ATPL Aero & Systems key terms

The maximum load expected in service over the aircraft's life. Structures must withstand it without permanent deformation, and it forms the basis for the ultimate load calculation.

The limit load multiplied by the prescribed safety factor, typically 1.5. The structure must sustain this load for a short period without failing, although permanent deformation is permitted.

The ratio of lift produced to the aircraft's weight, expressed in g. It quantifies structural and manoeuvring stress and defines the certified flight envelope.

The speed above which full deflection of a single flight control may overstress the structure. At or below it, the aircraft can be manoeuvred to limit load without structural damage.

A design philosophy where failure of one load path or element is carried by redundant structure until the defect is found at inspection, preventing catastrophic loss of the airframe.

A component designed and life-limited so it is removed from service before the accumulated fatigue cycles can cause cracking. Used where redundancy is impractical, such as landing gear legs.

A measure of a fluid's resistance to flow, falling as temperature rises. It determines leakage, pressure drop and response of actuators, so fluid grade must suit the system's operating temperature range.

Maintaining a positive pressure of air or nitrogen above the fluid in the reservoir. This prevents pump cavitation at altitude and ensures positive feed to the pumps.

Formation and collapse of vapour bubbles in hydraulic fluid where local pressure falls below vapour pressure. It causes noise, erosion and loss of pump delivery.

A device that converts mechanical drive into fluid flow and pressure. Engine-driven pumps supply normal demand, while electric pumps provide backup or ground operation.

A valve that opens to return excess fluid to the reservoir when system pressure exceeds a set value, protecting components and lines from overpressure.

A gas-charged vessel connected to the hydraulic system that stores pressurised fluid. It smooths pressure pulses, absorbs thermal expansion and provides emergency pressure for limited actuation.

The difference between cabin pressure and outside ambient pressure, limited by the fuselage structure. It sets the maximum cabin altitude at any given aircraft altitude.

The equivalent pressure altitude inside the pressurised cabin. It is kept below a maximum value, typically 8000 ft, to protect occupants from hypoxia.

A motorised valve in the cabin wall that modulates the rate air leaves the cabin, thereby controlling cabin pressure and cabin altitude.

Hot compressed air taken from a gas turbine compressor stage and used for air conditioning, pressurisation, anti-icing and engine starting.

A refrigeration unit in which compressed air is cooled by expansion through a turbine, producing cold air for cabin supply without a refrigerant circuit.

A generator producing direct current, or a transformer-rectifier converting AC to DC. Direct current supplies essential and battery-backed loads in the electrical distribution.

A machine producing alternating current from engine or APU drive. Constant-speed drives or variable-frequency output keep the supply within the equipment's accepted frequency range.

A common electrical conductor to which multiple circuits are connected, allowing power to be distributed to several loads from one supply point.

A resettable protective device that interrupts a circuit automatically when current exceeds its rating, protecting wiring from overheating and fire.

A control system in which pilot inputs are converted to electrical signals that drive hydraulic actuators, with computers shaping the commands and providing envelope protection.

A powered actuator that moves a flight control surface in response to an electrical or hydraulic command, often with feedback for position and rate control.

A motor or actuator that moves a control surface or the control linkage under autopilot command, allowing automatic flight path and heading control.

A device fitted to a nose landing gear to damp out rapid self-excited oscillation of the wheel about its steering axis during taxi and landing.

A system that senses wheel deceleration and modulates brake pressure to prevent wheel lock and maintain directional control and braking efficiency on wet or icy runways.

Loss of braking effectiveness caused by overheating of brake friction surfaces, often after repeated heavy braking, and requiring cooling before full braking returns.

The shock-absorbing leg, usually an oleo with gas and hydraulic fluid, that carries landing and taxi loads and dissipates their energy.

Arrangement allowing air to enter and leave a tank as fuel is used or expands, preventing structural overpressure or vacuum and permitting normal fuel flow.

A valve arrangement permitting fuel from one tank to feed an engine on the opposite side, used to correct imbalance or to supply an engine after a pump failure.

Fuel remaining in tanks and lines that cannot be delivered to the engines under normal operating conditions, and therefore excluded from usable fuel calculations.

The system, often capacitance or ultrasonic, that measures fuel mass or volume in each tank and displays it to the crew for management and endurance planning.

Electrical heating of the pitot probe to prevent ice blockage of the pressure inlet, which would otherwise cause erroneous airspeed indications.

Use of hot bleed air or electrical heating on surfaces such as wing leading edges and engine intakes to prevent ice formation or remove accumulated ice.

A continuous sensing element in an engine or cargo compartment that changes resistance or continuity with temperature rise, triggering a fire warning.

A pressurised container of extinguishing agent discharged into a designated zone through a distribution system when fire is confirmed.

A sensor in cargo or avionics compartments that responds to smoke particles or obscuration and provides an alert so suppression can be activated.

The absolute pressure in a piston engine's induction manifold, used with RPM as the primary indication of power output for a fixed-pitch or constant-speed propeller engine.

A propeller whose blade pitch is varied by a governor to maintain a selected RPM as power and airspeed change.

Adjustment of the fuel-to-air ratio delivered to a piston engine, set rich for high power and leaned for efficient cruise or high-altitude operation.

Fuel flow divided by thrust or power produced. A lower value indicates greater efficiency and is used to compare engine performance and plan fuel burn.

Temperature of the gas leaving the turbine or entering the exhaust, monitored to protect engine limits and to help set thrust and detect faults.

The ratio of turbine discharge pressure to compressor inlet pressure, used as an indication of thrust produced by a turbojet or turbofan engine.

The forward force produced by accelerating a mass of air rearward through the engine, balanced in steady flight by aircraft drag.