CAA NZ Basic Turbine Knowledge (Helicopter) Exam flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Brayton cycle

The constant-pressure thermodynamic cycle of a gas turbine: compression, constant-pressure heat addition, expansion through the turbine, and exhaust. Its efficiency rises with pressure ratio and turbine entry temperature.

Specific fuel consumption

Fuel mass flow divided by net shaft power, usually kg per kWh. It falls as pressure ratio and turbine entry temperature rise, and rises steeply at low power settings.

Free power turbine

A turbine stage not mechanically coupled to the gas generator, driving the output shaft independently. It lets rotor rpm vary while the gas generator keeps its own speed.

Life-limited part

A component that must be removed from service at a published hours or cycles limit regardless of condition. Its life is tracked against the part, not the whole engine.

Compressor surge

A violent reversal of airflow through the compressor, heard as bangs with fluctuating rpm and rising EGT. It can damage blades and requires reducing power and landing.

Compressor map surge line

The boundary on a compressor map separating stable operation from surge. Acceleration bleed valves and variable inlet guide vanes keep the operating point clear of it at low rpm.

Compressor fouling

Dirt, salt or insect deposits on compressor blades reduce mass flow and raise EGT for the same power. Washing can restore performance if detected early.

Primary combustion zone

The front part of the combustion chamber where fuel is atomised and burned near stoichiometric ratio. Dilution air downstream cools the gas before it reaches the turbine.

Turbine blade cooling

Internal passages, film cooling and thermal barrier coatings keep blade metal below gas temperature, which exceeds the alloy melting point. Cooling air comes from compressor bleed.

Hot start

A start in which EGT exceeds its limit because fuel is introduced too early or in excess. Action is fuel off, continue motoring to cool, then investigate before retry.

Hung start

A start where rpm stalls below idle, often from weak starter, low battery or bleed air, or a fuel scheduling fault. Fuel is shut off and the engine motored.

Wet start

Fuel is introduced but ignition does not occur, leaving fuel in the combustion chamber. Motoring the engine to purge before any relight prevents an explosive ignition.

Synthetic turbine oil

Turbine engines use synthetic oils such as the MIL-PRF-23699 class, not piston-engine ashless dispersant oils. Mixing grades requires manufacturer approval.

Chip detector

A magnetic plug in the oil system that collects ferrous particles as an early warning of bearing or gear distress. Findings require inspection, not just cleaning.

Oil consumption limit

A published maximum oil use per hour. A sudden rise can indicate seal leakage into the gas path, which also raises EGT and may produce exhaust smoke.

Fuel control unit

A hydromechanical or electronic unit that meters fuel using power lever position, compressor delivery pressure, rpm and temperature to schedule acceleration and limit turbine temperature.

Torque indication

The primary power indication in a helicopter, measured at the engine output or gearbox input. Transmission torque limits, not engine limits, often govern rated power.

EGT spread

The difference between highest and lowest exhaust gas temperature readings across the turbine. A wide spread suggests partially blocked fuel nozzles or combustion problems.

Capacitor discharge ignition

A high-energy ignition system that stores charge in a capacitor and discharges it through the igniter plug. It provides reliable light-up in turbine engines.

Continuous ignition

Ignition selected continuously for relight, anti-ice operation or heavy precipitation. It is normally off after light-up because continuous use wears igniter plugs.

Light-up confirmation

A rise in EGT within a set time after fuel and ignition are selected. If no rise occurs, fuel is closed and the engine motored to clear unburnt fuel.

Starter cut-out speed

The rpm at which the engine becomes self-sustaining and the starter disengages. Before this, the starter must accelerate the gas generator to a minimum light-up speed.

Governor droop

The small decrease in governed rpm as load increases. Isopower or electronic governing reduces droop so rotor rpm stays closer to the selected value.

Overspeed protection

An independent governor or electronic limiter that cuts fuel or trips the freewheel if engine or rotor speed exceeds limits, protecting the transmission after load loss.

Main gearbox oil system

A separate lubrication system with its own pressure, scavenge, cooler, chip detector and temperature limits. Loss of pressure or a chip warning requires landing as soon as possible.

Transmission torque limit

The maximum torque the main gearbox and drive train can accept, published in the flight manual. It may be lower than the engine's maximum output.

Rotor brake

A ground-only device that holds the rotor stationary. Running the engine with it applied causes high torque and possible transmission damage.

Freewheel unit

A sprag or roller-ramp one-way clutch that transmits torque from engine to rotor and overruns when the rotor turns faster, enabling autorotation without driving the engine.

Autorotation drive path

During autorotation the rotor drives the transmission while the freewheel overruns, so no engine drag reaches the rotor. Rotor rpm is controlled by collective.

EGT exceedance record

The peak exhaust gas temperature and its duration must be recorded after an exceedance. The maintenance manual gives inspection thresholds based on that record.

Compressor wash

An on-condition maintenance task that removes deposits from compressor blades to restore mass flow and lower EGT. It must be recorded and followed by a performance check.

Maintenance release

A certification, made by a suitably licensed engineer after maintenance, that the aircraft is airworthy and ready for flight. It is required before flight following maintenance.