EASA Module 14 key terms

The ideal thermodynamic cycle describing a gas turbine: air is compressed, heat is added at constant pressure by combustion, the gas expands through a turbine, and heat is rejected. It sets the theoretical efficiency limit for turbojet and turbofan engines.

The ratio of compressor delivery pressure to compressor inlet pressure. Higher values raise thermal efficiency and reduce specific fuel consumption, but are limited by compressor surge margin and turbine temperature capability.

The gas temperature entering the high-pressure turbine nozzle guide vanes, typically 1200-1700 K in modern engines. It is the dominant limit on thrust and efficiency and drives blade cooling design and material selection.

Fuel mass flow per unit of thrust (or shaft power) produced, usually kg/(N·h) or kg/(kW·h). It quantifies engine economy: lower values mean the engine converts fuel into useful output more efficiently.

The mass flow of air through the fan duct divided by the mass flow through the engine core. High bypass ratios (5-12) improve propulsive efficiency and reduce noise, which is why modern transport turbofans use them.

The fraction of engine power output converted into useful thrust work on the aircraft. It rises as exhaust velocity approaches flight velocity, so large mass flows accelerated gently (high bypass, turboprop) are more efficient than small, fast jets.

The percentage margin between an operating point on the compressor characteristic and the surge line at the same corrected speed. Adequate margin prevents unstable flow reversal during acceleration, crosswind starts, or inlet distortion.

A violent, low-frequency instability in which forward flow through the compressor breaks down and reverses, causing loud bangs, thrust loss, and possible blade damage. It can be triggered by exceeding the surge line, inlet distortion, or abrupt throttle changes.

The stationary aerofoil ring immediately upstream of a turbine rotor. It accelerates and turns the hot gas to the correct angle and velocity for the rotor blades, and is usually the most highly cooled component in the engine.

A cooling method in which compressor bleed air is discharged through small holes in a hot component to form an insulating layer of cooler air over its surface. It is used on turbine blades, vanes, and combustor liners.

The combination of internal convection passages, impingement, and external film cooling that keeps turbine blades below their material temperature limit. Cooling air is bled from the compressor, so excessive use penalises engine efficiency.

Compressor bleed air routed to hot-section components such as turbine blades, vanes, discs, and combustor liners. It protects materials from thermal distress but is not available for combustion, so its use reduces overall engine efficiency.

Movable vanes at the compressor inlet whose angle is scheduled with engine speed to match airflow direction to the first rotor stage. They widen the stable operating range and improve surge margin at low and high speeds.

A valve that releases compressor air overboard or to the bypass duct at low engine speeds. It prevents the front stages from surging when the rear stages cannot pass the airflow demanded by the operating line.

The reaction force produced by accelerating a mass of air rearward through the engine. In a turbofan it is mainly the momentum change of the bypass stream plus the core exhaust, minus the ram drag of incoming air.

The difference between gross thrust from the exhaust nozzles and ram drag from the intake momentum of incoming air. It is the useful force actually available to propel the aircraft at a given flight speed.

The sequence of induction, compression, power, and exhaust strokes completed in two crankshaft revolutions. Each stroke moves the piston between top dead centre and bottom dead centre and is controlled by valve timing.

The crankshaft angles at which the intake and exhaust valves open and close, usually given relative to top or bottom dead centre. Correct timing, including overlap, governs volumetric efficiency and cylinder filling.

The crank angle before top dead centre at which the spark occurs. It must advance with engine speed and retard with manifold pressure to keep peak cylinder pressure just after top dead centre, avoiding detonation and lost power.

Abnormally rapid, uncontrolled combustion of the end gas in the cylinder, producing a sharp pressure spike and audible knock. It is caused by excessive advance, high intake temperature, low octane fuel, or over-lean mixture, and can damage pistons and bearings.

Ignition of the fuel-air charge before the spark occurs, usually from a hot spot such as a glowing spark plug electrode, carbon deposit, or overheated exhaust valve. It can cause severe power loss and rapid mechanical damage.

A measure of a fuel's resistance to detonation, referenced to iso-octane and n-heptane blends. Higher ratings allow higher compression ratios or boost without knock; aviation grades use lean and rich mixture test ratings.

The ratio of the actual volume of air inducted per cycle to the theoretical swept volume of the cylinder. It falls with increasing speed and throttle loss, and is improved by tuned intake and exhaust systems.

The absolute pressure in the engine intake manifold, measured in inches of mercury or hectopascals. It indicates the charge density available to the cylinders and is the primary control parameter for a boosted piston engine.

A device in which exhaust gas drives a turbine that turns a compressor to pressurise the intake charge. It allows a piston engine to maintain power at altitude, where air density falls, and is controlled by a wastegate.

A valve that bypasses exhaust gas around the turbocharger turbine to limit boost pressure. It opens when the desired manifold pressure is reached, protecting the engine from overboost and detonation.

A heat exchanger that cools compressed intake air after the turbocharger or supercharger before it enters the cylinders. Cooler, denser air increases charge mass and reduces the risk of detonation.

A self-contained, engine-driven ignition generator that produces high-voltage pulses for the spark plugs without relying on the aircraft battery. Aircraft engines normally have two independent magnetos for redundancy.

A device that introduces the ignition spark into the combustion chamber through electrodes separated by an air gap. Its heat range must suit the engine so the tip runs hot enough to self-clean but not hot enough to pre-ignite.

Full Authority Digital Engine Control: a computer system that schedules fuel flow, ignition, and variable geometry without mechanical backup, using sensors and actuator loops. It provides precise control, protection limits, and fault reporting.

The computing unit within a FADEC system that processes inputs such as throttle position, pressures, temperatures, and speeds, then commands fuel and geometry actuators. It usually has dual channels with fault detection and reversion logic.

The component that delivers a precisely controlled fuel flow to the combustor in response to commands from the engine control unit. It typically combines a fuel pump, metering valve, and shut-off valve.

A hydromechanical or electronic device that schedules fuel flow to the engine as a function of throttle position, compressor speed, and ambient conditions. It maintains the correct acceleration and steady-state schedule.

A device that atomises fuel into fine droplets and distributes it within the combustion chamber for complete, stable burning. Types include simplex, duplex, and air-spray designs, and blockage causes hot spots and uneven temperature.

The section where fuel is mixed with compressed air and burned at approximately constant pressure to raise gas temperature before the turbine. Designs include can, can-annular, and annular types, with annular being standard on modern engines.

Extinction of the combustion flame, causing sudden loss of thrust. It can result from fuel starvation, excessive airflow during rapid throttle changes, water ingestion, or very low combustor pressure at high altitude.

The procedure of restoring combustion after a flameout, using the ignition system and starter or windmilling airflow within the engine's relight envelope. Pilots must descend to an altitude where relight is possible.

The high-energy exciter and igniter plugs that initiate combustion during start and provide automatic relight in flight. Turbine igniters differ from piston spark plugs in that they are designed for short, high-energy discharge.

A transducer that measures parameters such as N1, N2, exhaust gas temperature, fuel flow, oil pressure, or bleed pressure for control and indication. Sensor accuracy and redundancy directly affect engine protection and dispatch reliability.

The temperature of gas leaving the turbine, measured by thermocouples and displayed in the cockpit. It is the main indicator of turbine thermal load and is used to set take-off power limits and detect deterioration.

The cockpit control that commands engine thrust by signalling the FADEC or fuel control unit. Detents typically mark idle, maximum continuous, and take-off, and the lever may include reverse and beta ranges.

The angle between the blade chord line and the plane of rotation, which determines the advance per revolution. It is varied in flight to keep the blade at an efficient angle of attack across speed and power settings.

The angle at which a propeller blade is set relative to the hub reference plane, measured at a stated radius. It differs from pitch because it is a geometric setting, while pitch describes the resulting helical advance.

A propeller whose blade angle is automatically adjusted by a governor to hold a selected rotational speed as load or power changes. It lets the engine run at efficient rpm while the blades absorb varying power.

A device that senses propeller speed and adjusts blade angle through oil pressure or counterweights to maintain the selected rpm. It increases pitch when speed rises and decreases pitch when speed falls.

Rotating the blades to a high pitch angle nearly parallel to the airflow so that a stopped or failed engine produces minimum drag. It is used after engine failure in multi-engine aircraft and requires an accumulator or stored pressure.

A device that holds the propeller stationary when the engine is shut down, preventing windmilling and reducing wear. It is often used with feathering and must release before start to avoid damage.

A protective device that limits propeller or engine speed by increasing blade pitch or reducing fuel if the normal governor fails. It prevents destructive overspeed and is checked during ground runs.