Aircraft systems

Aircraft systems glossary

Plain-language definitions for common aircraft systems terms and abbreviations, organized by ATA chapter.

48 of 48 terms

ACARS: A digital datalink used to send short messages (position reports, weather requests, maintenance data) between the aircraft and ground systems.
Accumulator: A gas-charged reservoir that stores hydraulic pressure to dampen pressure surges or provide a limited reserve, such as for emergency braking.
ADIRU: A unit that combines air data (speed, altitude) computation with inertial reference (attitude, position) in a single line-replaceable unit.
Aileron: A control surface near the wingtip that moves opposite on each wing to roll the aircraft.
Anti-Ice: A system (typically hot bleed air or electrical heating) that runs continuously in icing conditions to prevent ice from forming on a protected surface.
Anti-Skid: A braking system function that modulates brake pressure on a wheel approaching a skid, to keep it near peak braking friction.
APU: A small gas-turbine engine, usually in the tail, that supplies electrical power and bleed air on the ground (and sometimes in flight) without the main engines running.
Autopilot: The system that flies the aircraft's pitch, roll, and yaw axes automatically to a selected mode, without the pilot holding the controls.
Autothrottle: The system that moves the thrust levers, or commands thrust electronically, to hold a selected speed or thrust target.
Battery Bus: A DC electrical bus fed directly by the battery, kept live even with all generators and external power off, for essential loads.
BITE: Self-test functionality inside a line-replaceable unit that detects and stores fault information, retrievable by maintenance without external test equipment.
Bleed Air: Compressed air tapped off an engine compressor stage (or from the APU), used to supply pneumatic services such as air conditioning, anti-ice, and engine start.
Boost Pump: An electrically-driven pump inside a fuel tank that provides positive-pressure fuel feed to the engines and to any transfer or crossfeed system.
CMS: The onboard computer that collects BITE fault reports from other systems and presents a consolidated maintenance summary for troubleshooting.
CPC: The computer that schedules cabin altitude against aircraft altitude and drives the outflow valve to maintain it.
Crossfeed Valve: A valve that, when open, lets fuel from one side's tanks feed an engine on the other side — used to correct imbalance or to feed all engines from fewer tanks.
De-Ice: A system that operates cyclically to remove ice that has already accumulated, rather than preventing it from forming in the first place.
ECAM: An Airbus-style display system that presents engine/system parameters and, on a warning, brings up the associated synoptic and drives the crew through the procedure.
EGT: The temperature measured at or near the turbine exhaust, used to monitor engine health and to protect against exceeding a structural temperature limit.
EICAS: A Boeing-style display system that presents primary engine parameters and system status, and annunciates cautions/warnings, on dedicated cockpit displays.
Elevator: A control surface on the horizontal tail (or, on some types, a full moving tailplane) that controls pitch.
Engine-Driven Pump: A hydraulic pump mounted on and driven by the engine accessory gearbox, providing the primary hydraulic pressure source for that system.
FADEC: A digital computer that has full control authority over engine (or APU) fuel scheduling and operating limits, replacing older hydro-mechanical fuel controls.
Fire Detection Loop: A sensing element run through an engine or APU compartment that signals a fire or overheat warning when it detects a rise in temperature.
Flight Director: Command bars on the primary flight display that show the pitch and roll needed to satisfy the selected autopilot mode, whether or not the autopilot is engaged.
Fly-By-Wire: A flight control architecture where pilot inputs are sent electronically to flight control computers, which command the actuators, rather than through direct mechanical linkages.
FMS: The computer that holds the flight plan, computes performance, and provides lateral/vertical guidance to the autopilot and flight director.
FQIS: The system of tank-mounted probes and a processing unit that measures fuel quantity in each tank and reports it to the cockpit displays.
GCU: The unit that regulates a generator's voltage, brings it on line, and trips it off for protection faults such as over- or under-voltage.
IDG: An engine-driven generator with a built-in constant-speed drive, so it outputs a stable frequency despite engine speed changes.
Igniter: A high-energy spark plug that ignites the fuel-air mixture in the combustor during engine start and, when selected, for continuous ignition in adverse conditions.
IRS: A self-contained system of accelerometers and gyros (or ring-laser/fiber-optic gyros) that computes attitude, heading, and position without external signals, after an alignment.
N1: The rotational speed of an engine's low-pressure spool (fan and low-pressure turbine), usually shown as a percentage of a reference speed and used as the primary thrust-setting parameter.
Outflow Valve: The valve that controls how fast cabin air leaves the fuselage, and so sets cabin pressure in conjunction with the pressurization controller.
Pack: The unit that cools and conditions bleed air (or, on some types, electrically-compressed air) before it enters the cabin distribution system.
PCU: A hydraulic (or electro-hydraulic) actuator that moves a flight control surface in response to a command from the flight control system.
Pre-cooler: A heat exchanger that cools hot bleed air, using fan air as the cooling source, before it is distributed to the pneumatic system.
PSEU: The unit that interrogates proximity sensors around the landing gear, doors, and other moving components, and reports their position states to other systems.
PTU: A unit that lets one hydraulic system drive a hydraulic motor which pressurizes a second, otherwise-unpowered system, without transferring fluid between them.
QAR: A recorder that captures the same kind of flight parameter data as the protected flight data recorder, but on removable media for routine flight-data-monitoring programs.
Radio Altimeter: A system that measures height above the terrain directly below the aircraft by timing a reflected radio signal, used mainly at low altitude for approach and landing.
Ram Air: Outside air taken in through an inlet opened to the airstream, used to cool pack components or, in some hydraulic systems, as an emergency pump drive.
Spoiler: A panel on the upper wing surface that deploys to reduce lift and add drag, used for roll control augmentation, speedbrake, and ground lift-dump after landing.
Squib: An electrically-fired cartridge that ruptures a fire extinguisher bottle's discharge disc, releasing its agent into the protected zone.
Thrust Reverser: A mechanism that redirects engine exhaust (or fan) flow forward to help decelerate the aircraft after landing.
Transponder: The system that replies to ground radar interrogations with the aircraft's identity, altitude, and (for ADS-B) position, for air traffic control.
TRU: A unit that converts AC electrical power to DC to feed DC buses and battery charging.
Weight-on-Wheels: A sensor or switch that indicates whether the aircraft's weight is on the landing gear, used to inhibit or enable ground- and air-only functions.

General reference definitions, not reproduced from any manufacturer manual, and not a substitute for type-specific training or your approved maintenance data. Terminology and implementation details vary by aircraft type.

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