Aviation glossary

Airborne collision avoidance system. It monitors nearby transponder-equipped traffic and gives alerts when a conflict is predicted.

Automatic dependent surveillance–broadcast. An aircraft equipped with ADS-B Out broadcasts information such as its identity, position and velocity. The position typically comes from GNSS.

Above ground level. A height given in AGL is measured from the ground beneath the point in question.

A movable surface near the outer trailing edge of a wing. The two ailerons usually move in opposite directions to roll the aeroplane.

Aeronautical information regulation and control. It coordinates planned changes to aeronautical information so charts, databases and procedures can switch on common effective dates.

An instrument that shows indicated airspeed. A conventional ASI uses the difference between pressure from the pitot tube and static pressure.

An aerodrome available for use if a landing at the intended aerodrome becomes impossible or inadvisable.

An instrument that indicates altitude or height from static pressure and the pressure setting selected. With QNH set, it indicates altitude above mean sea level.

Mid-level cloud that appears as rounded patches or rolls, often with gaps or shaded areas.

A grey or blue-grey mid-level sheet of cloud. Through thinner parts, the sun may look as though it is shining through frosted glass.

Marks an amended TAF issued to update the previous forecast.

The angle between an aerofoil’s chord line and the relative airflow. It can change even when the aircraft’s pitch attitude stays the same.

The signed distance from a reference datum to the centre of gravity of an item being included in a loading calculation.

Accelerate-stop distance available. The take-off run available plus the length of any declared stopway.

Wingspan squared divided by wing area. A long, narrow wing has a higher aspect ratio than a short, broad wing of the same area.

Air traffic control. A service that manages air traffic to help prevent collisions and keep traffic moving in an orderly way.

Automatic terminal information service. It provides recurring aerodrome information such as weather and the runway in use.

Approved training organisation. A provider authorised by the competent authority to deliver specified pilot training courses.

Airline transport pilot licence. In the UK, its privileges include acting as pilot-in-command in commercial air transport, subject to the applicable requirements.

Also called an artificial horizon. It shows the aircraft’s pitch and bank relative to the horizon.

Marks a METAR produced by an automated observing system without human intervention.

A system that moves the flight controls to fly the aircraft in the selected modes, such as holding a heading or altitude.

Becoming. The forecast conditions change to new prevailing conditions during the stated period.

Broken cloud. The reported layer covers five to seven oktas. In BKN015, the cloud base is 1,500 feet above aerodrome level.

Mist. Water droplets in the air reduce visibility, but visibility remains at least 1,000 metres.

A common connection that distributes electrical power to several circuits.

The curvature of an aerofoil, described by the shape of its mean line relative to its chord line.

Calibrated airspeed. Indicated airspeed corrected for instrument and position errors.

Ceiling and visibility OK. It indicates visibility of at least 10 km, with no cloud of operational significance and no significant weather for aviation.

The height of the lowest cloud layer reported as broken or overcast, or the reported vertical visibility when the sky is obscured.

The point through which the aircraft’s weight can be treated as acting. Loading and fuel burn can move it.

An imaginary straight line from the leading edge to the trailing edge of an aerofoil section.

High cloud made up of small white grains or ripples, usually without shading.

A thin, high veil of ice-crystal cloud. It can form a halo around the sun or moon.

High cloud made of ice crystals, typically seen as thin, wispy strands.

A licence qualification to fly a class of aircraft, such as single-engine piston aeroplanes, within the privileges of the rating.

Turbulence outside convective clouds, often associated with strong wind shear near a jet stream.

A designated area beyond the take-off run over which an aeroplane can make part of its initial climb.

The difference between a magnetic compass reading and the magnetic heading, caused by magnetic influences in the aircraft.

Fuel planned to cover unforeseen factors that could increase consumption on the planned flight.

Airspace in which air traffic control service is provided according to the airspace class. The class determines which services and flight requirements apply.

Marks a corrected METAR that replaces a report containing an error.

Commercial pilot licence. It allows a pilot to exercise commercial flying privileges, subject to the aircraft category, class or type ratings held and the type of operation.

Crew resource management. Using the crew, aircraft systems and other available support effectively through skills such as communication, coordination and decision-making.

The part of the wind that blows across the runway or intended track. The crosswind component acts at right angles to that line.

Control area. Controlled airspace extending upwards from a specified level above the surface.

Control zone. Controlled airspace that starts at the surface and extends to a specified upper limit, usually around an aerodrome.

A deep convective cloud that can produce thunderstorms, heavy showers and hail. A mature cloud often spreads into an anvil at the top.

Detached clouds with flat bases and rounded tops. With enough vertical growth they can become towering cumulus.

The reference plane from which distances are measured in aircraft mass and balance calculations.

Estimating a position from a known position using direction, speed and elapsed time.

The specified altitude on a 3D instrument approach at which the missed approach begins if the crew has not established the required visual reference.

The height on a 3D instrument approach at which the missed approach begins if the crew has not established the required visual reference. It is expressed as a height rather than an altitude.

Pressure altitude corrected for non-standard temperature. A higher density altitude corresponds to less dense air, which generally reduces take-off and climb performance.

The temperature to which air must cool to become saturated, assuming its moisture content and pressure stay the same.

Distance measuring equipment. It measures the straight-line distance between the aircraft and a ground station. This is called slant range.

The aerodynamic force that acts parallel to the relative airflow, opposing the aircraft’s motion through the air.

The mass of an aircraft ready for its intended operation, excluding usable fuel and traffic load. Crew and their baggage are normally included.

Declared training organisation. A provider that declares its intended training to the competent authority and may deliver courses within the scope of the declaration.

Drizzle. It can be combined with FZ, as in FZDZ for freezing drizzle.

A movable surface on the tailplane that controls pitch, raising or lowering the nose.

The tail assembly of an aircraft. On a conventional aeroplane it includes the tailplane, fin and their control surfaces.

Turning a propeller’s blades towards alignment with the airflow to reduce drag when the engine is shut down or has lost power.

Few clouds. The reported layer covers one or two oktas, or eighths of the sky.

Fog. In a standard aerodrome report, unqualified FG indicates visibility below 1,000 metres due to suspended water droplets or ice crystals.

The vertical stabilising surface at the tail. It helps the aircraft maintain directional stability and usually carries the rudder.

Fuel planned to remain at landing. For commercial aeroplane operations under EU rules, it is calculated using a specified holding time and conditions.

Flight information region. A defined volume of airspace where flight information and alerting services are provided.

A movable wing surface, usually at the trailing edge. Extending flaps increases the maximum lift the wing can produce and generally increases drag.

Display cues that show the pilot how to steer towards a selected flight path or attitude while flying manually.

A pressure-based level referenced to the standard altimeter setting of 1013.25 hPa. The number expresses the indicated level in hundreds of feet.

Information submitted to air traffic services about an intended flight or part of a flight, such as the route, levels and destination.

From. A TAF group that introduces new prevailing conditions from the specified day and time, replacing the preceding forecast conditions.

Flight management system. An onboard system that helps the crew manage the route, navigation and performance of the flight.

The main body of an aircraft. In a conventional aeroplane it houses the flight deck and cabin or cargo spaces, and the wings and tail attach to it.

Freezing. A descriptor used in codes such as FZRA, freezing rain, or FZFG, freezing fog.

The horizontal distance covered in still air for each unit of height lost in a glide. At 10 to 1, the aircraft travels ten units forward for one unit down.

The vertical guidance of an ILS. It shows whether the aircraft is above or below the approach descent path.

Global navigation satellite system. A general term for satellite-based positioning, navigation and timing systems such as GPS and Galileo.

Discontinuing an approach or landing and climbing away. An instrument approach procedure includes a published missed approach route.

Hail in a METAR or TAF. The code GS is used for small hail or snow pellets.

A circle whose plane passes through the centre of the Earth in a spherical model. Its shorter arc gives the shortest surface route between two points on it.

A change in airflow around a wing close to the ground. It reduces induced drag for a given amount of lift.

The aircraft’s horizontal speed over the ground. A headwind reduces it for a given true airspeed when the wind is aligned with the track.

The code for small hail or snow pellets in a METAR or TAF.

The direction in which the aircraft’s nose points, measured from a reference such as true or magnetic north.

An instrument that displays the aircraft’s heading. A conventional unslaved gyro version needs periodic alignment with the magnetic compass.

Flying a prescribed pattern to remain within a defined area while awaiting further clearance. The pattern is referenced to a holding fix.

The geographical point used as the reference for a holding pattern.

A device that stores hydraulic energy under pressure. It can smooth pressure fluctuations or supply fluid briefly when demand rises.

Breathing more than the body needs, which lowers carbon dioxide levels and can cause symptoms such as dizziness or tingling.

Too little oxygen reaching the body’s tissues. In aviation, reduced oxygen pressure at altitude can impair judgement and performance.

Haze. Fine dry particles suspended in the air reduce visibility.

Indicated airspeed. The airspeed shown on the airspeed indicator before corrections for instrument and position error.

Instrument flight rules. The rules governing flight conducted under instrument procedures. An IFR flight can also take place in clear weather.

Instrument landing system. A radio navigation system that uses a localiser for lateral guidance and a glideslope for vertical guidance.

Instrument meteorological conditions. Weather conditions below the applicable minima for visual meteorological conditions.

Drag that accompanies the production of lift. For the same aircraft configuration and lift, it increases as airspeed decreases.

A published sequence flown by reference to instruments. It provides obstacle clearance on the way to landing and includes a missed approach.

Instrument rating. A rating that allows a pilot to fly under instrument flight rules within the scope of the rating.

The assembly that supports the aircraft on the ground during taxi, take-off and landing. It is also called the undercarriage.

Landing distance available. The runway length declared usable for the ground run of a landing aeroplane.

The component of aerodynamic force perpendicular to the relative airflow.

In a manoeuvre, the ratio of aerodynamic lift to aircraft weight. A level turn requires a load factor greater than one.

The part of an ILS that gives left or right guidance relative to the runway approach course.

The ratio of true airspeed to the local speed of sound. Mach 1 is the local speed of sound.

An instrument that uses the Earth’s magnetic field to indicate heading. Magnetic influences within the aircraft can cause compass deviation.

The angle between true north and magnetic north at a given place. It is expressed as east or west.

The radiotelephony distress signal. It indicates grave or imminent danger requiring immediate assistance.

Minimum descent altitude. On a two-dimensional instrument approach or a circling approach, the altitude below which descent requires the specified visual reference.

Minimum equipment list. An operator’s document listing equipment that may be temporarily inoperative at the start of a flight and the conditions that apply.

A routine coded aerodrome weather report. It describes observed conditions such as wind, visibility and cloud at the time of the report.

The part of an instrument approach procedure that specifies the path to fly if the approach cannot be completed to a landing.

The point on an instrument approach by which the published missed approach must be initiated if the approach cannot continue to landing.

In a mass and balance calculation, an item’s mass multiplied by its arm. Adding the moments and dividing by total mass gives the centre-of-gravity arm.

Mean sea level. An average sea-level reference used for elevations and altitudes.

No cloud detected. In an AUTO report, it means the observing equipment detected no cloud.

Navigation display. A screen that presents navigation information such as the aircraft’s position, route and waypoints.

A thick grey cloud layer associated with widespread, steady rain or snow.

No significant change expected during the two-hour trend forecast appended to a METAR.

A time-sensitive notice about an aeronautical change or hazard, such as a runway closure or a navigation aid outage.

Non-precision approach. An instrument approach procedure designed for lateral guidance only.

No significant cloud. It indicates no cloud of operational significance when CAVOK does not apply. In a METAR, it also means vertical visibility is unrestricted.

No significant weather. In a forecast change group, it indicates that previously forecast significant weather is expected to end.

One eighth of the sky. Cloud amount describes the fraction covered by the reported layer, from one okta to eight for complete coverage.

Overcast. The reported cloud layer covers all eight oktas.

The radiotelephony urgency signal. It concerns a safety problem that needs attention but does not require immediate assistance.

Drag from moving the aircraft through the air apart from the drag associated with producing lift. It includes skin friction, form drag and interference drag.

The flight crew licensing annex of the Aircrew Regulation. It sets out requirements for pilot licences, ratings and certificates, including skill tests. The EU and UK versions apply in their respective jurisdictions.

Performance-based navigation. It specifies the navigation performance and functions needed for a route, procedure or airspace.

Primary flight display. A screen that brings together information such as attitude, airspeed and altitude in the pilot’s main field of view.

A forward-facing probe that senses total air pressure. The airspeed system compares that pressure with static pressure.

Private pilot licence. A UK PPL for aeroplanes lets the holder act as pilot-in-command on non-commercial flights, within the privileges of the licence and any class or type ratings held.

The altitude in the standard atmosphere that corresponds to the measured air pressure. It is what an altimeter indicates when set to standard pressure.

A 30 per cent probability of the conditions in the associated forecast group. It can be combined with TEMPO.

A 40 per cent probability of the conditions in the associated forecast group. It can be combined with TEMPO.

An altimeter pressure setting referenced to an aerodrome or runway threshold. With QFE set, the altimeter reads approximately zero at that reference elevation.

An altimeter pressure setting that makes the instrument indicate the aerodrome elevation at its reference point. In flight it gives an indicated altitude above mean sea level.

Rain in a METAR or TAF. A minus sign means light rain, and a plus sign means heavy rain.

An instrument that uses radio signals reflected from the surface below to measure the aircraft’s height above that surface.

An instruction to repeat the specified part of a radio message as received so the sender can check it.

An ACAS alert that tells the crew which vertical manoeuvre to make or which vertical rates to avoid to reduce collision risk.

A line that crosses meridians at a constant angle. Following it means keeping a constant true track.

Area navigation. It lets aircraft follow a chosen path within the limits of the navigation systems available, including paths that do not pass directly over ground aids.

Required navigation performance. An area navigation specification that includes on-board performance monitoring and alerting.

A radio acknowledgement that the previous transmission was received. It conveys receipt only.

A movable surface on the fin that controls yaw, turning the nose left or right about the vertical axis.

Runway visual range. The distance along a runway over which a pilot on its centreline can see the runway markings or lights that define it.

Reduced vertical separation minima. In designated airspace, eligible aircraft can be separated vertically by 1,000 feet between flight levels 290 and 410 inclusive.

A radio request to repeat all or a specified part of the previous transmission.

Scattered cloud. The reported layer covers three or four oktas.

Showers. It precedes the precipitation type, as in SHRA for rain showers.

Standard instrument departure. A published IFR route that connects a departure aerodrome or runway to the en-route phase of flight.

An advisory about observed or forecast weather hazards along a route, such as severe turbulence or volcanic ash, that may affect aircraft safety.

A practical assessment in which a candidate demonstrates the skills required for the issue of a pilot licence, rating or certificate.

A movable section at the wing’s leading edge. When extended, it opens a slot that helps the airflow stay attached at a higher angle of attack.

Snow. For example, +SN indicates heavy snow.

A mistaken sense of the aircraft’s attitude, position or motion. The body’s balance cues can be misleading when outside visual references are poor.

A special coded aerodrome weather report issued when conditions change enough to meet specified reporting criteria.

A VFR flight cleared by air traffic control to operate in a control zone when weather is below VMC minima.

A panel that rises from the upper surface of a wing to reduce lift and increase drag. Spoilers can assist roll control or reduce lift after touchdown.

A reduction in lift as airflow separates from a wing that has exceeded its critical angle of attack.

Standard instrument arrival. A published IFR route that connects the en-route phase to a point where an instrument approach can begin.

The surrounding air pressure. Aircraft instruments use it to determine quantities such as altitude and vertical speed.

The initial tendency of an aircraft after a disturbance. Positive static stability means its initial response is towards its original equilibrium.

A prepared area beyond the take-off run where an aeroplane can stop if a take-off is rejected.

A low cloud layer made up of broad patches or rolls, often with darker shading or gaps.

A low, fairly uniform grey cloud layer that may produce drizzle.

Terminal aerodrome forecast. A coded forecast of expected weather at and near an aerodrome over a stated period.

The horizontal tail surface, also called the horizontal stabiliser. It contributes to pitch stability and often carries the elevator.

True airspeed. The speed of the aircraft relative to the air mass through which it is flying.

Fuel planned for use before take-off, including the expected ground operation.

Traffic alert and collision avoidance system. A family of systems that includes TCAS II, an implementation of ACAS II.

A TAF change group indicating temporary fluctuations in the conditions during the stated period.

The force produced by the propulsion system, such as a propeller or jet engine.

Terminal control area, also called a terminal manoeuvring area. A control area normally established where air traffic routes meet near one or more major aerodromes.

Take-off distance available. The take-off run available plus the length of any declared clearway.

Take-off run available. The length of runway declared usable for the ground run of a departing aeroplane.

A strongly developing cumulus cloud with tall, cauliflower-shaped towers. It may grow into cumulonimbus.

The direction of an aircraft’s path over the ground. Wind can make the track differ from the heading.

An ACAS alert that draws attention to potentially conflicting traffic and helps the crew locate it.

The combined mass of passengers, baggage, cargo, any specialist equipment carried on board and any ballast.

Aircraft equipment that replies to surveillance interrogations. Depending on the mode, it can provide an identification code, pressure altitude or other flight data.

A small movable surface on a flight control. Adjusting it can reduce the force the pilot needs to hold on the controls.

Fuel planned to fly from take-off at the departure aerodrome to landing at the destination.

Thunderstorm. TSRA means a thunderstorm with rain.

A gas turbine engine that drives a fan. The fan moves air through a bypass duct around the engine core and contributes to thrust.

A gas turbine engine that drives a propeller. The propeller supplies most of the thrust.

An instrument that shows the direction and rate of turn. Its separate slip indicator, often a ball in a curved tube, shows whether the aircraft is in balanced flight.

A licence qualification to fly a specified aircraft type or group of variants covered by the same rating.

Upset prevention and recovery training. It develops the skills to avoid, recognise and recover from an aircraft upset, such as an unintended extreme attitude or airspeed.

Take-off decision speed. It sets the latest speed for taking the first action to reject within the calculated accelerate-stop distance and the minimum speed for continuing after the specified critical engine failure.

Take-off safety speed. A speed selected to meet the required climb performance with the critical engine inoperative.

In the vicinity of the aerodrome. VCTS indicates a thunderstorm nearby.

An instrument that shows how quickly an aircraft is climbing or descending, usually in feet per minute.

Visual flight rules. The rules governing flight under visual procedures, including applicable visibility and cloud clearance requirements.

Visual meteorological conditions. Weather that meets the applicable visibility and cloud criteria for visual flight. VMC describes the weather, while VFR describes the rules being used.

VHF omnidirectional range. A ground-based radio navigation aid that lets aircraft determine their bearing from the station.

Rotation speed. The speed at which the pilot initiates rotation during take-off.

Variable wind direction. VRB03KT means wind at three knots with no single reported direction.

Vertical visibility when the sky is obscured. VV002 means vertical visibility of 200 feet.

A defined geographical point used to describe an area navigation route or flight path.

A radio acknowledgement that a message was understood and will be complied with. The word comes from “will comply”.

The angle between the desired track and the heading needed to hold it in a given wind.

A change in wind speed or direction over a short horizontal or vertical distance.

A structural member that helps give the wing its aerofoil shape and transfers loads into the skin and spars.

A main structural beam running along the span of a wing. It carries much of the wing’s bending load.

The mass of the loaded aircraft excluding usable fuel. It is the dry operating mass plus the traffic load.