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A glossary of aviation terms and abbreviations, part 1 of 2...

Aviation related terms and definitions used in association with civil flight obtained from the Code of Federal Regulations, Federal Aviation Regulations, and many other sources (general abbreviations and those specific to Aerofiles can be found on its Codes page). Cross references are in CAPITAL letters.

ABSOLUTE ALTITUDE - The measureable height of an aircraft above the actual terrain.

ABSOLUTE CEILING - The maximum altitude above sea level at which an aircraft can maintain level flight under Standard Air conditions.

ACCELERATED STALL - Any stall made to occur at other than 1g.

ACCESSORY GROUP - Mechanical and electrical units mounted on an engine necessary for its operation, such as starter, magnetos, fuel pumps, etc.

ADCOCK RANGE - National low-frequency radio navigation system (c.1930-c.1950) replaced by an omnirange (VOR) system. It consisted of four segmented quadrants broadcasting Morse Code "A" (dot-dash) and "N" (dash-dot) signals in opposing quadrants so that pilots could orient their position relative to a "beam" broadcasting a steady tone, and a Morse Code station identifier. Using a "build-and-fade" technique, a pilot could (ideally) pinpoint his location by the strength or weakness of a signal. Read about it here.

ADF - Automatic Direction Finding via automated radio.

ADIABATIC LAPSE RATE - The rate at which ascending air cools and descending air warms, given that no heat is added or taken away. The rate for dry air (DALR) is based on 5.4° F per 1000' altitude (1° per 100 meters). Moist or saturated rates (SALR) vary with barometric pressures and temperatures, and must be adjusted for accuracy.

ADVERSE YAW - Yaw generated when the ailerons are used. The lifting wing generates more drag, causing an airplane to yaw (turn) toward it.

AGL - Above Ground Level, as a measurement of altitude above a specific land mass, and differentiated from MSL.

AILERON - The movable areas of a wingform that control or affect the roll of an aircraft by working opposite one another—up-aileron on the right wing and down-aileron on the left wing.
French: aileron small wing, diminutive of aile, from Latin: ala, wing. The word "aisle" also derives from the same root.

AIR AMBULANCE - Air charter of a private jet, helicopter or turbo-prop certified to provide air transport to medical patients.

AIR CHARTER - Act of renting or leasing a jet or plane for transport of cargo or passengers.

AIR DEFENSE IDENTIFICATION ZONE (ADIZ) - The area of airspace over land or water, extending upward from the surface, within which the ready identification, the location, and the control of aircraft are required in the interest of national security.

Domestic Air Defense Identification Zone - An ADIZ within the United States along an international boundary of the United States.

Coastal Air Defense Identification Zone - An ADIZ over the coastal waters of the United States.

Distant Early Warning Identification Zone (DEWIZ) - An ADIZ over the coastal waters of the State of Alaska. ADIZ locations and operating and flight plan requirements for civil aircraft operations are specified in FAR Part 99.

AIRFOIL - The shape of any flying surface, but principally a wing, as seen in side-view ("cross-section"). Its characteristics are Center of Pressure (CP), DRAG (CD), LIFT (CL), Lift-Drag Ratio (L/D), and Moment (CM).

AIR ROUTE TRAFFIC CONTROL CENTER (ARTCC) or "CENTER" - A facility established to provide air traffic control service to aircraft operating on IFR flight plans within controlled airspace and principally during the en route phase of flight. When equipment capabilities and controller workload permit, certain advisory/assistance services may be provided to VFR aircraft.

AIRSPACE CONFLICT - A near mid-air collision, intrusion, or violation of airspace rules.

AIR SPEED INDICATOR - An instrument or device that measures the air speed of an aircraft through an air mass, but not its ground speed.

AIR TAXI - An aircraft operator who conducts operations for hire or compensation in accordance with FAR Part 135 in an aircraft with 30 or fewer passenger seats and a payload capacity of 7,500# or less. An air taxi operates on an on-demand basis and does not meet the "flight scheduled" qualifications of a commuter.

AIR TRAFFIC CONTROL (ATC) - A service operated by the appropriate authority to promote the safe, orderly, and expeditious flow of air traffic.

AIRPORT TRAFFIC CONTROL TOWER (ATCT) - A terminal facility that uses air/ground communications, visual signaling, and other devices to provide ATC services to aircraft operating in the vicinity of an airport or on the movement area. Authorizes aircraft to land or takeoff at the airport controlled by the tower or to transit the Class D airspace area regardless of flight plan or weather conditions (IFR or VFR). A tower may also provide approach control services (radar or non-radar).

AIRWORTHINESS - A term used to describe both the legal and mechanical status of an aircraft with regard to its readiness for flight.

ALCLAD - Trademark name of Alcoa for high-strength sheet aluminum clad with a layer (approximately 5.5% thickness per side) of high-purity aluminum, popularly used in airplane manufacture.

ALPHABET (PHONETIC) - Devised for reasons of clarity in aviation voice radio, this is the current NATO version in global use:


The original, from early in World War 2, was:

ALTIMETER - An adjustable aneroid-barometric cockpit instrument used to measure an aircraft's altitude.

ALTITUDE - The height expressed in units of distance above a reference plane, usually above mean sea level or above ground level.

AMPHIBIAN, AMPHIBION - A SEAPLANE or FLOATPLANE with retractable wheels for use on land, as well. The latter spelling was used in the '20s and '30s but has since fallen out of favor.

ANGLE OF ATTACK - The acute angle at which a moving airfoil meets the airstream.

ANGLE OF INCIDENCE - The angle at which an airfoil is normally fixed in relation to the longitudinal axis of an aircraft.

ANHEDRAL - The downward angle of a wing in relation to a horizontal cross-section line; aka CATHEDRAL. See DIHEDRAL.


APRON - The hard-surfaced or paved area around a hangar.

ARM - In aircraft weight and balance, as well as load distribution, it is the distance from the CENTER OF GRAVITY (c/g) to some point. For computations, arms measured forward from the c/g are positive (+n) and those measured aft of the c/g are negtive (-n).


ARTIFICIAL HORIZON - A vacuum-powered panel instrument that displays pitch and roll movements about the lateral and longitudinal axes; aka Attitude Indicator.

ASPECT RATIO - The ratio of the span to the chord of an airfoil—a high-aspect ratio wing has wide span and narrow chord, and vice-versa for a low-aspect ratio wing.


ATC - (1) Aircraft license as Approved Type Certificate, see ATC; (2) AIR TRAFFIC CONTROL.


AUTOGYRO, AUTOGIRO - An aircraft, often wingless, with unpowered rotary airfoil blades that auto-rotate and serve as wings as they move through the air when mounted on a powered aircraft (or, in some cases, a glider). The latter spelling is a trademark of the Autogiro Corporation.

AUTO-ROTATION - Automatic rotation of rotary blades from a helicopter in an unpowered glide or the forward movement of an autogyro.

AUTOMATIC TERMINAL INFORMATION SERVICE (ATIS) - Continuous broadcast of recorded non-control information in selected terminal areas, to improve controller effectiveness and to relieve frequency congestion by automating repetitive transmissions of essential but routine information.

AVIONICS - The computers and other electronic systems that monitor and control an aircraft's electrical and mechanical systems.

AZIMUTH - The direction or angle between the radar site and an aircraft; measured clockwise from north in a horizontal plane.

BALANCED CONTROL SURFACE - A movable control surface, as an aileron or rudder, having an added physical extension or weights forward of the hinge-point to reduce forces on a joystick or yoke. See ELEPHANT EARS (2).

BEARING - The horizontal direction to or from any point, usually measured clockwise from true north (true bearing) magnetic north (magnetic bearing), or some other reference point, through 360°.

BERNOULLI EFFECT or LAW or THEOREM - Since the pressure of a fluid is proportional to its velocity, airflow over the upper surface of an airfoil causes suction [lift] because the airstream has been speeded up in relation to positive pressure of the airflow on the lower surface.

BLEED AIR - Hot air at high pressure, usually from the bypass section of a gas turbine engine, for de-icing, heating, and other uses.

BOUNDARY-LAYER CONTROL - The design or control of slotted or perforated wings with suction methods to reduce undesirable aerodynamic effects caused by the boundary layer—that region adjacent to the boundary where shear stresses dominate in the airflow over a wingform.

BUMPED COWLING - An engine FAIRING, generally circular, with welts or compound shapes in its surface to accommodate cylinder heads.

CABANE STRUT - Wing strut attached to the fuselage.

CALIBRATED AIRSPEED (CAS) - The indicated airspeed of an aircraft, corrected for position and instrument error. CAS is equal to true airspeed in standard atmosphere at sea level. Compare INDICATED AIRSPEED and TRUE AIRSPEED.

CAMBER - The convex or concave curvature of an airfoil.

CANARD - An arrangement in which the horizontal stabilizer and elevators of an aircraft are mounted in front of the main wing(s).

CAT - Clear-Air Turbulence.


CAVU - Ceiling and Visibility Unlimited; ideal flying weather.

CEILING - (1) The heights above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as "broken," "overcast," or "obscuration," and not classified as "thin" or "partial". (2) The maximum height above sea level in STANDARD AIR attainable by an aircraft under given conditions—see ABSOLUTE CEILING, SERVICE CEILING.

CENTER - An Air Route Traffic Control Center (ARTCC).

CENTER OF GRAVITY (c/g) - The longitudinal and lateral point in an aircraft where it is stable; the static balance point.

CERTIFICATED AIRPORT - An airport operating under FAR Part 139. The FAA issues airport operating certificates to all airports serving scheduled or unscheduled air carrier aircraft designed for more than 30 passenger seats. Certificated airports must meet minimum safety standards in accordance with FAR Part 139.

CHORD - The measurable distance between the leading and trailing edges of a wingform.

CLASS G AIRSPACE (Uncontrolled Airspace) - Airspace not designated as Class A, B, C, D or E CONTROLLED AIRSPACE.

COAMING - A padded, protective rim around an open cockpit.

COLLECTIVE PITCH - A cockpit control that changes the PITCH of a helicopter's rotor blades; used in climbing or descending.

COLLECTOR RING - A circular duct on a radial engine into which exhaust gases from its cylinders are safely discharged.

COMMON TRAFFIC ADVISORY FREQUENCY (CTAF) - A frequency designed for the purpose of carrying out airport advisory practices while operating to or from an airport without an operating control tower. The CTAF may be a UNICOM, Multicom, FSS, or tower frequency and is identified in appropriate aeronautical publications.

COMMUTER - An air carrier operator operating under 14 CFR 135 that carries passengers on at least five round trips per week on at least one route between two or more points according to its published flight schedules that specify the times, day of the week, and places between which these flights are performed. The aircraft that a commuter operates has 30 or fewer passenger seats and a payload capability of 7,500# or less.

COMPASS COURSE - A bearing as indicated by the horizontal angle between the compass needle and the centerline of the aircraft. A Compass Course is equal to a True Course ± variation and deviation; also equal to a Magnetic Course ± deviation.

COMPASS NORTH - The North point at which a liquid compass needle points, rather than Geographical, or True, North. Compare MAGNETIC NORTH.

CONE OF SILENCE - Refer to the Adcock page.

CONSTANT-SPEED PROPELLER - A hydraulically-controlled propeller that governs an engine at its optimum speed by the blade pitch being increased or decreased automatically.

CONTACT FLIGHT - Navigation in which altitude and flight path can be maintained by visual reference to the ground and its landmarks. Similar to VFR.

CONTOUR FLIGHT - Contact Flight in and around mountainous areas following visual reference to the terrain's countours.

CONTRAIL - Trailing streak of condensed water vapor created in the air by heat from an aircraft engine at high altitudes; aka VAPOR TRAIL.

CONTROLLED AIRSPACE - An airspace of defined dimensions within which air traffic control service is provided to IFR flights and to VFR flights in accordance with the airspace classification. Controlled airspace is a generic term that covers Class A, B, C, D, and E airspace. Controlled airspace is also that airspace within which all aircraft operators are subject to certain pilot qualifications, operating rules, and equipment requirements in FAR Part 91. For IFR operations in any class of controlled airspace, a pilot must file an IFR flight plan and receive an appropriate ATC clearance. Each B, C, and D airspace area designated for an airport contains at least one primary airport around which the airspace is designated (for specific designations and descriptions of the airspace classes, refer to FAR Part 71).

Class A (formerly PCA - Positive Control Area) generally, that airspace from 18,000' mean sea level (MSL) up to and including flight level (FL) 600 (60,000' pressure altitude), including the airspace overlying the waters within 12 nautical miles of the coast of the 48 contiguous States and Alaska. Unless otherwise authorized, all persons must operate their aircraft under IFR.

Class B (formerly TCA - Terminal Control Area) Generally, that airspace from the surface to 10,000' MSL surrounding the nation's busiest airports in terms of airport operations or passenger enplanements. The configuration of each Class B airspace area is individually tailored and consists of a surface area and two or more layers (some Class B airspace areas resemble upside-down wedding cakes), and is designed to contain all published instrument procedures once an aircraft enters the airspace. An ATC clearance is required for all aircraft to operate in the area, and all aircraft that are so cleared receive separation services within the airspace. The cloud clearance requirement for VFR operations is "clear of clouds."

Class C (formerly ARSA - Airport Radar Service Area) Generally, that airspace from the surface to 4,000' MSL above the airport elevation surrounding those airports that have an operational control tower, are serviced by a radar approach control, and that have a certain number of IFR operations or passenger enplanements. Although the configuration of each Class C area is individually tailored, the airspace usually consists of a surface area with a 5 nautical mile (nm) radius, an outer circle with a 10 nm radius that extends from 1,200' to 4,000' above the airport elevation and an outer area. Each person must establish two-way radio communications with the ATC facility providing air traffic services prior to entering the airspace and thereafter maintain those communications while within the airspace. VFR aircraft are only separated from IFR aircraft within the airspace.

Class D (formerly ATA - Airport Traffic Area and CZ - Control Zone) Generally, that airspace from the surface to 2,500' MSL above the airport elevation surrounding those airports that have an operational control tower. The configuration of each Class D airspace area is individually tailored and when instrument procedures are published, the airspace will normally be designed to contain the procedures. Arrival extensions for instrument approach procedures may be Class D or E airspace. Unless otherwise authorized, each person must establish two-way radio communications with the ATC facility providing air traffic services prior to entering the airspace and thereafter maintain those communications while in the airspace. No separation services are provided to VFR aircraft.

Class E (formerly General Controlled Airspace) Generally, if the airspace is not Class A, B, C, or D, and is controlled airspace, it is Class E airspace. Class E airspace extends upward from either the surface or a designated altitude to the overlying or adjacent controlled airspace. When designated as a surface area, the airspace will be configured to contain all instrument procedures. Also in this class are Federal airways, airspace beginning at either 700' or 1,200' AGL used to transition to/from the terminal or enroute environment, enroute domestic, and offshore airspace areas designated below 18,000' MSL. Unless designated at a lower altitude, Class E airspace begins at 14,500' MSL over the USA, including that airspace overlying the waters within 12 nautical miles of the coast of the 48 contiguous States and Alaska, up to, but not including 18,000' MSL, and the airspace above Flight Level 600.

CONVENTIONAL GEAR - Having two main landing wheels at the front and a tailwheel or tailskid at the rear as opposed to having a tricycle gear with three main wheels. Such an aircraft is popularly called a TAILDRAGGER.

COWL, COWLING - A circular, removable FAIRING around an aircraft engine (generally radial) for the purposes of streamling or cooling; aka RING COWLING.

COWL FLAP - A controllable louvre to regulating airflow through an engine's cowling.

CRAB - A rudder-controlled yawing motion to compensate for a crosswind in maintaining a desired flight path, as in a landing approach.


DEAD RECKONING - In pioneer flight before radio, beacons, and accurate maps, flying distances much by instinct and guesswork, and referring to whatever landmarks were below, was quite routine. The "dead" part simply meant "straight," as in the nautical "dead ahead," and pilots often relied heavily on the IRON COMPASS for cross-country flights over unfamiliar territory.

DEADSTICK - Descending flight with engine and propeller stopped.

DECALAGE - The difference in angular settings [Angles of Attack] of the wings of a biplane or multiplane.

DECISION HEIGHT - With respect to the operation of aircraft, the height at which point a decision must be made during an instrument approach to either continue the approach or to execute a missed approach [abort].

DELTA-WING - A triangularly-shaped aircraft wing having a low aspect ratio, a sharply-tapered leading edge, a straight trailing edge, and a pointed tip.


DEPARTURE STALL - A stall in the takeoff configuration with power.

DEVIATION (MAGNETIC) - The error of a Magnetic Compass due to inherent magnetic influences in the structure and equipment of an aircraft.


DIHEDRAL - The acute angle, usually upward, between the wing of an airplane and a horizontal cross-section line. Opposite of ANHEDRAL.

DIRECTIONAL GYRO - A panel instrument providing a gyroscopic reading of an aircraft's compass heading.

DIRIGIBLE - A lighter-than-air craft capable of being propelled and steered for controlled flight.
Latin: dirigo,to steer.

DME - Distance Measuring Equipment, a radio navigation device that determines an aircraft's distance from a given ground station, as well as its groundspeed and time to/from the station.

DOPE - Preservative and pigmented coloring for fabric aircraft covering and paints, generally nitrate lacquer but generically used to denote all early shellac and coal-tar mixtures on up to present-day acrylics.

DORSAL FIN - A lateral fin/rudder extension on the top of a fuselage. Opposite of VENTRAL FIN.

DOWNWASH - The air deflected perpendicular to the direction of movement of an airfoil.

DRAG - The resisting force exerted on an aircraft in its line of flight opposite in direction to its motion. Compare THRUST.

DRAG WIRE - A wire designed to resist DRAG forces, usually running from a forward inboard point to an outboard aft point.

DRIFT - The angle between the heading of an aircraft and its Track [flight path] over the ground as affected by winds.

DRY WEIGHT - The weight of an engine exclusive of any fuel, oil, and coolant.

DURAL - Originally a tradename for a wrought aluminum-copper alloy created by Bausch Machine Tool Co, now fallen into generic use as any aluminum alloy containing 3.0-4.5% copper, 0.4-1.0% magnesium, and 0.1-0.7% manganese. Alcoa's version is commonly referred to as "Duraluminum," popularly used in aircaft manufacture.

DZUS FASTENER - ("Zoose") Very singular to aircraft, a patented slotted screw that binds to a wire for fast release and securing of cowlings and inspection plates, requiring no special tool other than a quarter coin.

EARTH-INDUCTOR COMPASS - One whose indications depend on the current generated in a coil revolving in the earth's magnetic field.

ELEPHANT EAR - (1) An air intake characterized by twin inlets, one on each side of the fuselage. (2) A type of balanced aileron in which the outer edges are noticeably larger than the control itself. See BALANCED CONTROL SURFACE and example Travel Air 4000.

ELEVATOR - The movable part of a horizontal airfoil which controls the pitch of an aircraft, the fixed part being the STABILIZER.

ELEVON - A hinged device on the rear portion of an aircraft wing combining the functions of an elevator and an aileron. Usually found on delta-wing aircraft, it can be moved in the same direction on either side of the aircraft to obtain longitudinal control, or differentially to obtain lateral control. Also see FLAPERON.

ELT - Emergency Locator Transmitter

EMERGENCY - (a) Life-Threatening: A situation or occurrence of a serious nature, developing suddenly and unexpectedly and demanding immediate action to prevent loss of life. (b) Operational: An unforeseen combination of circumstances that calls for immediate action, but not life-threatening.

EMPENNAGE - An aircraft's tail group, includes rudder and fin, and stabilizer and elevator.
Old French: empenner, to feather an arrow, from Latin penna, feather.

EN ROUTE AIR TRAFFIC CONTROL SERVICES - Air traffic control services provided aircraft on IFR flight plans, generally by centers, when these aircraft are operating between departure and destination terminal areas. When equipment, capabilities, and controller workload permit, certain advisory/assistance services may be provided to VFR aircraft.

ETA - Estimated Time of Arrival.

ETD - Estimated Time of Departure.

FAIRING - An added streamlining structure or auxiliary member, most often of light metal, whose only purpose is to reduce drag. Fairings are not load-bearing and, therefore, are not meant to carry any principal air loads placed on the airplane structure.

FBO - Fixed-Base Operator. A commercial operator supplying fuel, maintenance, flight training, and other services at an airport.


FEATHERING - In the event of engine failure, the process of adjusting a controllable-pitch propeller to a pitch position where the blade angle is about 90° to the plane of rotation in order to to stop its windmilling and lessen drag.

FEDERAL AVIATION ADMINISTRATION (FAA) - The U.S. government agency responsible for air safety and operation of the air traffic control system. The FAA also administers a program that provides grants from the Airport and Airway Trust Fund for airport development.


FAR Part 91  -  General Aviation (portions apply to all operators)
FAR Part 103 - Ultralight Vehicles
FAR Part 105 - Parachute Jumping
FAR Part 108 - Airplane Operator Security
FAR Part 119 - Certification: Air Carriers and Commercial Operators
FAR Part 121 - Domestic, Flag, and Supplemental Air Carriers and Commercial Operators of Large Aircraft
FAR Part 123 - Travel Clubs
FAR Part 125 - US Civil Airplanes, seating 20 or more passengers or a maximum payload capacity 6,000# or more
FAR Part 127 - Air Carriers using helicopters for scheduled interstate flights (within the 48 contiguous states)
FAR Part 129 - Foreign Air Carrier and Foreign Operators of US registered aircraft engaged in common carriage
FAR Part 133 - Rotorcraft External Load Operations
FAR Part 135 - Air Taxi Operators and Commercial Operators
FAR Part 137 - Agricultural Aircraft Operations
FAR Part 141 - Pilot School

FERRY FLIGHT - A flight for the purpose of (1) returning an aircraft to base; (2) delivering an aircraft from one location to another; (3) moving an aircraft to and from a maintenance base. Ferry flights, under certain conditions, may be conducted under terms in a special flight permit. They do not carry revenue passengers.

FIN - The fixed part of a vertical airfoil that controls the yaw of an aircraft; the movable part being the RUDDER. Sometime referred to as Vertical Stabilizer.

FIREWALL - A fire-resistant bulkhead that isolates the engine from other parts of an airplane's structure.

FISHTAILING - A rudder-controlled side-to-side [yawing] motion to reduce air speed, generally prior to landing.

FIVE-BY-FIVE (5x5) - In radio jargon, affirms that a radioed transmission was received as clear and loud. Rated one to five, with the first figure for clearness and the second second for loudness.

FLAP - A movable, usually hinged AIRFOIL set in the trailing edge of an aircraft wing, designed to increase LIFT or DRAG by changing the CAMBER of the wing or used to slow an aircraft during landing by increasing lift. Also see FOWLER FLAP, SLOTTED FLAP, and SPLIT FLAP.

FLAPERON - A control surface combining the functions of a FLAP and an AILERON.

FLARE - A simple maneuver performed moments before landing in which the nose of an aircraft is pitched up to minimize the touchdown rate of speed.

FLIGHT CREWMEMBER - A pilot, flight engineer, or flight navigator assigned to duty in an aircraft during flight time who holds a valid Federal Aviation Administration (FAA) Airman's Certificate and flight physical.

FLIGHT ENVELOPE - An aircraft's performance limits, specifically the curves of speed plotted against other variables to indicate the limits of speed, altitude, and acceleration that a particular aircraft cannot safely exceed.

FLIGHT LEVEL (FL) - A level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each is stated in three digits that represent hundreds of feet—flight level 250 represents a barometric altimeter indication of 25,000', flight level 255 as 25,500'.

FLIGHT PLAN - Specified information relating to the intended flight of an aircraft, filed orally or in writing with an FSS or an ATC facility.

FLIGHT SERVICE STATION (FSS) - Air traffic facilities which provide pilot briefing, enroute communications and VFR search and rescue services, assist lost aircraft and aircraft in emergency situations, relay ATC clearances, originate Notices to Airmen, broadcast aviation weather and NAS information, receive and process IFR flight plans, and monitor NAVAIDs. In addition, at selected locations, FSSs provide Enroute Flight Advisory Service (Flight Watch), take weather observations, issue airport advisories, and advise Customs and Immigration of transborder flights.

FLOATPLANE - A water-based aircraft with one or more mounted pontoons, as differentiated from a hulled SEAPLANE [Flying Boat], but often used generically.

FLYING WIRES - Interplane bracing wires that help support wingloads when the plane is in flight. Direction of travel is upward and outward from the fuselage to the interplane struts. Also known as LIFT WIRES, the opposite of LANDING WIRES.

FORCED LANDING - A landing necessitated by failure of engines, systems, components, or incapacitation of a crewmember, which makes continued flight impossible, and which may or may not result in damage.

FOWLER FLAP - Trademark name of a split-flap attached to a wing's trailing edge with a system of tracks and rollers to roll the FLAP backward and downward, increasing the wing area. Named for its inventor, USAAF engineer Harland D Fowler.

FRISE AILERON - A type of aileron that has a beveled leading edge projecting beyond its inset hinges. When lowered, it forms an extension of the wing surface; when raised, its nose protrudes below the wing, increasing DRAG and reducing YAW. Named for its inventor, British engineer Leslie George Frise.


FUSELAGE - An aircraft's main body structure housing the flight crew, passengers, and cargo and to which the wings, tail and, in most single-engined airplanes, engine are attached.
French: fuselé, tapering.

g or G  see  LOAD FACTOR

GAP - The distance between two adjacent wings of a biplane or multiplane.

GCA - Ground-Controlled Approach; part of ILS.

GENERAL AVIATION - That portion of civil aviation which encompasses all facets of aviation except air carriers holding a certificate of public convenience and necessity from the Civil Aeronautics Board and large aircraft commercial operators.

GEOGRAPHIC NORTH - The northern axis around which the Earth revolves; aka "Map North" and TRUE NORTH. Also see MAGNETIC NORTH.

GLASS COCKPIT - Said of an aircraft's control cabin which has all-electronic, digital and computer-based, instrumentation.

GLIDER - An unpowered aircraft capable of maintaining altitude only briefly after release from tow, then gliding to earth. Compare SAILPLANE.

GLIDE SLOPE - (1) The angle between horizontal and the glide path of an aircraft. (2) A tightly-focused radio beam transmitted from the approach end of a runway indicating the minimum approach angle that will clear all obstacles; one component of an instrument landing system (ILS).

GPS - Global Positioning System; satellite-based navigation.

GREEN LIGHT - Approval for landing. A carryover expression from days when aircraft for the most part had no radios, and communication from a control tower was by means of a light-gun that beamed various green, red, and yellow signals to pilots in the air and on the ground. A popular phrase, "3 green lights," refers to the instrument panel lights verifying that all three gears (in a tri-gear aircraft) are down and locked for safe landing.

GROSS WEIGHT - The total weight of an aircraft when fully loaded, including fuel, cargo, and passengers; aka Takeoff Weight.

GROUND CONTROL - Tower control, by radioed instructions from air traffic control, of aircraft ground movements at an airport.


GROUND EFFECT - Increased lift generated by the interaction between a lift system and the ground when an aircraft is within a wingspan distance above the ground. It affects a low-winged aircraft more than a mid- or high-winged aircraft because its wings are closer to the ground; aka GROUND CUSHION.

GROUND MISHAP - An aircraft mishap in which there is no intent to fly; however, the power plants and/or rotors are in operation and damage incurred requiring replacement or repair of rotors, propellers, wheels, tires, wing tips, flaps, etc., or an injury is incurred requiring first aid or medical attention.

GROUNDSPEED - The actual speed that an aircraft travels over the ground—its "shadow speed"; it combines the aircraft's AIRSPEED and the wind's speed relative to the aircraft's direction of flight.

GULL-WING - Descriptive of wing in frontal view bent as the wing of a seagull; a distinctive shallow, inverted "V" shape—see Stinson SR-10 or inverted gull-wing Vought F4U.

GYROPLANE - A rotorcraft whose rotors are not engine-driven, except for initial starting, but are made to rotate by action of the air when the rotorcraft is moving and whose means of propulsion, usually a conventional propeller, is independent of the rotor system. Similar to AUTOGYRO.

HANGAR - An enclosed structure for housing aircraft. Originated with lake-based floating homes of the original German Zeppelins in which they were "hung" from cables, which explains the erroneous, oft-seen spelling of "hanger."
French: hangar, shed, outbuilding, from Latin: angarium, shed.

HAZARD - Any condition, act or set of circumstances that exposes an individual to unnecessary risk or harm during aviation operations.

HEADING - The direction in which the longitudinal axis of the airplane points with respect to true or magnetic north. Heading is equal to course plus or minus any wind correction angle.

HELICOPTER - A wingless aircraft acquiring its lift from revolving blades driven by an engine about a near-vertical axis. A ROTORCRAFT acquiring its primary motion from engine-driven rotors that accelerate the air downward, providing a reactive lift force, or accelerate the air at an angle to the vertical, providing lift and thrust.

HIGH BLOWER - A blower-type supercharger set at high rpm.

HIGH PERFORMANCE AIRPLANE - An airplane that has more than 200 horsepower or that has a retractable landing gear, flaps, and controllable propeller.

HIGH-SPEED STALL - Any stall made to occur at more than 1g, such as pulling out of a dive or while turning.

HORIZONTAL STABILIZER - The airfoil or small wing at the rear of the aircraft that balances lift forces generated by the wings and increases stability. In some airplanes, the stabilizer is fixed and the elevator attached to its trailing edge. In other airplanes, the entire stabilizer can move, acting as the elevator, which controls the up or down movement of the airplane's nose. A trim system minimizes the force needed to hold the elevator in the proper position for climb, cruise, descent, and other flight conditions.

HORSEPOWER - The motive energy required to raise 550# one foot in one second, friction disregarded.

HOVER LANDINGS - Landings which do not meet the definition of toe-in, single-skid, or step-out landings – characterized by the necessity to maintain a substantial amount of hover power while the landing gear is in contact with the surface. This is normally due to the nature of the surfaces such as swampy ground, tundra/muskeg, snow, lava rock, etc. During these landings, the potential CG shifts are not as hazardous as in the previously mentioned landings (i.e., toe-in, one-skid); however, the pilot remains alert and on the controls as opposed to a flat surface/flat pitch landing stability.

HYPERSONIC - Speed of flight at or greater than Mach 5.0, exceeding SUPERSONIC.

HYPOXIA - Deprivation of oxygen, aka "altitude sickness," which can adversely affect human judgment and movement at altitudes above 12,000' with symptoms ranging from light dizziness to unconsciousness, even death at the extreme. It was a principal reason for pressurized aircraft.

IFR - Instrument Flight Rules, governing flight under instrument meteorological conditions.

IFR AIRCRAFT/IFR FLIGHT - An aircraft conducting flight in accordance with instrument flight rules.

ILS - Instrument Landing System. A radar-based system allowing ILS-equipped aircraft to find a runway and land when clouds may be as low as 200' (or lower for special circumstances).

INCIDENT - An occurrence other than an accident, associated with the operation of an aircraft which affects or could affect the safety of operations.

INCIDENT WITH POTENTIAL - An incident that narrowly misses being an accident and in which the circumstances indicate significant potential for substantial damage or serious injury. Final classification will be determined by the USDA-FS National Aviation Safety and Training Manager or the AMD Aviation Safety Manager, as appropriate.

INDICATED AIRSPEED (IAS) - A direct instrument reading obtained from an air speed indicator uncorrected for altitude, temperature, atmospheric density, or instrument error. Compare CALIBRATED AIRSPEED and TRUE AIRSPEED.

INSTRUMENT METEOROLOGICAL CONDITIONS (IMC) - Meteorological conditions expressed in terms of visibility, distance from clouds, and ceiling less than minima specified for visual meteorological conditions (VMC).

IRON COMPASS - Railroad tracks, favored by pilots of yore as a dependable aid to surface navigation before radio.

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