REFERENCE

EASA ATPL Glossary

66 key aviation terms and definitions covering all 13 EASA ATPL subjects.

A

ATPLAir Law: Airline Transport Pilot Licence — the highest level of aircraft pilot licence issued by a civil aviation authority. Required to act as pilot-in-command of aircraft over 5,700 kg MTOM or multi-crew aircraft.
Angle of AttackPrinciples of Flight: The angle between the chord line of the aerofoil and the relative airflow. As angle of attack increases, lift increases up to the critical angle of attack (stall angle), beyond which lift decreases sharply.
ATISCommunications: Automatic Terminal Information Service — a continuous broadcast of recorded non-control information (runway in use, weather, NOTAMs) on a dedicated frequency to reduce controller workload at busy aerodromes.
ACASRadio Navigation: Airborne Collision Avoidance System — the ICAO term for the airborne safety net that warns of collision risk independently of ATC. ACAS II (implemented as TCAS II) is required for aeroplanes above 5,700 kg MTOM or carrying more than 19 passengers.
AirworthinessAir Law: The condition of an aircraft, including its components, that makes it fit and safe to fly in accordance with its approved type design. A valid Certificate of Airworthiness (CofA) and an Airworthiness Review Certificate (ARC) must be maintained for legal operation.

C

CASGeneral Navigation: Calibrated Air Speed — indicated airspeed corrected for instrument and position error. Used as the basis for performance calculations at lower altitudes.
Centre of Gravity (CG)Mass & Balance: The point through which the total weight of the aircraft acts. The CG must remain within the certified CG envelope throughout the flight for stable and controllable flight.
Clear Air TurbulenceMeteorology: CAT — turbulence occurring in cloud-free air, most often near the tropopause in association with jet streams and strong vertical wind shear. Because it is not visible and not detected by weather radar, it is a notable hazard at cruise levels.
Critical Mach NumberPrinciples of Flight: Mcrit — the free-stream Mach number at which airflow over some part of the aerofoil first reaches the local speed of sound (Mach 1). Beyond Mcrit, shock waves form, drag rises sharply (drag divergence), and high-speed handling problems can appear.
Coffin CornerPrinciples of Flight: The narrow band at high altitude where the low-speed stall and the high-speed (Mach buffet) boundaries converge, leaving very little margin between minimum and maximum permissible speed. Also called the aerodynamic ceiling.

D

DMERadio Navigation: Distance Measuring Equipment — an interrogator–transponder system that measures slant range distance from the aircraft to a ground station, accurate to ±0.5 NM or 3% of distance.
DriftGeneral Navigation: In navigation, the angular difference between the track made good and the heading steered, caused by wind. Also the lateral distance an aircraft is displaced from its intended track.
DeviationGeneral Navigation: The error in a magnetic compass caused by the aircraft's own magnetic fields. Unlike variation, deviation differs on each heading and is recorded on the compass correction card.
Density AltitudePerformance: Pressure altitude corrected for non-standard temperature. High density altitude (hot, high, humid conditions) reduces aircraft performance — engine power, lift, and propeller efficiency all decrease.
Dutch RollPrinciples of Flight: A coupled lateral-directional oscillation in which the aircraft rolls and yaws out of phase, common on swept-wing aircraft with strong dihedral effect. It is damped artificially by a yaw damper.
Decision HeightOperational Procedures: DH — the wheel height above the runway threshold elevation on a precision approach (or approach with vertical guidance) at which a go-around must be initiated if the required visual reference is not established. Referenced to a radio altimeter or the approach baro source.

E

ECQBAir Law: European Centralised Question Bank — the single EASA database of theoretical knowledge examination questions used across all EU member states for ATPL, CPL, and IR certification.
ETOPSOperational Procedures: Extended-range Twin-engine Operational Performance Standards — regulations governing twin-engine aircraft operations over routes where diversion to an alternate aerodrome takes longer than 60 minutes at single-engine speed.

G

GNSSRadio Navigation: Global Navigation Satellite System — the generic term for satellite-based navigation systems including GPS (USA), GLONASS (Russia), Galileo (EU), and BeiDou (China).

H

HypoxiaHuman Performance: A state of oxygen deficiency in the body tissues sufficient to cause physiological impairment. Altitude hypoxia is the most common form in aviation, occurring when cabin altitude exceeds ~10,000 ft.
Holding PatternOperational Procedures: A racetrack-shaped flight path used to delay an aircraft already in flight while keeping it within a protected area. A standard hold uses right-hand turns, 1-minute legs at or below 14,000 ft (1.5 minutes above), and is entered via a direct, parallel, or teardrop entry depending on the inbound heading.

I

ISAMeteorology: International Standard Atmosphere — a model of how atmospheric properties (temperature, pressure, density) vary with altitude. ISA assumes sea-level temperature of 15 °C and pressure of 1013.25 hPa.
IMCOperational Procedures: Instrument Meteorological Conditions — meteorological conditions below the minima required for VMC, requiring the pilot to navigate by instruments under IFR.
IFROperational Procedures: Instrument Flight Rules — a set of regulations under which a pilot operates an aircraft in weather conditions generally not suitable for visual reference to the ground.
ILSRadio Navigation: Instrument Landing System — a precision approach system providing lateral guidance (localiser) and vertical guidance (glideslope) to runway threshold. Categorised I, II, and III by minimum visibility requirements.

M

MTOMPerformance: Maximum Take-Off Mass — the maximum certificated mass at which an aircraft may take off, as specified in the aircraft flight manual.
Mach NumberPrinciples of Flight: The ratio of the aircraft's TAS to the local speed of sound. At Mach 1 the aircraft is travelling at the speed of sound; at Mach 0.85 it is at 85% of the speed of sound at that altitude.
METARMeteorology: Meteorological Aerodrome Report — a routine weather observation issued hourly (or half-hourly at busy aerodromes) covering wind, visibility, weather phenomena, clouds, temperature, dew point, and altimeter setting.
MELOperational Procedures: Minimum Equipment List — an operator-specific document, approved by the authority, that lists the items of equipment that may be temporarily inoperative while still allowing dispatch, together with any operational or maintenance conditions. It is derived from, and never less restrictive than, the MMEL.
MMELOperational Procedures: Master Minimum Equipment List — the baseline list produced by the aircraft manufacturer and approved by the certifying authority, from which each operator builds its own MEL. The MMEL applies to the aircraft type; the MEL applies to the individual operator.
Mach TuckPrinciples of Flight: A nose-down pitching tendency that develops as a swept-wing aircraft accelerates beyond its critical Mach number, caused by a rearward shift of the centre of pressure and reduced downwash at the tailplane. Mach trim systems counter it automatically.
MSAGeneral Navigation: Minimum Sector Altitude — the lowest altitude that provides at least 1,000 ft of obstacle clearance within a sector (typically 25 NM) around a navigation aid or aerodrome reference point, shown on approach charts for emergency use.
MDAOperational Procedures: Minimum Descent Altitude — the lowest altitude (referenced to mean sea level) to which descent is authorised on a non-precision approach without the required visual reference. Unlike a Decision Height, the aircraft must not descend below the MDA until visual.

N

NDBRadio Navigation: Non-Directional Beacon — a radio navigation aid transmitting an unmodulated carrier in the LF/MF band (190–1750 kHz). Used with an Automatic Direction Finder (ADF) to determine bearing to the station.
NOTAMAir Law: Notice to Airmen — a notice containing information essential to personnel concerned with flight operations that is not known far enough in advance to be published by other means.

P

Pressure AltitudeInstrumentation: The altitude indicated when the altimeter sub-scale is set to 1013.25 hPa. Used above the transition altitude and for performance calculations.
PBNRadio Navigation: Performance-Based Navigation — an ICAO framework that specifies aircraft RNAV/RNP performance requirements (such as RNAV 1 or RNP APCH) for a given airspace or procedure, defined in a navigation specification rather than by individual sensors.

Q

QNHInstrumentation: Altimeter sub-scale setting that causes the altimeter to indicate altitude above mean sea level when the aircraft is on the ground at a known elevation. Derived from the station pressure corrected to sea level.
QFEInstrumentation: Atmospheric pressure at aerodrome elevation (or at runway threshold). With QFE set, the altimeter reads height above the aerodrome reference point.

R

Ram Air TurbineAirframe & Systems: RAT — a small turbine deployed into the airflow to provide emergency hydraulic and/or electrical power when normal engine-driven or APU sources fail. It is a last-resort backup that powers essential flight controls and instruments.
RVSMOperational Procedures: Reduced Vertical Separation Minimum — the application of 1,000 ft vertical separation (instead of 2,000 ft) between FL290 and FL410, which roughly doubles the number of usable cruising levels. It requires approved, accurate altimetry and altitude-hold equipment.
RNAVRadio Navigation: Area Navigation — a method of navigation that allows aircraft to fly any desired flight path within the coverage of ground- or space-based navaids (or self-contained systems), rather than tracking directly to and from them. A core component of Performance-Based Navigation.

S

StallPrinciples of Flight: An aerodynamic condition in which the angle of attack exceeds the critical (stalling) angle, causing a sudden loss of lift due to airflow separation over the upper wing surface.
SIGMETMeteorology: Significant Meteorological Information — an advisory message concerning weather phenomena that may affect the safety of aircraft operations, such as severe turbulence, icing, tropical cyclones, or volcanic ash.
Spatial DisorientationHuman Performance: A state in which a pilot has an incorrect perception of the aircraft's position, attitude, or motion relative to a fixed reference (the surface of the Earth).
SELCALCommunications: Selective Calling System — an HF/VHF system that allows a ground station to alert the crew of a specific aircraft, reducing the need for continuous listening watch on HF frequencies during oceanic operations.
Squall LineMeteorology: A line of active thunderstorms, often forming ahead of or along a cold front, that can extend for hundreds of nautical miles. Squall lines produce severe turbulence, hail, icing, lightning, and windshear, and generally cannot be safely penetrated.
Swept WingPrinciples of Flight: A wing angled rearward (or, on variable-geometry types, adjustable in sweep) to delay the onset of compressibility effects and raise the critical Mach number, improving high-speed cruise efficiency at the cost of lower-speed handling characteristics.
Service CeilingPerformance: The pressure altitude at which the maximum achievable rate of climb falls to a defined small value (commonly 100 ft/min for jets, 500 ft/min may be used otherwise) in a given configuration, marking the practical maximum operating altitude.

T

TASGeneral Navigation: True Air Speed — the actual speed of the aircraft through the air mass. Derived from CAS by correcting for altitude and temperature effects on air density.
TAFMeteorology: Terminal Aerodrome Forecast — a concise statement of expected meteorological conditions at an aerodrome during a specified period (typically 9, 24, or 30 hours).
TropopauseMeteorology: The boundary between the troposphere and the stratosphere, typically at 36,000 ft (11 km) at mid-latitudes in ISA conditions. Characterised by a cessation of temperature decrease with altitude.
Time of Useful Consciousness (TUC)Human Performance: The time between exposure to an oxygen-deficient atmosphere and the point at which a person can no longer take appropriate corrective action. Decreases rapidly with altitude.
TCASRadio Navigation: Traffic Collision Avoidance System — an airborne system that interrogates the transponders of nearby aircraft and warns the crew of conflicting traffic. TCAS II issues Traffic Advisories (TA) and Resolution Advisories (RA, a vertical climb/descend command). It is an implementation of ACAS II and is mandatory for most large transport aircraft.
Transition AltitudeInstrumentation: The altitude at or below which the vertical position of an aircraft is referenced to QNH (altitude). Climbing through it, the altimeter is set to the standard 1013.25 hPa and the aircraft then flies flight levels above the transition level.

V

VMCOperational Procedures: Visual Meteorological Conditions — meteorological conditions (visibility, distance from cloud) equal to or better than the minima specified for VFR flight.
VFROperational Procedures: Visual Flight Rules — a set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going.
VORRadio Navigation: VHF Omnidirectional Range — a short-range radio navigation system operating in the 108–117.95 MHz band. Provides bearing information (radials) from the station; accurate to ±1° in good conditions.
VariationGeneral Navigation: The angular difference between True North and Magnetic North at a given location. Applied to convert between true and magnetic headings. "Variation West — magnetic best; variation East — magnetic least."
V1Performance: Take-off decision speed — the speed at or below which the pilot must initiate the first action to stop the aircraft within the accelerate-stop distance if an engine failure occurs.
VrPerformance: Rotation speed — the speed at which the pilot initiates the rotation to take-off attitude during the take-off run.
V2Performance: Take-off safety speed — the minimum speed at which the aircraft complies with handling criteria associated with the take-off climb after engine failure.
VNEPerformance: Velocity Never Exceed — the maximum speed, marked by the red line on the airspeed indicator, beyond which structural failure or loss of control may occur. It must never be exceeded in any operation.

W

Wake TurbulencePrinciples of Flight: Disturbed air generated behind an aircraft, dominated by two counter-rotating wingtip vortices. Vortex strength increases when the generating aircraft is heavy, clean, and slow. ICAO/EASA separation minima behind heavier categories exist to mitigate the hazard.
WindshearMeteorology: A sudden change in wind speed and/or direction over a short distance, horizontally or vertically. Low-level windshear on approach or departure — often from thunderstorm downbursts or frontal activity — is the most operationally significant form.

Z

Zero Fuel Weight (ZFW)Mass & Balance: The weight of the aircraft with all payload and crew but with no usable fuel. Structural limits apply to ZFW because wing bending relief from fuel is absent.