may be set to measure indicated altitude from any arbitrarily chosen level.

pressure altitude
The altitude indicated when the altimeter is set to 29.92 inches of mercury or 1013.2 millibars. Pressure altitude is used in several important calculations, including the determination of density altitude, true altitude, and true airspeed. In the United States, aircraft operating at 18,000 feet (5,486 meters) or higher fly at "flight levels," which are pressure altitudes. When operating at or above FL180, the pilot should set the altimeter to 29.92 to display pressure altitude.

prevailing easterlies
The broad current or pattern of persistent easterly winds in the tropics and polar regions.

prevailing visibility
In the United States, the greatest horizontal visibility that is equaled or exceeded throughout half of the horizon circle; it need not be a continuous half.

prevailing westerlies
The dominant west-to-east motion of the atmosphere, centered over middle latitudes of both hemispheres.

primary flight instruments
The six instruments displayed on the standard instrument cluster: airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator, and vertical speed indicator.

private pilot certificate
In the United States, a pilot certificate issued to a person who does not intend to act as a pilot in command for compensation or hire. To be eligible for a private pilot certificate, a person must be at least 17 years old and hold a current third-class medical certificate. To earn a private pilot certificate, a person must acquire at least 40 hours of flight time, including 20 hours of flight instruction and 20 hours of solo flight time. The training must include basic maneuvers, cross-country navigation, and other specific tasks.

procedure turn
A predefined turn during an instrument approach that reverses the aircraft’s course and puts it on the final approach heading to the runway.

prognostic chart
A chart of expected or forecast weather conditions.

propeller
Often called a "prop;" a rotating airfoil that is driven by an aircraft engine. The propeller produces thrust when turning and either pushes or pulls the aircraft through the air.

propwash
The air blown back from an airplane propeller (or rotor, in the case of helicopters).

pushback
The act of being pushed back from an airport terminal gate. This is usually done by hooking a small tug to the nose wheel of a large aircraft and pushing it backwards into the taxi lane. In Flight Simulator, press SHIFT+P to push back from the gate.

---

---

radar
A device for detecting distant objects by reflecting radio waves from their surfaces. The British first integrated radar into their military defenses, and its value during the Battle of Britain was enormous. Today, radar is the primary tool for air traffic control. Some aircraft also carry weather radar, designed to detect precipitation and wind shears associated with thunderstorms and other hazards.

radar altitude
The altitude of an aircraft determined by radar or radio altimeter. It is the actual distance between an aircraft and the ground or water surface directly beneath it.

radarsonde observation
An observation in which winds are determined by tracking a balloon-borne target with radar.

radial
One of the 360 "spokes," one for each degree in a circle, radiated from a VOR station. To fly a specific course, the pilot tunes the appropriate VOR station and selects a radial to fly. An indicator in the cockpit, called an OBI, shows the aircraft's position relative to that radial.

radial engine
A type of reciprocating piston engine in which the cylinders are arranged like the spokes of a wheel.

radiation fog
Fog characteristically formed over land at night or near daybreak when cooling of the earth's surface lowers the air temperature near the ground to or below the initial dew point. Radiation fog is especially common on calm, clear nights. It is relatively shallow and may be dense enough to hide the entire sky.

radio stack
The area where the COM, NAV, and transponder radios are installed in an aircraft instrument panel. Radios are usually arranged in a vertical column, or stack.

radiosonde
A balloon-borne instrument for measuring pressure, temperature, and humidity aloft.

rate of climb
The speed (usually measured in feet per minute) at which an aircraft is climbing. The term sometimes is stretched to include the rate of descent. The rate of climb is read on the vertical speed indicator (VSI).

rate of climb indicator
See vertical speed indicator (VSI).

rating
In the United States, an endorsement added to a pilot certificate that specifies the classes of aircraft (single-engine land, instrument, multiengine land, helicopter, and so forth) that a pilot may fly and whether the pilot is allowed to act as pilot in command when the weather does not meet the minimums specified for flight under visual flight rules (VFR).

rate of sink
See sink rate.

rawinsonde observation
A combined winds-aloft and radiosonde observation. Winds are determined by tracking the radiosonde by radio direction finder or radar.

recreational pilot certificate
In the United States, a pilot certificate issued to a person who intends to fly only for recreation in basic aircraft. To be eligible for a recreational pilot certificate, a person must be at least 17 years old and hold a current third-class medical certificate. To earn a recreational pilot certificate, a person must acquire at least 30 hours of flight time, including 15 hours of flight instruction. Recreational pilots are subject to many restrictions. For example, they may not carry more than one passenger and must remain within 50 nautical miles of an airport where they received flight instruction. Recreational pilots are restricted to basic single-engine aircraft with no more than 180 horsepower, four seats, and fixed landing gear. They also cannot fly at night, above 10,000 feet (3000 meters), or when the visibility is less than 3 sm.

relative humidity
The ratio, usually expressed in percent, between the amount of water present in the air and the maximum amount that could be present at a given temperature. Warm air can hold more water vapor than cool air. If temperature increases and the amount of water vapor in the air remains constant, relative humidity decreases. If temperature drops while the amount of water vapor stays constant, relative humidity increases. Air with 100 percent relative humidity is said to be "saturated." The temperature at which the air reaches 100 percent relative humidity is called the "dew point."

relative wind
The speed and direction of air striking an airfoil; that is, the air flow caused by an aircraft or airfoil's movement through the air. The relative wind blows parallel and opposite to the aircraft's flight path. The angle between the relative wind and the chord line is the angle of attack. Lift acts perpendicular to the relative wind.

resolution
The level of graphics detail displayed on a computer screen.

retractable gear
Landing gear that can be retracted into the fuselage of an aircraft. Though they are more complex than fixed-gear, the ability to retract the gear provides a significant reduction in drag.

retreating blade stall
A condition in which a helicopter's rotor blades on the left side of the rotor disk (as viewed from above) exceed their critical angle of attack and stall. This is most apparent during high forward speed and is the primary limiting factor on a helicopter's maximum speed. Recovery from a retreating blade stall entails lowering the collective to reduce the rotor blade's angle of attack and slowing down.

reverse thrust
An engine's thrust directed forward to help slow the aircraft's forward motion. On turbojet aircraft, this is accomplished by the use of moveable devices that are extended across the engine's exhaust path or by movement of the cowling on high-bypass turbofans. On turboprop engines, this is accomplished by movement of the propeller blades into a position that directs thrust forward.

ridge
In meteorology, an elongated area of relatively high atmospheric pressure; usually associated with and most clearly identified as an area of maximum anticyclonic curvature of the wind flow. In soaring, rising terrain that is often accompanied by rising air.

ridge lift
An area of rising air created when wind blows against the side of a ridge and is deflected upward. In such conditions, sailplanes can stay aloft for hours by flying parallel to a ridge and riding those rising currents of air.

rigid
One of three main types of main rotor systems used in modern helicopters. A rigid rotor system utilizes three or more rotor blades that flap independently to compensate for dissymmetry of lift. Unlike the fully articulated system, the rigid system is hingeless, and the rotors can lead and lag independently.
Rigid rotor systems are typically made of composite materials and titanium (and are, therefore, expensive) and tend to give a rough ride. They resist low G conditions and ground resonance, and are less expensive to maintain than other rotor systems.

rime ice
White or milky and opaque granular deposits of ice formed by the rapid freezing of supercooled water droplets on exposed parts of an aircraft. Rime ice forms when drops are small, usually in stratiform clouds or light drizzle. The liquid portion remaining after initial impact freezes before the drop spreads over the aircraft surface. The small frozen droplets trap air between them, giving the ice a milky appearance. Rime ice is lighter than clear ice, but its irregular shape and rough surface quickly degrade the efficiency of airfoils.

roger
Used to acknowledge that the speaker has understood a radio transmission he or she has received.

roll
Rotation about an aircraft's longitudinal axis. The pilot controls roll, or the bank of the wings, with the ailerons. A roll is also an aerobatic maneuver in which an airplane rotates completely around its longitudinal axis.

roll cloud
A dense and horizontal roll-shaped accessory cloud located on the lower leading edge of a cumulonimbus. Roll clouds can also be found near a rapidly developing cumulus cloud. They indicate turbulence.

rotary engine
Type of piston engine, common in World War I aircraft, with a stationary crankshaft and cylinders that rotate around the crankshaft. Rotary engines were lighter-weight, more reliable, and more easily cooled than many contemporary radial and inline piston engines, but they had high fuel consumption and their rotational inertia sometimes made an aircraft difficult to maneuver.

rotation
Pulling back on the control yoke or stick to raise the nose of an aircraft during the takeoff roll.

rotation speed (Vr)
The speed at which the pilot should pull back on the control yoke or stick to begin raising an aircraft's nose during takeoff.

rotor
The rotating airfoil that provides lift or antitorque capabilities on a helicopter. Can also refer to the moving part of an axial-flow compressor in a turbine engine.

rotor cloud
A turbulent cloud formation found in the lee of some large mountain barriers, the air in the cloud rotates around an axis parallel to the range; indicative of possible violent turbulence.

roundout
See flare.

rudder
A movable control surface usually mounted on the vertical stabilizer of the tail. The rudder moves the aircraft about its vertical, or yaw, axis. It does not, however, turn the airplane. It is used primarily to balance forces in turns and to counteract yawing motions induced by the propeller during flight. A pilot moves the rudder by applying pressure to the left or right rudder pedal. The pedals are mounted on the floor of the cockpit. In normal maneuvering, the pilot uses simultaneous aileron and rudder pressures to maintain balanced or coordinated flight.

runway
A prepared surface designed for departing or landing aircraft.

run-up
Before taking the runway for takeoff, most pilots of piston-powered aircraft "run up" the engine(s) to a high rpm setting to test the magnetos and carburetor heat, and to check vacuum system suction, among other things. Better to find problems on the ground before takeoff, than in the air.

runway visual range (RVR)
The horizontal distance (in feet or meters) that a pilot should be able to see down the runway from the approach end. RVR is based on either the sighting of high intensity runway lights or on the visual contrast of other objects, whichever yields the greatest visual range.

---

S turn
A taxiing technique employed by pilots of tailwheel aircraft. Due to the attitude of a taildragger when it’s sitting on the ground, the nose of the aircraft limits forward visibility. By alternately turning the aircraft left and right, the pilot can see ahead of the aircraft.

Also refers to a technique used in flight during training. Student pilots are taught to S turn across a fixed ground reference (such as a road), in order to gain proficiency in making uniform turns.

sailplane
A highly efficient unpowered aircraft that can maintain or gain altitude by riding thermals or other rising air.

Santos-Dumont, Alberto
1873–1932. Brazilian-born inventor of dirigibles and airplanes who made the first powered flight in Europe. Santos-Dumont made his first balloon flight early in 1898; later that year, he became the first to fly a balloon equipped with an engine. In 1906, he flew his boxlike "14-bis" flying machine 722 feet (220 meters) for the first powered flight outside the United States. Santos-Dumont retired from aviation in 1910. In 1932, depressed over the military use of aircraft, he took his own life.

saturated adiabatic lapse rate
The rate of decrease of temperature with height as saturated air is lifted with no gain or loss of heat from outside sources. This value varies with temperature, and is greatest at low temperatures.

scan
Refers to the methodical examination of flight instruments during IFR flight. Can also refer to a methodical examination of the sky around an aircraft when looking for traffic.

scud
Small detached masses of stratus fractus clouds below a layer of higher clouds, usually nimbostratus. More generally, "scud" is any low-level clouds or fog. Pilots who fly at low altitude beneath clouds are said to be "scud running."

sea fog
A type of advection fog formed when air that has been lying over a warm surface is transported over a colder water surface.

see and avoid
The responsibility of pilots flying in visual meteorological conditions (VMC) to look for other aircraft and avoid them.

semimonocoque
A kind of fuselage construction that distributes the structural load of the aircraft between its skin and the framework.

semirigid
One of three main types of main rotor systems used in modern helicopters. A semirigid rotor system utilizes two rotor blades that flap together as a unit to compensate for dissymmetry of lift. Semirigid rotor systems are relatively inexpensive to maintain but are susceptible to low G conditions, and their flapping characteristics can lead to mast bumping.

settling with power
Also called a retreating blade stall. A condition in which a helicopter's rotor blades on the left side of the rotor disk (as viewed from above) exceed their critical angle of attack and stall. This is most apparent during high forward speed and is the primary limiting factor on a helicopter's maximum speed. Recovery from a retreating blade stall entails lowering the collective to reduce the rotor blade's angle of attack and slowing down.

severe icing
Icing, regardless of type, that accumulates so rapidly that even aircraft equipped with anti-icing or deicing equipment must divert immediately.

severe turbulence
In aviation weather reports and pilot reports, turbulence that causes large, abrupt changes in altitude, attitude, or both. Severe turbulence usually causes wide variations in indicated airspeed. The aircraft may be momentarily out of control. Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. On large aircraft, food service and walking are impossible.

shaft horsepower (SHP)
A power output measurement for turboprop engines. SHP is determined by propeller rpm, and the torque power output of exhaust and shaft forces is called Equivalent Shaft Horsepower (ESHP).

shock wave
Created when airflow changes abruptly from subsonic to supersonic speed, causing an enormous increase in air pressure and temperature. When an aircraft travels above Mach 1, shock waves bend backward to form a cone. If this cone reaches the earth's surface, it causes an explosive sound called a sonic boom.

shower
Precipitation from a cumuliform cloud. A shower is characterized by the suddenness with which it begins and ends, by a rapid change of intensity, and usually by rapid change in the appearance of the sky. Showery precipitation may be in the form of rain, ice pellets, or snow.

SIGMET advisory (significant meteorological information)
An advisory of concern to all aircraft that covers severe weather conditions.

Sikorsky, Igor
1889–1972. Russian-born aircraft designer and manufacturer considered to be "the father of the helicopter." Sikorsky built his first helicopter at age 20, and in 1913, he built the world's first four-engine airplane. After moving to the United States in 1919, he formed an aircraft company with other Russian émigrés. Sikorsky's flying boats, although a technical success, were a financial failure, so with support from United Air Transport, he returned to helicopters. In 1939, his VS-300 became the first practical helicopter.

sink rate
Negative vertical velocity, usually expressed in feet per second.

simplified directional facility
A navigational aid used for nonprecision approaches. Similar to an ILS, but not as accurate.

skid
A turn in which the rate of turn is too great for the angle of the bank. In a skid, the ball at the bottom of the turn coordinator moves to the outside of the turn. To correct a skid, use the ailerons to increase the bank, reduce rudder pressure, or both, in the direction of the turn.

slat
A movable auxiliary airfoil on the leading edge of a wing. The slat extends into the flow of air and creates a gap that allows air to flow smoothly over the top of the wing, delaying the stall at high angles of attack.

slewing
In Flight Simulator, a method of rapidly changing aircraft position, direction, location, or altitude without flying.

slip
A turn in which the rate of turn is too slow for the angle of the bank. In a slip, the ball at the bottom of the turn coordinator moves to the inside of the turn. To correct for a slip, use the ailerons to decrease the bank, increase rudder pressure, or both, in the direction of the turn. Pilots also use forward slips and side slips to correct for crosswinds during landings and to increase an aircraft's rate of descent without increasing its airspeed.

slow flight
Flight at an aircraft’s minimum controllable airspeed.

solo
Flight during which only one pilot is flying the aircraft.

speed brakes
A type of secondary control on a highly streamlined aircraft designed to allow it to descend rapidly without an excess buildup of airspeed. Speed brakes produce drag without affecting lift or changing the aircraft’s pitch.

spin
A steep, spiraling descent during which an aircraft is stalled and rotating rapidly. The characteristic rotation is the result of a strong yawing moment that occurs when one wing is stalled while the other still produces some lift.

spoilers
Panels on an aircraft's wings that disrupt the flow of air over the wing surface. Spoilers reduce the wing's lift and increase drag. They enable a jet aircraft to make a rapid descent without building excess speed. They are also used immediately after landing to "dump" lift and increase braking efficiency. Sailplane pilots also use spoilers during descent and landing to control their rate of descent.

spool up
To increase power in a jet engine.

spot plane
A view in Flight Simulator that allows the user to see the airplane they are flying as though they were viewing it from another plane flying alongside.

squall
A sudden increase in wind speed by at least 15 knots to a peak of 20 knots or more and lasting for at least one minute. The essential difference between a gust and a squall is the duration of the peak speed.

squall line
A narrow band of active, often severe, thunderstorms not associated with a front. Squall lines often form ahead of a cold front in moist, unstable air, but they can also develop in unstable air far removed from any front. Squall lines present the single most-intense weather hazard to aircraft. They usually develop rapidly and reach maximum intensity during the late afternoon and early evening. Squall lines may also be too long to fly around and too wide and severe to fly through, creating a true barrier in the sky.

stability (aircraft)
Generally, a measure of how an object reacts after it is disturbed by an outside force. Aircraft stability is classified as three types. Positive stability is the tendency to return to steady-state flight after a change in attitude or power. Aircraft are generally designed to exhibit positive stability. An aircraft with neutral stability would remain in the new attitude after being disturbed and would not return to its initial condition. An aircraft with negative stability would diverge from its initial condition, with the oscillations increasing with time.

Stability can also be classified as "static" and "dynamic." Static stability is the initial tendency to return to equilibrium. Dynamic stability refers to the dampening of oscillations over time.

stability (atmospheric)
A measure of the vertical movement of air within an air mass. More specifically, it is a state of the atmosphere in which the vertical distribution of temperature is such that a parcel of air resists displacement from its initial level.

stabilizer
See horizontal stabilizer, vertical stabilizer.

stall
A sudden loss of lift caused by a disruption of the normal smooth flow of air over the upper surface of a wing. A stall is an aerodynamic phenomenon and has nothing to do with the engine. A stall occurs when a wing's angle of attack reaches a specific value, called the critical angle of attack. It is this angle between the wing and the oncoming air—not the airplane's speed, weight, or pitch attitude relative to the horizon—that determines when a wing stalls. In fact, an airplane can stall at any airspeed and in any attitude.

stall speed
The speed at which an aircraft enters a stall under a specified set of conditions. Although an airplane always stalls when the wing's angle of attack reaches the critical angle of attack, the speed at which the stall occurs depends on the aircraft's weight, load factor, the amount of thrust being produced, angle of bank, position of flaps and landing gear, and other factors.
In fact, an airplane can stall at any airspeed—because regardless of speed, a stall occurs only when the angle between the relative wind (parallel and opposite to the aircraft's flight path) and the wing reaches the critical angle of attack.

standard atmosphere
A hypothetical atmosphere based on climatological averages. The most important constants defined in the standard atmosphere are: a surface temperature of 15 C (59 F) and a surface pressure of 1,013.2 millibars (29.92 inches of mercury) at sea level; a lapse rate in the troposphere of approximately 2 C (3.6 F) per 1,000 feet (6.5 C per kilometer), a drop in pressure of approximately 1 inch of mercury per 1,000 feet (110 millibars per 1,000 meters), a tropopause of approximately 36,000 feet (11 kilometers) with a temperature of –56.5 C; and an isothermal lapse rate in the stratosphere to an altitude of approximately 80,000 feet (24 kilometers).

standard conditions
An arbitrary standard established as a baseline for calculations used in meteorology, aviation, and aerodynamics. A set of standard conditions exists for each altitude. At sea level, standard conditions are defined as 29.92 inches of mercury (1,013 millibars) and 59 F (15 C).

Standard Terminal Arrival Route (STAR)
A published procedure that provides an efficient route for arrival at an airport while on an IFR flight plan. STARs are designed to facilitate air traffic control.

Standard Instrument Departure (SID)
A published procedure that provides an efficient route for departure from an airport while on an IFR flight plan. SIDs are designed to facilitate air traffic control.

standard pressure
An arbitrary standard established as a baseline for calculations used in meteorology, aviation, and aerodynamics. A standard pressure is defined at all altitudes. At sea level, the standard is 29.92 inches of mercury or 1,013 millibars.

standard rate turn
A turn of either 3 degrees or 1.5 degrees per second. Small aircraft typically use the 3 degrees per second rate, which results in a 360-degree turn after 2 minutes. Large, fast aircraft typically use the slower rate, which results in a 360-degree turn after 4 minutes. Pilots use the standard rate turn while flying under Instrument Flight Rules (IFR). It provides a steady, predictable, and easy-to-control rate of turn. Standard rate turns are also useful if the heading indicator fails. The pilot can make accurate turns to specific headings by banking at the standard rate and timing the turn.

standard temperature
An arbitrary standard used by aeronautical engineers and pilots in calculations to determine aircraft performance, true airspeed, true altitude, and so forth. The standard temperature is part of the definition of the International Standard Atmosphere (ISA). At sea level, the standard temperature is defined as 59 F (15 C). The standard temperature of dry air drops by about 3.5 F (2 C) per 1,000 feet (305 meters). The standard temperature drops to about 32 F (0 C) at about 7,500 feet (2,300 meters).

standardized instrument cluster
An industry-accepted standard for arranging the six most-commonly used flight instruments on an aircraft instrument panel. The instruments are arranged in two rows. The top row contains the airspeed indicator, attitude indicator, and altimeter. The bottom row contains the turn coordinator, heading indicator, and vertical speed indicator.

stationary front
A front in which neither air mass is replacing the other. In such cases, the surface winds tend to blow parallel to the frontal zone. The slope of a stationary front is normally shallow, although it may be steep, depending on wind distribution and density difference.

statute mile (sm)
A distance of 5,280 feet (1,609 meters). In the United States, statute miles are used to measure visibility in weather reports. Speeds and distances are measured and reported in nautical miles (6,076 feet or 1,852 meters) or nautical miles per hour (knots).

steady-state flight
Flight in which the opposing pairs of forces (lift and weight, thrust and drag) are balanced. An aircraft is considered to be in steady-state flight when it flies at a constant altitude and airspeed, or when it flies at a constant airspeed and is climbing or descending at a constant rate.

steam fog
Fog formed when air blows from a cold surface (either land or water) over warmer water.

stick
A tubular control in some aircraft, usually between the pilot’s knees, used to control the aircraft about its roll and pitch axis (the same function as a yoke).

stick shaker
A mechanism that shakes the control column in some aircraft to warn the pilot of an impending aerodynamic stall.

STOL
Abbreviation for "short takeoff and landing." It describes aircraft capable of operating from airfields with short runways.

stop and go
Similar to a touch and go landing, except the aircraft makes a complete stop.

straight-and-level flight
One of the more difficult maneuvers to master. Like a balancing act, straight-and-level flight requires that you make smooth, small corrections to keep from wobbling all over the sky. To master straight-and-level flight, you must hold a constant altitude and hold a constant heading.

stratiform
Clouds with extensive horizontal development. Stratiform clouds develop in stable air and, therefore, are composed of small water droplets.

stratocumulus
A low, predominantly stratiform cloud. It is usually a mosaic of gray and whitish patches or layers. The layers may or may not merge, and such clouds are rounded or roll-shaped with relatively flat tops.

stratosphere
The region of the earth's atmosphere above the troposphere and tropopause, beginning at an altitude of 5–10 mi (8–16 km), depending on latitude and season. The stratosphere is a region of relatively uniform temperatures and winds, extending to about 30 miles (48 kilometers), where it meets the mesosphere.

stratus clouds
A low, gray cloud layer or sheet with a fairly uniform base formed in stable air. Stratus clouds sometimes appear in ragged patches. They seldom produce precipitation, but may produce drizzle or snow grains.

student pilot certificate
In the United States, a pilot certificate issued to a pilot in training. To be eligible for a student pilot certificate for powered aircraft, a person must be at least 16 years old and hold at least a current third-class medical certificate. A student pilot certificate is valid for 24 months. Student pilots cannot carry passengers, act as pilot in command of an aircraft for compensation or hire, or make international flights (except between certain places in Alaska and Canada). Student pilots can fly only when visibility is at least 3 sm (5 sm at night) and must maintain visual contact with the ground. Student pilots also must receive specific endorsements from a certified flight instructor before flying solo, making cross-country flights, or operating an aircraft in certain types of controlled airspace. Those endorsements must be renewed every 90 days.

sublimation
The changing of ice directly to water vapor, or water vapor to ice, bypassing the liquid state in each process. Snow or ice crystals result from the sublimation of water vapor directly to the solid state.

subsonic
Speeds below the speed of sound, that is, less than Mach 1.

supercharger
A compressor used to increase the density of the air or the air-fuel mixture supplied to a piston engine. As an aircraft climbs, the density of the air entering the engine decreases. Without a supercharger or a turbocharger, an engine gradually loses power. A normally aspirated, or "unboosted," engine is most efficient at about 8,000 feet (2,438 meters). By the time an aircraft reaches 18,000 feet (5,486 meters), the density of the air is about half its value at sea level, and a supercharger or turbocharger is required for efficient operation of the engine.

supercooled water
Water droplets colder than 0 C (32 F). When supercooled droplets strike an exposed object, such as an aircraft's wings and other structures, the impact induces freezing. Supercooled water droplets often are in abundance in clouds at temperatures between 0 and –15 C with decreasing amounts at colder temperatures. However, strong vertical currents may carry supercooled water to great heights where temperatures are much colder. Supercooled water has been observed at temperatures colder than –40 C.

supersonic
Speed that exceeds the speed of sound; Mach 1 or greater.

surface inversion
An inversion with its base at the surface, often caused by cooling of the air near the surface as a result of terrestrial radiation, especially at night.

surface visibility
Visibility observed from eye-level above the ground.

synchrophaser
A device that automatically adjusts the speed of the propellers of all engines on a multiengine aircraft so that a master blade on each propeller keeps the same relative position in its rotation to the master blade of each of the other propellers.

Propellers out of synch result in higher cabin-noise levels and a sort of washing-machine-like sound. During World War II, pilots flying with their props out of synch earned the nickname "Washing Machine Charlie."

---

tachometer
The instrument that shows the speed of rotation of the engine. It is marked in revolutions per minute (rpm). Engines that produce more than about 180 horsepower usually have constant-speed propellers that can change the blade angle to make more efficient use of engine power throughout a wide range of airspeeds.

taildragger
An aircraft that has its main wheels mounted ahead of the center of gravity and a small pivoting or steerable wheel supporting the aft fuselage. There is no nosewheel, as with tricycle-gear aircraft. Taildraggers were the norm during the early years of aviation and are sometimes referred to as conventional-gear aircraft. They are trickier to handle on the ground than tricycle-gear aircraft and require special training and skill.

tailwind
A wind that blows in the same direction as the aircraft is traveling.

takeoff roll
The portion of the takeoff during which the airplane accelerates on the runway.

taxi
To move an aircraft under its own power on the ground.

temperature layer
In Flight Simulator, a user-specified weather option that includes the air temperature at a specific altitude and the day-night variation in temperature. You select temperature options in the Advanced Weather dialog box. You can create four temperature layers to see how temperature affects density altitude and aircraft performance.

temperature-dew point spread
The difference between air temperature and dew point temperature. As this difference narrows, clouds and fog are more likely to appear. Pilots must also be aware that even if the spread at ground level is quite large, as temperature drops with altitude, the spread narrows, and clouds may form. Pilots should be especially alert for fog whenever the temperature-dew point spread is 5 F (2.8 C) or less and decreasing.

terminal velocity dive brakes
See spoilers.

tetrahedron
A large wind-direction indicator made of lightweight material in the shape of pyramid.

thermal
A rising column of air caused by the sun heating the earth's surface. Sailplane pilots use thermals to climb or maintain altitude.

threshold
The beginning of the runway surface that is usable for landings. The threshold may coincide with the physical end of the runway or be displaced if part of the runway is not usable for landings. The threshold is marked by a single white line on visual runways or by eight parallel white lines arranged in two groups of four on either side of the centerline of an instrument runway.

throttle
The cockpit control that most directly determines the power output of the engine. In a piston engine, the throttle actually controls the amount of air entering the carburetor or induction system. The carburetor, or fuel metering system, mixes the appropriate amount of fuel with the air to create a combustible mixture. When fully "open," the throttle allows the maximum amount of air to enter the system to produce maximum power. When the throttle is "closed," only a small amount of air enters the system and the engine produces minimum power.

thrust
The forward force generated by a propeller or jet engine that moves the aircraft forward through the air. One of the four major forces, thrust is opposed by drag.

thunderstorm
In general, a local storm invariably produced by a cumulonimbus cloud and always accompanied by lightning and thunder.

tie-down
The spot on an airport ramp where an aircraft is parked and secured with ropes or chains. Airplanes are designed to fly, so they must be tied down to prevent catastrophe in windy weather.

tilt-rotor
A vertical-takeoff aircraft with rotors that can tilt from the horizontal to the vertical. An example of this type of aircraft is the V-22 Osprey.

torque
A force that produces or attempts to produce rotation.

touch and go
A landing during which the aircraft does not come to a complete stop before power is applied and the aircraft takes off again. Often done for pilot practice and training.

tower
A building on an airport from which an air traffic controller directs traffic within the immediate airport traffic area. The top of the tower, called a cab, is usually glassed in on all sides giving a 360-degree view.

tower visibility
Prevailing visibility determined from the control tower.

towering cumulus
A rapidly growing cumulus in which height exceeds width.

trace icing
Icing, regardless of type, that accumulates at about the same rate as it dissipates by sublimation. Trace icing is not considered hazardous, even to aircraft without anti-icing or deicing equipment, unless the airplane remains in icing conditions for more than one hour.

track
The path followed by an aircraft while in flight.

trade winds
Prevailing, almost continuous, winds blowing with an easterly component from the subtropical high pressure belts toward the intertropical convergence zone. They blow northeast in the Northern Hemisphere, southeast in the Southern Hemisphere.

traffic
Aircraft other than yours that present a potential for occupying the same airspace.

traffic pattern
The traffic flow prescribed for aircraft landing at and taking off from an airport. Also called a "circuit" in Britain. A complete landing pattern includes a downwind leg, a base leg, and a final leg. Aircraft taking off typically fly an upwind leg and crosswind leg to depart the airport traffic pattern.

trainer
Aircraft used for pilot training. They are usually (not always) single-engine light aircraft. Popular models for pilot training include the Cessna 152, 172, and 182.

transmissometer
An instrument system that measures the transmission of light through the atmosphere. The transmission value is converted either automatically or manually into visibility, runway visual range (RVR), or both.

translating tendency
The tendency of a helicopter to drift in the direction of the tail rotor thrust.

transonic
Speeds close to the speed of sound in which both subsonic and supersonic airflow conditions exist. Transonic speeds range from about Mach 0.8 to Mach 1.4.

transponder
An airborne transceiver that receives signals from air traffic control (ATC) radar and replies with a preset identification code, or "squawk," set by the pilot. A computer uses the code to display information about each aircraft on an air traffic controller's radar display. Most aircraft today are also equipped with altitude encoders, which transmit the aircraft's altitude along with the transponder code. In the United States, aircraft operating under visual flight rules (VFR) squawk code 1200 unless they are using air traffic control services and are assigned a specific code by ATC. Under instrument flight rules (IFR), all aircraft are assigned transponder codes by ATC.

transverse flow effect
A decrease in lift in the aft portion of a helicopter rotor disk when in forward flight or hovering in a wind.

tricycle-gear
A landing gear system that includes a nosewheel assembly and two main gear assemblies. Because the center of gravity of a tricycle-gear airplane lies ahead of the main gear, this geometry is much more stable on the ground than the so-called conventional gear or "taildragger" arrangement, which has two main gear assemblies and a tailwheel.

trim
To adjust a movable tab on a control surface, usually the elevator, to relieve pressure on the flight controls. Trim is necessary because as an aircraft changes speed, the amount of air flowing over the control surfaces varies. Without trim, a pilot would have to hold forward or back pressure on the yoke or column to maintain a specific airspeed or pitch attitude. Larger aircraft also have aileron and rudder trim.

tropopause
A thin layer in the upper atmosphere that forms the boundary between the troposphere and stratosphere. The tropopause is usually characterized by an abrupt change in lapse rate. The height of the tropopause varies from about 65,000 feet (19.8 kilometers) over the equator to 20,000 feet (6,096 meters) or lower over the poles. Temperature and wind vary greatly in the vicinity of the tropopause. Maximum winds generally occur at levels near the tropopause. These strong winds create narrow zones of wind shear, which often generate hazardous high-altitude turbulence.

troposphere
The layer of the atmosphere from the surface to an average altitude of about 7 miles (11 kilometers). Most weather occurs within the troposphere. Temperature generally decreases with altitude in the troposphere at an average rate of 2 C (3.6 F) per 1,000 feet (305 meters). The height of the troposphere varies with latitude and seasons. It slopes from about 20,000 feet (6,096 meters) over the poles to about 65,000 feet (19.8 kilometers) over the equator. It is higher in summer than in winter. A very thin layer called the tropopause marks the boundary between the troposphere and the next highest layer, the stratosphere.

trough
In meteorology, an elongated area of relatively low atmospheric pressure. A trough is usually associated with and most clearly identified as an area of maximum cyclonic curvature of the wind flow.

true airspeed (TAS)
An aircraft's actual speed through the surrounding air. As an aircraft climbs, the surrounding air becomes less dense. Therefore, indicated airspeed tends to decrease as altitude increases. To determine how fast the airplane is really moving through the air, the pilot calculates TAS based on the aircraft's current pressure altitude and the outside air temperature. A pilot must know TAS to solve navigation problems and file flight plans. As a rule of thumb, at a given indicated airspeed, true airspeed increases about 2 percent for each 1,000 feet (305 meters) of altitude. Therefore, an aircraft flying at an indicated airspeed of 100 knots at 10,000 feet (3,050 meters) is actually flying at about 120 knots through the air.

true altitude
An aircraft's actual height above mean sea level. True altitude is the altitude shown on the altimeter, corrected for nonstandard temperature. If the temperature at a particular indicated altitude is warmer than the standard temperature for that altitude, an aircraft's altimeter senses higher-than-normal pressure and shows an altitude lower than the aircraft is actually flying. If the temperature is cooler than the standard temperature for a particular altitude, the altimeter senses lower-than-normal pressure and shows an altitude higher than the aircraft is actually flying. Variations from nonstandard temperature usually cause only small errors in the altimeter. When flying without visual references over mountainous terrain, however, or when temperatures aloft differ significantly from the standard temperatures, a pilot should check the aircraft's true altitude to ensure obstacle clearance.

true north
A heading, or course, pointed directly at the earth's geographic North Pole. The earth's magnetic North Pole is offset from the geographic North Pole, so course lines drawn on a chart are usually referenced to lines of longitude, which indicate true north. To solve navigation problems, pilots convert true courses, or headings, into magnetic courses and headings that they can fly by referencing the aircraft's compass. The difference between true north and magnetic north is called "variation." The actual value of the variation depends on your location. Many weather reports and forecasts indicate wind direction in reference to true north.

turbine
The fan-like portion of a jet engine or turboprop engine that compresses the incoming air.

turbine inlet temperature (TIT)
Air temperature measured as it enters the turbine inlet guide-vanes or the first stage of a turbocharger or a turbine engine. TIT is the highest temperature inside a turbocharged engine and is a limiting factor of the amount of power an engine can produce.

turbocharger
A device on a piston engine that compresses the air entering the engine to maintain power output at high altitudes. Exhaust gas from the engine spins a turbine in the turbocharger at high speed. A compressor attached to the same shaft as the turbine compresses the air entering the intake manifold, "boosting" the engine's power as the density of the outside air decreases with altitude. Turbochargers increase the temperature of the air entering the engine, so pilots must monitor engine temperatures carefully.

turbofan
A jet engine in which most of the air entering the engine is accelerated by a large fan and does not pass through the combustion chamber of the engine. Turbofan engines have largely replaced turbojet engines, in which most of the air entering the engine passes through the combustion chamber. Turbofan engines are much more efficient and significantly quieter than turbojets.

turbojet
A jet engine in which most of the air entering the engine passes through several compressing turbines and then enters the combustion chamber. In a turbojet engine, the exhaust stream produces most of the engine's thrust. Turbojet engines have largely been replaced by more efficient and quieter turbofan engines.

turboprop
A jet engine that drives a propeller to create thrust. Turboprop aircraft are less noisy and burn less fuel than turbojet aircraft, but they are also efficient only at speeds up to about 640 kilometers per hour (400 mph). Also called a propjet.

turn coordinator
One of the six primary flight instruments, the turn coordinator shows the rate of turn and the quality of a turn—whether the aircraft is slipping, skidding, or in a balanced turn. In most modern light aircraft, the turn coordinator has replaced the "needle and ball," which served the same function.

TWEB
Abbreviation for "transcribed weather broadcast." These recorded weather reports and forecasts, prepared by a Flight Service Station (FSS) in the United States, are broadcast over many navigation facilities, especially VORs. Pilots can monitor TWEB broadcasts while en route to stay up-to-date with the latest weather information.

type certificate
An official document that certifies that an aircraft or engine design has met all the criteria specified in the regulations governing the certification and testing of aircraft. In the United States, the FAA issues normal, utility, transport, aerobatic, experimental, limited, restricted, and provisional type certificates.

type rating
In the United States, an authorization added to a pilot certificate that permits a person to act as pilot in command of a specific type of aircraft, usually a heavy, turbojet-powered, or rotary wing aircraft. Type ratings are required for all aircraft with a maximum certified takeoff weight of more than 12,500 pounds (5,670 kilograms) and all turbojet-powered aircraft, regardless of maximum takeoff weight. A type rating is also required to act as pilot in command of a helicopter if the operation also requires that the pilot hold an airline transport pilot certificate.

---

ultralight
Single-seat, lightweight aircraft designed for recreational flying. Also known as "microlights" in Britain. In the United States, ultralight aircraft are not certified by the FAA and no pilot certificate is required to fly them. According to United States regulations (FAR Part 103), an aircraft qualifies as an ultralight if it has an empty weight of no more than 254 pounds(115 kilograms), if it is equipped with an engine, can carry no more than 5 gallons (18.9 liters) of fuel, cannot fly more than 55 knots in level flight at full power, and has a power-off stall speed no greater than 24 knots.

uncontrolled airspace
Airspace in which air traffic control does not provide services and in which an ATC clearance is not required to operate, regardless of weather conditions.

uncoordinated flight
Slipping or skidding flight caused by yawing forces produced by a turn, by the action of the propeller and slipstream, or by an imbalance of power in a multiengine aircraft. The inclinometer (ball) of the turn coordinator shows whether the yawing forces are properly balanced. If the ball is on the inside of a turn, the airplane is slipping. If the ball moves to the outside of a turn, the airplane is skidding.

under the hood
See hood.

undercast
A solid cloud layer as viewed from above the layer.

UNICOM
A privately owned radio station on an airport used to give advisories to pilots. UNICOM cannot be used to control traffic.

updraft
A localized upward current of air.

upslope fog
Fog formed when air flows upward over rising terrain and is cooled to or below its initial dew point. Once the upslope wind ceases, the fog dissipates. Unlike radiation fog, upslope fog can form under cloudy skies. It often is quite dense and extends to high altitudes.

upwind
That leg of a standard traffic pattern aligned with the runway on takeoff. The aircraft is said to be on the upwind leg until it is turned 90 degrees onto the crosswind leg.

useful load
The maximum allowable weight of an aircraft minus its empty weight. The useful load includes the weight of the fuel, oil, crew, passengers and their baggage, and any cargo carried.

UTC (Universal Coordinated Time)
The official time used in air navigation. Also known as Zulu time, or Greenwich Mean Time (GMT). UTC is based on the 24-hour clock; for example, 6 P.M. is 1800 hours. Departure and arrival times, air traffic control clearances, the valid time of weather reports and forecasts, and other time-sensitive information in aviation is expressed in UTC.

---

variometer
An instrument that helps you know when you're in rising air and how strong the lift is. Unlike a vertical speed indicator in a powered aircraft, which indicates the vertical speed of the aircraft, a compensated variometer indicates the vertical speed of the air through which a glider is moving.

vector
A heading given by an air traffic controller to a pilot for the purpose of navigation or traffic avoidance.

vertical axis
An imaginary vertical line running through the center of an aircraft. Rotation about the vertical axis is called "yaw" and is controlled by the rudder.

vertical speed indicator
One of the six basic flight instruments, the vertical speed indicator shows an aircraft's rate of climb or descent, usually in feet per minute. Also known as the rate of climb indicator (RCI) or vertical velocity indicator (VVI). Large aircraft are typically equipped with a sophisticated version of this instrument, called an "instantaneous vertical speed indicator" (IVSI) that reacts immediately to changes in altitude.

vertical stabilizer
The vertical tail surface on an aircraft; sometimes called a "fin." The vertical stabilizer is fixed on most aircraft. With the attached rudder, the vertical stabilizer provides directional stability by controlling movement about the vertical (yaw) axis.

vertigo
A state of temporary spatial confusion resulting from misleading information sent to the brain by various sensory organs; also called "spatial disorientation." Vertigo typically occurs when a pilot cannot see the ground or other references due to clouds or darkness. Turns and other maneuvers can cause the vestibular system—a set of tubes and other sensory organs in the inner ear—to send conflicting signals about the aircraft's orientation and movement. Without visual references to overcome those sensations, the pilot quickly becomes disoriented and unable to tell whether the airplane is flying straight and level, turning, climbing, or descending. To avoid or overcome vertigo, a pilot must rely on the flight instruments to verify the aircraft's attitude and maintain control.

very high frequency (VHF)
The portion of the radio spectrum used in civil aviation for primary radio navigation and communications. Voice communications are assigned frequencies between 118.0 and 136.975 MHz. VORs operate on frequencies between 108.0 and 177.95 MHz.

very high frequency omnidirectional radio (VOR) range
A ground-based radio transmitter that sends signals in 360 radials. Some of these radials define airways, but pilots can track any radial to fly a specific path over the ground. VORs operate on frequencies between 108.0 to 177.95 MHz in the VHF band. Although satellite-based navigation systems are rapidly coming into widespread use, VORs still remain the primary electronic navigation system in use today.

VFR sectional charts
Navigational charts published for use by pilots flying under visual flight rules (VFR). The charts depict navaids, landmarks, Victor airways, terrain elevations, and other important information relevant to VFR flight.

Victor airways
Airways delineated by navigational radio signals for low routes (below 18,000 feet). Sometimes referred to as highways in the sky. Jet airways are used for high-altitude routes.

view-limiting device
See hood.

virga
Water or ice particles falling from a cloud, usually in wisps or streaks, that evaporate before reaching the ground.

virtual airline
Airlines created on the World Wide Web by and for pilots of flight simulators. Virtual airlines simulate the world of commercial aviation by hiring, training, and assigning routes to virtual pilots. There has been a great increase in interest in these airlines in recent years, and some operations are quite elaborate.

visibility
The ability to see and identify prominent, unlighted objects by day and prominent, lighted objects at night. Visibility is expressed in units of distance (statute miles, hundreds of feet, or meters). Atmospheric conditions determine the distance.

visual approach slope indicator
A lighting system that indicates an airplane's position relative to the desired glideslope to a particular runway.

visual flight rules (VFR)
The "rules of the road" that govern flight when the visibility and ceiling allow pilots to navigate and avoid obstacles and other aircraft by visual reference. In the United States, VFR flight is generally permitted when the visibility is at least 3 miles (5 kilometers) and a pilot is able to operate at a safe altitude that is at least 500 ft (152 meters) below and 1,000 feet (304 meters) above any clouds. In addition, the pilot must remain at least 2,000 feet (608 meters) horizontally from any clouds. Specific requirements for VFR depend on the type of airspace, time of day, and height above the terrain.

visual meteorological conditions
Weather conditions that permit flight under visual flight rules (VFR). In controlled airspace in the United States, VMC conditions generally mean that the ceiling is at least 1,000 feet (305 meters) and flight visibility is at least 3 miles (5 kilometers).

VOR approach
A nonprecision instrument approach in which a VOR is the initial approach fix.

VOR hold
A holding pattern over a VOR station.

VOR intercept
Tuning in, identifying, and turning toward a VOR station.

vortices
Rotating air currents created by wings and other airfoils that are producing lift. Vortices are typically created when high-pressure air below a wing circulates toward the low-pressure area above a wing. These vortices are the primary source of induced drag, a by-product of lift. Vortices created by large, heavy aircraft (called "wake turbulence") are a serious hazard to smaller aircraft, especially during takeoff and landing.

V-speeds
Used to designate speeds during specific flight conditions.

• VA - Design maneuvering speed; the maximum speed at which full control deflection can be made without overstressing the aircraft.
• VAPP - Approach climb speed; the airspeed used in the approach configuration.
• VFE - Maximum flap-extended speed; the maximum allowable speed with the flaps extended.
• VLE - Maximum landing gear-extended speed; the maximum allowable speed with the landing gear extended.
• VLO - Maximum landing gear-operating speed; the maximum speed at which the landing gear can be extended or retracted.
• VMC - Minimum control speed; the lowest speed at which the airplane is controllable when one engine is inoperative and the other engine is operating at full power.
• VMO/MMO - Maximum operating limit speed; the speed that cannot be exceeded in any flight condition. VMO is expressed in KIAS. VMMO is expressed in Mach number.
• VNE - Never-exceed speed (the red line on the airspeed indicator).
• VN - Maximum structural cruising speed; the maximum allowable airspeed in turbulent air (the lower limit of the yellow arc on the airspeed indicator).
• VR - Rotation speed; the speed at which the pilot raises the nose to lift off the runway during the takeoff roll.
• VREF - Approach speed (based on weight and conditions).
• VS - Stalling speed; the minimum steady flight speed at which the aircraft is controllable.
• VSO - Stalling speed; the minimum steady flight speed in the landing configuration.
• VTT - Target threshold speed.
• VX - Best angle of climb speed; the speed at which the aircraft will gain the most altitude in the least horizontal distance.
• VY - Best rate of climb speed; the speed at which the aircraft will gain the most altitude in the least amount of time.
• V1 - Takeoff decision speed; the speed at which it may not be possible to stop the aircraft on the runway in case of a rejected takeoff (RTO).
• V2 - Minimum takeoff safety speed; the minimum safe flying speed should an engine fail immediately after takeoff.

VTOL
Abbreviation for "vertical takeoff and landing," describing aircraft that can take off straight up and land straight down without the need of a runway. The most successful application of VTOL technology, based on the vectored-thrust engine, is the British Harrier jump jet.

---

Wagstaff, Patty
1951– . American aerobatic pilot. Wagstaff was the 1993 International Aerobatic Club Champion and a three-time United States National Aerobatic Champion. A member of the United States Aerobatic Team, she is also an aerobatics coach, an aerial competition judge, and a movie stunt pilot.

wake turbulence
Turbulence caused by a moving aircraft. More specifically, the powerful vortices generated by the wing tips of a large, heavy aircraft.

warm front
Any nonoccluded front in which warmer air replaces colder air.

waypoint
A predetermined navigational point in space along a route of flight. Waypoints can be electronically stored in some navigation devices, such as RNAV and GPS units.

weather areas
In Flight Simulator, regions with similar weather characteristics. Flight Simulator has a global weather area, and you can create two local weather areas. You can define the size of the local weather areas and specify the type of clouds, visibility, temperature, wind, atmospheric pressure, and other characteristics of each area.

weathercock
(Verb) To display a tendency to veer in the direction of the wind.

weathervane effect
The tendency of an aircraft to pivot around its center of gravity and point into the wind.

weight
One of the four basic forces at work on an aircraft in flight. Lift opposes weight (more accurately, the sum of all downward forces), which always acts directly toward the center of the earth. In most calculations, aeronautical engineers assume that all of an aircraft's weight is concentrated at a point called the center of gravity.

wet wing
An aircraft wing in which fuel is stored directly inside the skin of the wing as opposed to enclosing the fuel in a rubber bladder inside the wing.

Whitcomb, Richard
1921– American engineer. At the laboratory of the National Advisory Committee for Aeronautics (NACA), Whitcomb developed the "area rule" for reducing drag at transonic speeds by pinching the fuselage where it meets the wing. In 1955, he won aviation's prestigious Collier Trophy for inventing this "Coke-bottle effect" during his breakthrough redesign of the fuselage of the Convair YF-102.

Whittle, Frank
1907–1996. English engineer and RAF officer who invented the turbojet aircraft engine. Whittle published his theory of jet propulsion in 1928, but the British Air Ministry didn't become interested in the idea until 1938. Whittle developed a working jet aircraft by 1941; however, it wasn't until 1944 that the first military jet aircraft, the Gloster Meteor, was put into service.

wilco
Used in radio transmissions as shorthand for "will comply."

wind layer
In Flight Simulator, a user-specified weather option that includes wind speed, direction, gust factor, and turbulence between two altitudes. You select these options on the Wind tab in the Advanced Weather dialog box. You can create two wind layers in each weather area.

wind shear
A sudden, sharp change in wind direction or speed, often associated with a violent, downward burst of air called a microburst or a low-level temperature inversion. Wind shear is particularly dangerous to aircraft during takeoff and landing, where it can cause large variations in airspeed and lead to a rate of sink that can push an aircraft into the ground.

wind tunnel
A device for studying the effects of airflow on aircraft and other vehicles or structures. To simulate the conditions of flight, an aircraft model is mounted in the tunnel and subjected to a stream of air. Sensors gather data on lift and drag, pressure, and other forces acting on the model.

wing loading
An aircraft's weight divided by the area of its wings. For example, an aircraft weighing 4,000 pounds (1,814 kilograms) that has 400 square feet of wing area has a wing loading of 10 pounds per square foot.

winglets
Vertical extensions added to the ends of a main wing. Winglets reduce induced drag by increasing the effective aspect ratio of the wing, thereby decreasing the intensity of wingtip vortices produced as a by-product of lift.

wingspan
The distance from one wing tip to the other.

wingtip vortex
Rapidly rotating air at an aircraft's wing tips created when a wing produces lift. At the tips, high-pressure air below the wing spirals up to the low-pressure area above the wing. Large, heavy aircraft produce intense vortices that are extremely hazardous to small aircraft, especially during takeoff and landing. For example, a small aircraft that flies into a wingtip vortex can be rolled inverted because the rotation of the vortex exceeds the rolling capability produced by the ailerons. Also known as "wake turbulence."

Wright brothers
American aviation pioneers Wilbur (1867–1912) and Orville (1871–1948) Wright made the first powered flight in a heavier-than-air craft. They began experimenting with double-winged kites and gliders, and in 1900, made their first test flights at Kitty Hawk. In 1901, they built the first wind tunnel in the United States, testing over 200 models of wing surfaces. After building an engine for their Flyer, they made their historic flight on December 17, 1903.

---

---

yaw
Movement of an aircraft about its vertical axis, as when the nose turns left or right. Along with roll and pitch, yaw is one of an airplane's three basic movements. The vertical stabilizer and rudder are designed to control yaw.

Yeager, Chuck
1923– American Air Force officer and fighter pilot, test pilot, and the first person to fly faster than the speed of sound (1947). Yeager, who achieved the rank of brigadier general in 1968, received numerous decorations, including the Distinguished Service Medal, the Purple Heart, and the Presidential Medal of Freedom. Yeager made his most famous flight despite two broken ribs he sustained in a riding accident just days before.

yoke
See control yoke.

---

zeppelin
An airship with rigid internal bracing. The helium or hydrogen used to lift a zeppelin is usually contained in large cells. The largest of airships, zeppelins were used as bombers in World War I, but they caused little damage; instead, most were lost to accidents and enemy fire. In 1937, the "Hindenburg" disaster brought a swift end to the use of the rigid airship as a civil transport.

Zeppelin, Count Ferdinand von
1838–1917. German aeronaut, inventor, and army general who rode in observation balloons with Union forces during the American Civil War. In 1898, he founded an airship factory in Germany and constructed the first rigid type of airship. By 1910, his zeppelins carried passengers on excursions between German cities, and in 1912, the German navy ordered the first military zeppelin.

Zulu time
See Greenwich Mean Time.