CAA NZ Basic Turbine Knowledge (Helicopter) Exam 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 compressor outlet total pressure divided by its inlet total pressure. A higher pressure ratio raises cycle efficiency and power output but also raises compressor delivery temperature and turbine inlet temperature demands.

The proportion of fuel heat energy released in combustion that is converted into useful shaft or thrust work. It improves with higher pressure ratio and higher turbine entry temperature, and falls with losses such as tip clearance leakage.

The gas temperature at the inlet to the first turbine nozzle guide vanes. It is the most limiting temperature in the engine and is controlled by fuel flow scheduling and cooled turbine blading.

A violent breakdown of steady airflow through the compressor, where flow reverses momentarily and pressure fluctuates. It causes bangs, severe vibration and rapid temperature rise, and must be avoided by bleed valves and correct scheduling.

A localised separation of airflow from compressor blade surfaces, reducing pressure rise and efficiency. It may affect only some blades and can progress to full surge if not corrected.

Movable vanes at the compressor inlet that change airflow direction and swirl as engine speed changes. They keep blade incidence near optimum, widening the stable operating range and improving acceleration and part-power efficiency.

A valve that releases compressor air overboard or to another section during starting and acceleration. It reduces axial load and pressure ratio at low speed, helping prevent surge and stall until the engine reaches a stable speed.

A compressor that accelerates air outward through an impeller and then diffuses it to raise pressure. It is robust, gives a high pressure rise per stage and tolerates foreign object damage, which suits many small helicopter engines.

A compressor in which air flows generally parallel to the shaft through alternating rotor and stator rows. Each stage gives a modest pressure rise, so many stages are needed, but efficiency and mass flow are high.

A duct of increasing cross-sectional area that slows air or gas and converts kinetic energy into static pressure. It is used after a centrifugal impeller and at the compressor outlet before the combustion chamber.

A single continuous ring-shaped combustion space around the shaft, fitted with multiple fuel nozzles. It gives even temperature distribution, compact length and low pressure loss compared with can or can-annular designs.

The heat-resistant inner shell of the combustion chamber that contains the flame and shapes the airflow. It is cooled by compressor air through small holes and is inspected for cracking, burning and distortion.

A device that meters and atomises fuel into the combustion chamber so it mixes readily with air and burns cleanly. Blockage, carbon build-up or poor spray pattern causes hot spots, smoke and uneven temperature spread.

Stationary aerofoil vanes immediately upstream of the first turbine rotor. They turn and accelerate hot gas to the correct angle and speed, and are usually cooled because they see the highest gas temperature in the engine.

The section that extracts energy from hot high-pressure gas to drive the compressor and, in a turboshaft engine, the output shaft. It may be a single spool or split into gas generator and free power turbines.

A turbine that drives the output shaft but is mechanically independent of the gas generator, so it can run at a different speed. This arrangement lets rotor speed stay constant while engine speed varies.

The compressor, combustion chamber and compressor-driving turbine assembly that produces hot gas. Its speed and temperature are controlled by the fuel system and are the main parameters monitored for engine health.

The temperature of gas measured at the turbine exit or exhaust duct. It is a proxy for turbine entry temperature and is used to set power limits and detect deterioration or overheating.

The difference between the highest and lowest exhaust gas temperatures measured around the exhaust or between thermocouples. A widening spread can indicate a faulty fuel nozzle, liner distress or a failing thermocouple.

Recording and comparing engine parameters such as gas generator speed, torque and exhaust gas temperature over time at reference conditions. Gradual changes reveal deterioration, so maintenance can be planned before a limit is reached.

The turning moment delivered by the engine to the transmission, usually measured by a torque meter and displayed as a percentage of maximum continuous or take-off rating. It is a primary power limit in helicopters.

The mass of fuel burned per unit of power or shaft work produced per unit time. A lower value means greater efficiency and is used to compare engine performance and detect deterioration.

The unit that meters fuel to the engine in response to pilot demand and engine parameters such as compressor speed, turbine temperature and inlet conditions. It protects against over-temperature and over-speed during acceleration.

A fuel or rotor speed governor that uses flyweights, springs and hydraulic pressure to sense and correct speed. It provides stable speed control and is often backed up or trimmed by an electronic control unit.

An electronic system that computes and commands fuel flow and other functions across the whole engine operating range, with no mechanical backup for normal scheduling. It uses sensors, software schedules and built-in fault detection.

The automatic sequence of starter engagement, ignition, fuel introduction, light-up, acceleration to idle and starter disengagement. Timings and limits are defined so a hot or hung start is detected and aborted.

A unit that converts low-voltage supply into high-voltage pulses for the igniter plugs. It provides the spark for starting and, on many helicopters, a continuous relight facility for autorotation or adverse weather.

A plug that produces a high-energy spark inside the combustion chamber to ignite the fuel-air mixture. Its gap erodes with use, so it is inspected and replaced at defined intervals.

An abnormal start in which exhaust gas temperature rises above the permitted limit, usually from too much fuel or too little airflow. It requires immediate fuel shut-off and can seriously damage the turbine.

A start in which the engine lights up but fails to accelerate to idle speed within the normal time. Causes include weak starter, low battery, bleed valve or fuel scheduling faults, and it must be aborted before overheating.

Unintended extinction of the combustion flame, causing loss of power. It can follow fuel starvation, water ingestion, abrupt throttle movement or compressor surge, and requires an immediate relight attempt.

Systems that prevent gas generator or power turbine speed exceeding safe limits, by cutting fuel or limiting the control signal. In helicopters it also protects the rotor system from destructive over-speed.

A one-way clutch between the engine output and the main transmission that lets the rotor turn while the engine is stopped or slower. It allows autorotation and prevents the engine driving the rotor during a power-off descent.

Flight with the rotor driven by upward airflow rather than engine power, used after engine failure. The freewheeling unit disconnects the drive so the rotor keeps turning while the engine slows or stops.

The gearbox that reduces engine output speed to rotor speed and transmits torque, also driving the tail rotor and accessories. It is a critical life-limited component with its own temperature and chip monitoring.

A magnetic plug or sensor in an engine or gearbox oil system that collects ferrous particles and warns of internal wear. Debris found on inspection triggers further checks and possible component removal.

The system that supplies filtered, cooled oil to bearings and gears for lubrication and heat removal. It includes a tank, pumps, filter, cooler and pressure and temperature indications, and is monitored for pressure loss.

A drain plug with a magnetic core that captures ferrous metal particles in the oil. It is checked at scheduled servicing to detect abnormal wear before a component fails.

A scheduled inspection of the combustion chamber, nozzle guide vanes and turbine for cracking, burning, erosion and distortion. Findings determine whether components can continue in service or must be replaced.

Cleaning of the compressor with an approved fluid to remove salt, dirt and deposits that reduce performance. It restores efficiency and is normally done with the engine turned by the starter, following the maintenance manual.

Damage to compressor or turbine blades caused by ingested debris such as stones, birds or ice. It reduces performance and strength and must be assessed against limits before further flight.

Visual inspection of internal engine components through small ports using a flexible optical instrument. It allows combustion and turbine condition to be assessed without engine removal or disassembly.