CAA NZ AME Licence Exam - Turbine Engines (Subject 6) key terms

The ideal thermodynamic cycle describing a gas turbine: air is compressed, heat is added at constant pressure by burning fuel, the hot gas expands through a turbine, and heat is rejected. It sets the theoretical efficiency limit for the engine.

The ratio of compressor delivery pressure to compressor inlet pressure. Higher pressure ratio generally raises thermal efficiency and specific thrust, but increases compressor exit temperature and turbine entry demands.

The proportion of heat energy released by fuel that is converted into useful shaft or jet work. It rises with pressure ratio and turbine entry temperature and falls with losses such as tip clearance and cooling bleed.

Fuel mass burned per unit of thrust or shaft power per unit time, used to compare engine economy. Lower values mean better fuel efficiency and depend on cycle temperatures, pressure ratio and component efficiency.

The net reaction force produced by accelerating a mass of air and exhaust gas rearward. For a turbojet it is mass flow times velocity change plus the pressure thrust term at the nozzle.

The mass flow of air passing through the fan duct divided by the mass flow entering the core. High bypass ratios improve propulsive efficiency and reduce noise, typical of modern turbofan engines.

The fraction of the engine's kinetic energy output that appears as useful thrust rather than wasted residual jet velocity. It improves when a large mass of air is accelerated only slightly.

The gas temperature at the inlet to the first turbine stage, a primary limit on engine power. It is controlled by fuel scheduling and cooling design because it drives blade creep life.

The safety buffer between an operating point on the compressor map and the surge line. It shrinks with fouling, bleed extraction, inlet distortion or rapid throttle changes and is monitored during testing.

A violent breakdown of steady airflow in which flow reverses momentarily through the compressor, causing loud bangs, vibration and possible damage. It is prevented by bleed valves, variable stators and fuel scheduling limits.

Local separation of airflow from compressor blade surfaces, reducing pressure rise and efficiency. It may be rotating or static and can escalate into surge if not corrected by reducing power or opening bleeds.

A chart plotting pressure ratio against corrected mass flow for a compressor, with speed lines, efficiency contours and a surge line. It is used to predict matching between compressor and turbine.

A compressor in which air flows roughly parallel to the shaft through alternating rows of rotating blades and stationary vanes, each row giving a modest pressure rise. It offers high efficiency and large mass flow.

A compressor that flings air radially outward from an impeller into a diffuser, achieving a large pressure rise per stage. It is rugged and resistant to fouling, common in small engines and as a final stage.

Adjustable vanes at the compressor inlet that change airflow angle to match engine speed, improving surge margin at low power and efficiency at high power. They are moved by an actuation system under control unit command.

Movable stator blades in an axial compressor whose angle is scheduled with engine speed to keep the angle of attack on rotor blades near optimum. They extend the usable operating range and delay stall.

The section where fuel is injected, mixed with compressed air and burned continuously to raise gas temperature before the turbine. It must achieve stable, efficient combustion with low pressure loss and even exit temperature.

A single continuous ring-shaped combustion chamber surrounding the shaft, giving compact length, low pressure loss and uniform exit temperature. It has largely replaced can and can-annular designs in modern engines.

A device that atomises and delivers fuel into the combustion chamber in a controlled spray pattern. Poor atomisation causes hot streaks, carbon build-up and uneven exit temperature distribution.

Extinction of the combustion flame, causing sudden loss of power or thrust. It can follow fuel interruption, inlet icing, rapid throttle movement or water ingestion and requires relight procedures.

A high-energy spark plug, usually of the surface-discharge type, that initiates combustion during starting and provides a relight capability in flight. Two igniters are often fitted for redundancy.

A unit that stores electrical energy and discharges it as a high-voltage pulse to the igniter plug. It converts low aircraft DC supply into the high-energy spark needed for reliable light-up.

The coordinated steps of motoring the engine, igniting fuel, monitoring acceleration and releasing the starter at self-sustaining speed. Automatic start systems monitor limits and abort on hot or hung starts.

A start in which the engine lights but fails to accelerate to idle within the expected time, usually from weak starter, low fuel pressure or bleed leakage. It is aborted to avoid overheating.

A start in which exhaust gas temperature exceeds the permitted limit because too much fuel burns before adequate airflow exists. Causes include excessive fuel scheduling, slow rotation or a faulty fuel control unit.

The component that meters fuel flow to the engine in response to throttle position, compressor conditions and limits. It may be hydromechanical or electronic and schedules acceleration, deceleration and idle flow.

Full Authority Digital Engine Control, a computer system that controls fuel flow, variable geometry and starting without a hydromechanical backup for normal operation. It provides precise scheduling, protection limits and fault reporting.

The temperature of gas leaving the turbine, measured by thermocouples and used as the main indicator of engine thermal stress. It is compared with limits during start, take-off and climb.

Systematic recording and analysis of parameters such as exhaust gas temperature, fuel flow and vibration over time to detect deterioration before failure. Trends help plan maintenance and diagnose faults.

Visual examination of internal engine components such as compressor blades, combustor liners and turbine blades through ports, without disassembly. It detects cracks, burns, erosion and foreign object damage.

Damage to compressor or turbine blades from ingested debris such as stones, birds or ice. It shows as nicks, dents, tears or bent blades and can reduce performance or cause fatigue failure.

Permanent elongation of hot turbine blades under sustained centrifugal load and high temperature. It reduces tip clearance and can lead to rubbing or failure, so blades are life-limited by hours and cycles.

A turbine blade with internal passages that pass compressor bleed air to keep metal temperature below gas temperature. Film cooling holes discharge air over the surface, allowing higher turbine entry temperatures.

Stationary aerofoil vanes that turn and accelerate hot gas onto the first turbine rotor blades at the correct angle. They are highly loaded thermally and often use internal cooling and thermal barrier coatings.

The rearward duct that expands exhaust gas to a controlled exit area, converting remaining pressure into velocity. Fixed or variable area designs affect thrust and back-pressure on the turbine.

A system in which oil is scavenged from the engine bearings to an external tank, then pressure-fed back. It prevents oil foaming, allows attitude independence and is standard on turbine engines.

A pump that returns used oil from bearing compartments and the accessory gearbox to the oil tank. Scavenge capacity is normally greater than pressure supply to keep sumps from flooding.

A heat exchanger that removes heat from engine oil using fuel or air as the cooling medium. Fuel-cooled units also preheat fuel and prevent ice formation in the fuel system.

A rolling element or plain support that carries shaft radial and axial loads with minimal friction. Turbine bearings run hot and are lubricated and cooled by metered oil under pressure.

Measurement of engine vibration, usually by accelerometers on the casing, to detect imbalance, bearing wear or blade damage. Trends and frequency analysis help isolate the faulty component.

A gear train driven from the engine shaft that powers fuel pumps, oil pumps, generators, hydraulic pumps and the starter. It may be engine-mounted or mounted on the fan case.

Extraction of shaft power from the engine to drive accessories or, in turboprops and turboshafts, the propeller or rotor. The pad location and gear ratio determine speed and torque limits.

A ceramic layer applied to hot section parts to reduce metal temperature and oxidation. Coating loss or spalling raises component temperature and shortens life, so condition is checked at inspection.

The large front rotor of a turbofan that accelerates bypass air to produce most of the thrust. Fan blade damage or imbalance is a significant source of vibration and performance loss.