ATPL Aircraft General Knowledge key terms

The main longitudinal load-bearing member of a wing or tailplane; it carries bending and shear loads from the lifting surfaces inboard to the fuselage attachment points.

A chordwise structural member that gives the wing or tailplane its aerofoil shape and supports the skin; it transfers local air loads to the spars and stabilises the skin panels.

A longitudinal stiffening member running spanwise or lengthwise along the skin, riveted to frames or ribs; it carries axial loads and prevents the skin from buckling under compression.

A structural philosophy in which the skin carries most of the load, stabilised against buckling by stringers and frames; the fuselage behaves as a stiffened shell rather than a truss.

A design philosophy in which the structure tolerates the failure of one element by providing redundant load paths, so damage is detected before it threatens the whole structure.

A design approach in which a component is given a fixed, calculated service life and is withdrawn from use before fatigue cracking is expected, with no redundant load path assumed.

Progressive cracking caused by repeated cycles of stress well below the ultimate strength of the material; it starts at stress raisers such as holes, rivets or sharp corners.

Cracking that develops when a susceptible metal under sustained tensile stress is exposed to a corrosive environment; it can progress rapidly with little visible external warning.

Localised attack along the grain boundaries of a metal, often caused by improper heat treatment; it may spread internally and leave only slight surface evidence until the material is seriously weakened.

A pressure vessel containing a gas charge separated from the hydraulic fluid; it stores energy for peak demands, damps pressure pulses and maintains pressure during brief pump interruptions.

A spring-loaded valve that opens to return fluid to the reservoir when system pressure exceeds a set value, protecting components and seals from overpressure.

A device that automatically shuts off fluid flow to a downstream circuit when the flow rate exceeds a set limit, limiting fluid loss and preserving pressure for the rest of the system after a line rupture.

The difference between cabin pressure and ambient outside pressure, expressed in psi or hPa; it is limited by the structural strength of the fuselage and controlled by the outflow valve.

The equivalent altitude corresponding to the pressure maintained inside the pressurised cabin; it is normally kept at or below about 8,000 ft and is displayed on the cabin pressure controller.

A controllable valve in the pressure hull, usually on the aft bulkhead, that regulates how much cabin air escapes to atmosphere and thereby sets cabin pressure and cabin altitude.

A crew oxygen system in which the mask supplies oxygen only when the wearer inhales, and mixes cabin air with oxygen according to altitude; it conserves oxygen and prevents oxygen waste at low cabin altitudes.

A system that delivers oxygen under positive pressure during inhalation and may maintain slight positive pressure in the mask, used when cabin altitude is high or the mask seal is critical.

A simple system in which oxygen flows continuously through a mask or cannula regardless of breathing, typically used for passengers and often regulated by an altitude-sensitive valve.

The period after loss of cabin pressurisation during which a person remains capable of taking correct, purposeful action before hypoxia incapacitates them; it shortens rapidly as altitude increases.

A deficiency of oxygen reaching body tissues, causing impaired judgement, euphoria and loss of consciousness; at altitude it can develop without the person noticing their own deterioration.

Ice protection that uses heated air, usually bleed air from the engines, to warm the leading edges of wings, tailplane and engine intakes above freezing so that ice cannot form.

An inflatable rubber strip on a leading edge that is cyclically inflated with pneumatic pressure to crack and shed accumulated ice; it removes ice rather than preventing its formation.

A fluid ice protection system in which anti-icing fluid seeps through a porous leading-edge panel to form a film that prevents ice from adhering to the surface.

Electrical heating of the pitot tube, and often the static ports, to prevent ice blockage that would cause erroneous airspeed and altitude indications.

An opening on the fuselage side that supplies undisturbed static pressure to the altimeter, airspeed indicator and vertical speed indicator; blockage or icing causes serious instrument errors.

A continuous sensing element routed around an engine or compartment that responds to heat or flame and signals a fire warning; its resistance or continuity change triggers the warning circuit.

A designated region of the aircraft, such as an engine nacelle or APU compartment, provided with its own detection and extinguishing system and separated from other zones by firewalls.

A substance such as halon or an inert gas discharged into a fire zone to interrupt combustion by removing oxygen or breaking the chemical chain reaction; it is stored in pressurised bottles.

A conductive bar that distributes electrical power from generators, batteries or external power to the individual aircraft circuits connected to it.

An engine-driven AC generator combined with a constant-speed drive in one unit, so that it produces constant-frequency AC power regardless of engine speed.

A unit that converts AC power from the aircraft bus into DC power for battery charging and DC-operated equipment, typically providing 28 V DC.

A store of electrical energy used to start engines or the APU, to supply essential loads during generator failure and to provide emergency power for critical systems.

A ground connection point on the aircraft through which an external ground power unit can supply electrical power to the aircraft buses when the aircraft's own generators are not running.

Compressed air tapped from a turbine engine compressor stage and used for cabin pressurisation, air conditioning, thermal anti-icing and engine starting.

A refrigeration unit in which compressed air is cooled by expansion through a turbine that also drives a compressor; it provides conditioned air for the cabin without a refrigerant fluid.

A self-contained air-conditioning unit, usually one of two, that cools, filters and controls the pressure and temperature of bleed air before it enters the cabin.

The angle between the propeller blade chord line and the plane of rotation; it determines the angle of attack of the blade and thus the thrust produced at a given RPM.

A propeller whose blade pitch is automatically varied by a governor to keep engine RPM constant as power or airspeed changes, improving efficiency over a wide range of conditions.

Rotating propeller blades to approximately the flight path direction so that they present minimum drag and the engine stops windmilling after an engine failure.

Abnormally rapid, explosive combustion of the fuel-air mixture in a piston engine cylinder, causing high peak pressures, overheating and possible engine damage.

Ignition of the fuel-air mixture before the spark occurs, usually caused by a glowing hot spot such as a carbon deposit or overheated spark plug; it can quickly damage the engine.

A measure of a fuel's resistance to detonation; a higher rating indicates greater anti-knock quality and permits higher compression ratios or boost pressures.

The proportion by mass of fuel to air entering a piston or turbine engine combustion chamber; it determines combustion temperature, power output and exhaust gas characteristics.

A condition in which fuel vaporises in the lines or pump before reaching the engine, causing a lean mixture, rough running or complete loss of power, especially at high altitude or high temperature.

The temperature of the exhaust gases measured by probes in the exhaust stream; it indicates combustion efficiency and is used to set mixture in piston engines and monitor turbine engine limits.

A gas turbine engine in which air is compressed, mixed with fuel and burned, and the expanding gases drive a turbine that powers the compressor and produces thrust or shaft power.

A lubrication system in which oil is stored in a separate tank and pumped to and from the engine, allowing the engine to be mounted in any attitude and improving oil cooling.

A lubrication system in which oil is stored in a sump at the bottom of the engine and circulated by a pump; common in smaller piston engines and simple in construction.

A hinged control surface on the trailing edge of each wing, moving in opposite directions to produce roll about the longitudinal axis.

A hinged control surface on the trailing edge of the tailplane that changes the pitch attitude of the aircraft about its lateral axis.

A hinged control surface on the trailing edge of the fin that produces yaw about the vertical axis and is used to balance the aircraft and coordinate turns.

An actuator that moves a flight control surface, or the control linkage, in response to commands from the autopilot computer to hold or change the aircraft's flight path.

A system that computes and displays steering commands on the attitude indicator to guide the pilot toward a selected flight path, whether or not the autopilot is engaged.

A wheel-braking system that senses wheel deceleration or slip and modulates brake pressure to prevent wheel lock-up, maintaining steering control and shortening stopping distance.

A system that automatically applies and modulates wheel braking to a selected deceleration rate during landing or a rejected take-off, reducing pilot workload.

A hydraulic or friction device fitted to a nose landing gear to damp out rapid, self-excited oscillation of the nose wheel about its steering axis during ground running.

A system, usually hydraulically or electrically actuated, that turns the nose wheel in response to tiller or rudder pedal input to steer the aircraft on the ground.