This page is dedicated to increasing the comprehensibility of this site by providing definitions in "plain english" for some of the less common terms you may come across. If you notice a term that needs more explanation, please e-mail us and we will add that word to this list.

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ADC: Air Data Computer.

ADF: Automatic Direction Finder. Previously known as radio compass.

ADI: Attitude Direction Indicator. Artificial horizon.

AFCS: Automatic Flight Control System.

Aileron: Control surfaces on (usually) outer sections of wing trailing edges, controlling bank and roll of aircraft.

Airspeed Indicator (ASI): Instrument measuring speed of aircraft through air, usually expressed in knots.

Air Traffic Clearance: Approval by Air Traffic Control for aircraft to taxi, takeoff, climb, enter controlled airspace, descend or to land.

Air Traffic Control (ATC): System of directing all aircraft operating within designated airspace by radio. Divided into sectors such as Tower (aerodrome control for takeoffs and landings), Departures, Control (en route aircraft), and Approach.

Alt Sel: Altitude Selector.

Altitude: Height of aircraft as shown on altimeter adjusted to local barometric pressure.

Angle of attack (AoA): Angle at which wings meet airflow.

APU: Auxiliary Power Unit.

Artificial horizon (AH): Instrument displaying aircraft attitude in relation to real horizon.

Asymmetric flight: Multi engined aircraft flying with one engine inoperative.

ATIS: Automated terminal information service. Continuous, recorded radio transmission of meteorological conditions at airport.

Attitude: Lateral and longitudinal relationship of aircraft to horizon.

Attitude indicator: See artificial horizon and flight director.

Bunt: Sudden nose down maneuver of aircraft, usually producing uncomfortable negative G.

CAM: Cockpit Area Microphone.

CDU: Control/Display Unit.

CFDS: Centralised Fault Display System.

"Clean" (aircraft): Aircraft in normal cruising configuration, with high lift devices and undercarriage retracted.

CMD: Command.

Control Area: Designated area of airspace in which all aircraft movements are under radio direction of Air Traffic Control.

Control Zone: Designated airspace encompassing terminal area of an airport in which all aircraft movements are under radio direction of Tower Controller.

Co-ordinates: Latitude and longitude of a position anywhere on the earth's surface, estimated to one minute of an arc.

CRT: Cathode Ray Tube [Screen/Display].

CVR (Cockpit Voice Recorder): Sophisticated, "crash proof" tape recording equipment fitted to airline aircraft to record flight crew conversations and radio transmissions. The tape is a 30 minute closed loop which is continuously recycled, providing a complete audio record of the last 30 minutes of any flight.

Directional gyro (DG): Instrument accurately registering direction aircraft is heading. When aligned with compass, provides immediate indication of changes in magnetic heading.

DME: Distance Measuring Equipment. Radio navigation aid providing pilot with constant readout of distance from selected radio beacon.

ECAM: Electronic Centralised Aircraft Monitoring [System].

EFCS: Electronic Flight Control System.

EFIS: Electronic Flight Instrumentation System.

ELAC: Elevator/Aileron Computer.

Elevation: Height of terrain above mean sea level. Abbreviated MSL.

Elevators: Control surfaces at rear of horizontal tail (tailplane), controlling nose attitude of aircraft.

Endurance: Time (expressed in minutes) that an aircraft can theoretically remain in air before fuel is exhausted.

EPR: Engine Pressure Ratio. Measure of thrust developed in turbofan jet engines.

ETA: Estimated time of arrival.

ETD: Estimated time of departure.

FADEC: Full Authority Digital Engine Control.

FAA: Federal Aviation Administration.

FAC: Flight Augmentation Computer.

FADEC: Full Authority Digital Electronic Control.

FBW: Fly By Wire.

Fin: The vertical aerofoil member of an aeroplane's tail assembly or empennage. Provides directional stability in flight. Known as the vertical stabiliser in US aviation parlance (see also rudder).

FCDC: Flight Control Data Concentrator.

FCOM: Flight Crew Operating Manual.

FCU: Flight Control Unit.

FD: Flight Director.

FDR (Flight Data Recorder): Complex "crash proof" instrument fitted to airline aircraft to continuously record operating parameters during flight. Early FDRs using stylus scribing on metallic tape recorded only four parameters - airspeed, altitude, heading and vertical acceleration. Today's digital FDRs (DFDRs) simultaneously record some 70 aircraft performance parameters, including instrument readings, flight control movements, engine performance and secondary control settings.

Flaps: Adjustable surfaces on aircraft's wing trailing edge. When lowered, flaps increase lift of wing, thereby reducing stalling speed, and increase drag, steepening aircraft's glide angle.

Flight Director: Complex, computer controlled flying instrument combining inputs of other flying and radio navigation instruments in single large dial located directly in front of each pilot.

Flightplan: Document prepared by pilot on official form before departure, providing details of proposed flight - track to be followed, waypoints, computations of wind effects, headings and speeds for each leg, all-up weight at departure, and progressive fuel burn.

Flight Level (FL): Expression of height in hundreds of feet, based on standard barometric altimeter setting of 1013.2 millibars. Eg, 12,000 feet on standard altimeter setting would be FL120. Differs from altitude in that the latter is based on actual barometric altimeter setting for a particular area or airport.

FMC: Flight Management Computer.

FMGC: Flight Management/Guidance Computer.

FMS: Flight Management System.

FO: First Officer.

"G" (gravities): Expression of force acting on aircraft and its occupants in flight, measured in multiples of earth's gravitational force.

Glideslope: The vertical guidance portion of an Instrument Landing System. Usually at an angle of 3°,  the glideslope produces a straight line of descent to the runway touchdown zone.

GMT (Greenwich Mean Time): Standard world time used for navigation regardless of location of ship or aircraft. Now generally referred to as UTC (Co-ordinated Universal Time).

GPWS: Ground Proximity Warning System.

Ground speed: Actual speed of aircraft over ground. May be greater or less than airspeed, according to wind.

HDG: Heading.

HDG/SEL: Heading Selector.

Heavy: Added to the end of the callsign of any aircraft with a max takeoff weight of over 255,000 pounds in the United States. Aircraft currently in service in the United States which are considered heavy are: Boeing 707,747,767,767. Airbus A300, A310, A330, A340. Lockheed L-1011. McDonnell Douglas DC-8, DC-10, MD-11.

HF (High (radio) Frequency): Radio propagation in the frequency band from 3 to 30 MHz. Permits communication over long distances, but reception can be subject to atmospheric and electrical interference. Used by aircraft operating beyond range of VHF and UHF radio propagation.

HSI (Horizontal Situation Indicator): Instrument on the pilot's flight instrument panel capable of displaying position information in ILS, VOR or NAV modes. When selected in NAV mode, displays distance to the next waypoint in top left hand corner, ground speed top right hand corner, and plan view of aircraft's position left or right of track, thus providing instant information on aircraft's position and speed.

ILS (Instrument Landing System): Electronic approach aid which enables a pilot to carry out an approach for landing when weather conditions preclude visual contact with the ground.

IFR (Instrument Flight Rules): Stipulated procedures for navigating aircraft by reference to cockpit instruments and radio navigation aids alone. Enables flight regardless of visibility. Normal operating procedure for airline flights.

IMC (Instrument Meteorological Conditions): Weather conditions in which visibility is less than specified for visual flying, and in which flight is legally possible only under IFR.

Knot: One nautical mile per hour. Equivalent to 1.853km/h.

Lifeguard: A term attached to an airliner's radio callsign when the aircraft is transporting time sensitive medical materials/supplies, such as blood plasma, organs for transplant, etc. (Example: TWA Flight 800 was known as "Lifeguard TWA800")

Localizer: The horizontal guidance portion of an Instrument Landing System. Indicates to the pilot  whether he is to the left of, to the right of, or on course.

LOG: Localised VHF radio beacon providing accurate horizontal guidance.

Lowest Safe Altitude (LSA): Designated minimum altitude for particular air route, providing minimum of 1,000 feet clearance above underlying terrain.

MAC: Mean Aerodynamic Chord.

MSA (Minimum Safe Altitude): Altitude below which IFR aircraft may not descend unless specifically authorised to do so by ATC. Takes into account high terrain underlying an air route.

Mach number: Figure expressing relationship between true airspeed of aircraft and speed of sound.

Mayday (repeated three times): Radio telephony version of former morse code "SOS" distress call. Derived from the French "m'aidez" - "help me".

N1: RPM of Stage 1 fan of turbofan jet engine, expressed as a percentage of normal maximum fan speed.

N2: RPM of gas turbines of turbofan jet engine, expressed as a percentage of normal maximum turbine speed.

Nautical mile (nm): Measure of distance used for navigation in the air and at sea. Equal to one minute of an arc of latitude on the earth's surface. Is 800 feet longer than a statute mile and equivalent to 1.853km.

Nav: Navigation.

Navaid: Radio navigation aid.

ND: Navigation Display.

NDB: Non directional beacon. Ground based medium frequency radio transmitter sending continuous signals in all directions for use by aircraft fitted with ADF (radio compass).

NOTAM (Notice to Airmen): Message concerning changes to serviceability of aerodromes, radio and navigation facilities.

NTSB: National Transportation Safety Board.

Octas ("eighths"): Expression of cloud amount. Eight octas (or eighths) represents a completely overcast sky; four octas a half clouded sky.

PFD: Primary Flight Display.

Pitot-static system: System of instruments, connecting tubes and air sensors for measuring altitude, airspeed, and rate of climb or descent.

Precipitation: (Meteorological) Rain, hail, sleet or snow in or falling from cloud.

Preflight (inspection): "Walk around" inspection of aircraft by pilot, usually immediately prior to flight.

QFE: Code expression designating altimeter setting in millibars for particular airport. When set on subscale of altimeter, instrument reads aircraft's height above that airport.

QNH: Code expression designating altimeter setting in millibars - when set on subscale of aircraft's altimeter, instrument reads aircraft's height above mean sea level.

Radial: Bearing to or from VOR radio range.

Radio Compass: See ADF.

Radio Range: Type of radio beacon providing defined aircraft tracks to or from that navigation aid.

Rate One turn: Shallow standard rate turn used in instrument flight conditions.

RMI: Radio Magnetic Indicator.

RMP: Radio Management Panel.

RPM (rpm): Measure of engine speed expressed in revolutions per minute.

Rudder(s): Control surface(s) at rear of vertical tail (fin) controlling yawing movement of aircraft.

SAR: Search and Rescue.

SEC: Spoiler/Elevator Computer.

SFCC: Slat/Flap Control Computer.

Sigmet: Warning signal issued by Aviation Meteorological Service when weather conditions suddenly deteriorate.

Slats: Aerodynamic device fitted to leading edge of wings to delay onset of stall.

Spot height: Height noted on chart showing elevation of prominent mountain peak.

SSC: Side Stick Controller. Replaces a conventional flight yoke on Airbus aircraft.

Stalling speed: Low airspeed at which aircraft wings suddenly lose lift. No connection with engine "stall". Is absolute minimum airspeed at which aircraft can maintain flight.

Stick-shaker: Stall warning device which shakes aircraft's control columns as stalling speed is approached.

Tailplane: Horizontal aerofoil member of an aeroplane's tail assembly or empennage. Provides longitudinal stability in flight. Known as the stabilizer in US aviation parlance (see also elevators).

Threshold: The point at which a runway begins.

Transponder: Radio device fitted to aircraft which, when triggered off by certain radar wavelengths, emits a signal visible on ground radar screens. Signal usually includes additional information such as altitude of the aircraft.

Trim: Adjusting control of aircraft in climb, level flight and descent, so pilot is not required to maintain continuous pressure on elevators, ailerons or rudder.

T-VASIS: T Visual Approach Slope Indicator System.

UHF (Ultra High (radio) Frequency): Frequency band of 300 to 3000 MHZ. Aviation use confined mainly to military aircraft.

V (code): Schedule of indicated airspeeds stipulated for different phases of flight (see following).

V-1: Decision speed during takeoff. Aircraft is committed to fly when this speed is passed.

Vr: Rotation speed. Speed at which aircraft is "rotated" into liftoff attitude by raising the nosewheel off the runway.

V-2: Takeoff safety speed. Minimum control speed plus safety margin to allow for engine failure and other contingencies.

Vne: Never exceed speed.

Vref: Flap reference speed. Landing speed for stipulated number of degrees of flap extension.

VASIS: Visual approach slope indicator. System of lights located on ground on either side of runway to indicate correct angle of descent to approaching aircraft.

VSI (Vertical Speed Indicator): Instrument displaying rate of climb or descent in feet per minute.

VHF (Very High (radio) Frequency): In general use for inflight radio communications on air routes. Its frequency band from 30 to 300 MHZ is largely free from interference and static, but range is limited to "line of sight".

VFR (Visual Flight Rules): Stipulated flight procedure for navigating aircraft visually, clear of cloud, in Visual Meteorological Conditions.

VMC (Visual Meteorological Conditions): Weather providing specified range of visibility, making it possible for pilots to use visual means to avoid obstructing terrain and other aircraft.

VOR: Very High Frequency Omnidirectional Radio Range.