CAA NZ Basic Turbine Knowledge (Helicopter) Exam cheat sheet

Brayton (constant-pressure) cycle: intake compression, constant-pressure combustion, expansion through turbine, exhaust; thermal efficiency rises with pressure ratio and turbine entry temperature.

Ideal compressor work = cp(T2−T1); turbine work = cp(T3−T4); useful shaft work is turbine work minus compressor work, so every compressor efficiency gain directly increases net output.

Isentropic relation T2/T1 = (P2/P1)^((γ−1)/γ); for air γ ≈ 1.4, giving (γ−1)/γ ≈ 0.286.

Specific fuel consumption = fuel mass flow ÷ net shaft power (kg/kWh); it falls as pressure ratio and TET rise, but rises steeply at part-power.

Turboshaft output is nearly constant-speed shaft power; free-turbine (free power turbine) engines let rotor rpm vary independently of gas generator rpm.

Helicopter turboshafts are usually rated as take-off (5 min), maximum continuous, and occasionally 30-min or emergency power; each has its own torque/temperature limit.

Thermal efficiency ≈ 1 − 1/(pressure ratio)^0.286 for the ideal cycle; real engines lose to compressor/turbine inefficiency, pressure loss and cooling bleed.

Power available falls with altitude and temperature because mass flow falls; density altitude, not pressure altitude alone, governs performance.

Typical turboshaft modules: inlet/accessory gearbox, compressor, combustion section, gas generator turbine, free power turbine, exhaust, and reduction gearbox.

A free-turbine engine has no mechanical coupling between gas generator and power turbine; the power turbine drives the output shaft through its own bearing and reduction gearing.

Case splits, flange joints and splined couplings are torque-critical: manufacturer torque values and sequences apply, and locking devices must be replaced, not reused, where specified.

Hot-section parts (combustion liner, nozzle guide vanes, turbine blades) are life-limited by cycles or hours; life consumption is recorded against the part, not the engine.

Compressor bleed valves, anti-ice valves and variable geometry are externally mounted and can cause large power loss if stuck open or closed.

Engine mounts carry thrust and torque reaction; a failed or loose mount shows as abnormal vibration and can allow exhaust/tailpipe contact with airframe structure.

Borescope ports on combustion and turbine cases allow internal inspection without splitting the engine; findings are compared with the maintenance manual limits.

Centrifugal compressors give high pressure ratio per stage with a wide operating band; axial compressors give higher mass flow and efficiency in more, smaller stages.

Surge is a violent reversal of flow, heard as bangs with fluctuating rpm and high EGT; rotating stall is a local, less severe form that can precede surge.

Compressor map: surge line on the left, choke line on the right; acceleration bleed valves and variable inlet guide vanes keep the operating point clear of the surge line at low rpm.

Pressure ratio per stage is modest (about 1.2–1.4 axial); total pressure ratio is the product of stage ratios, not the sum.

Compressor fouling from dirt, salt and insect ingestion reduces mass flow and raises EGT for the same power; washing restores performance if caught early.

Foreign object damage to blades causes imbalance, vibration and possible surge; blended or replaced blades must meet limits in the maintenance manual.

Compressor bleed air supplies anti-ice, cabin heating and engine starting; excessive bleed demand reduces available power.

Combustion section: diffuser slows air, fuel is atomised and mixed, primary zone burns at near-stoichiometric ratio, dilution air cools gas to a turbine-safe temperature.

Annular combustors dominate modern turboshafts; can-annular and can types exist on older engines, with different flameout and relight characteristics.

Turbine entry temperature exceeds the melting point of the blade alloy; blade cooling (film, impingement, internal passages) and thermal barrier coatings are essential.

Nozzle guide vanes accelerate and turn hot gas onto the turbine rotor; they run hotter than the blades and often need replacement first.

Turbine blade creep and stress rupture are life-limited by temperature and time; exceeding TET limits consumes life even without visible damage.

EGT (T4/T5) is measured downstream of the turbine and lags actual combustion temperature; rapid EGT rise on start is the classic hot-start warning.

Flameout causes rpm and EGT to fall together; a hung start shows rpm stalled below idle with EGT rising, requiring immediate fuel shut-off.

Turbine oils are synthetic (MIL-PRF-23699 class); they are not interchangeable with piston-engine ashless dispersant oils, and mixing grades requires manufacturer approval.

Dry-sump system: pressure pump feeds bearings and gearbox jets, scavenge pumps return oil to tank, oil cooler controls temperature, chip detectors monitor wear.

Oil pressure must reach a minimum before start completion; low pressure at idle can indicate pump wear, airlock, or a blocked filter bypassing.

Chip detectors (magnetic plugs) and filter bypass indicators are the primary early warning of bearing or gear distress; findings require inspection, not just cleaning.

Oil consumption limits are published per hour; a sudden rise can indicate seal leakage into the gas path, which also raises EGT and can cause smoke.

Oil sample analysis (spectrometry) tracks iron, copper, aluminium and silicon trends; a single sample rarely condemns an engine, a trend does.

Oil level is checked with the engine stopped and within a specified time after shutdown; hot oil expands, so a hot check reads high.

Fuel control unit meters fuel by combining throttle (power lever) position, compressor delivery pressure (P3), rpm (N1/N2) and temperature to schedule acceleration and limit TET.

Hydromechanical units use bellows, diaphragms and servo valves; electronic engine control (EEC/FADEC) adds digital scheduling and fault recording on newer types.

Fuel nozzles atomise fuel for combustion; partially blocked nozzles create hot streaks and uneven EGT spread, visible as a wide EGT spread at high power.

Fuel contamination (water, ice, microbiological growth) causes flameout or blocked filters; pre-flight drains and filter differential-pressure indications are the defence.

Indications to cross-check: torque (or N2) as power, N1 gas generator rpm, T4/T5 EGT, fuel flow, oil pressure and temperature.

Torque is measured by a torque meter (e.g. oil-filled or strain-gauge shaft) and is the primary power indication for rotorcraft power limits.

Fuel flow divided by torque gives a trend indicator; a rising fuel flow for the same torque suggests compressor or turbine deterioration.

Turbine ignition is high-energy (capacitor discharge) exciter and igniter plug; energy is stored in a capacitor and discharged as a high-voltage spark.

Igniters are usually two per engine (or one igniter plus one spare port); normally only one is used for start, both for relight in adverse conditions.

Ignition is normally switched off after light-up and only re-selected for relight, anti-ice, or flight in heavy precipitation; continuous ignition wears plugs.

Igniter plug fouling or a failed exciter shows as no light-up (no EGT rise) with fuel flowing; check for wet igniter and spark before assuming fuel fault.

Ignition leads carry high voltage and can retain charge; observe the manufacturer's discharge/wait time before handling.

On start, light-up is confirmed by EGT rise within a set time (typically seconds) and N1 rise; if no rise, close the fuel and motor the engine to clear fuel.

Start sequence: starter motor cranks N1 to a minimum (e.g. 10–15%), fuel on, ignition on, light-up, starter cut-out at self-sustaining rpm, then acceleration to idle.

Hot start: EGT limit exceeded because fuel is introduced too early or too much; action is fuel off, continue motoring to cool, investigate before retry.

Hung start: rpm fails to accelerate to idle, often from weak starter, low battery/bleed air, or fuel scheduling fault; fuel off and motor.

Wet start: fuel introduced with no ignition; excess fuel in the combustion chamber can cause an explosive relight, so motor the engine to purge before retry.

FADEC/EEC start protection automatically cuts fuel if EGT or time limits are exceeded; the crew still monitors and can abort.

Governor and fuel control keep N2 (or power turbine) at the selected speed; droop is the small rpm drop with increasing load, and isopower governing reduces it.

Overspeed protection: independent overspeed governor or electronic limiter trips fuel or the freewheel, preventing rotor/engine overspeed on load loss.

Drive path: engine output shaft → freewheel unit → main gearbox input → main rotor mast and tail take-off; each stage has its own torque limit.

Main gearbox is usually oil-lubricated with its own pressure and scavenge system, chip detector, and temperature limits that differ from the engine's.

Torque limits are airframe limits, not engine limits: transmission torque (measured at the engine output or MGB input) governs rated power in helicopters.

Tail rotor drive uses a shaft with flexible couplings and intermediate gearboxes; failure modes include shaft whirling, coupling wear and bearing seizure.

MGB oil pressure loss or chip warning requires land as soon as possible; continued flight can lead to gear seizure and loss of drive.

Rotor brake is used on the ground only; attempting to start or run the engine with the rotor brake on causes high torque and possible damage.

Transmission oil temperature and pressure are separate indications from engine oil; both must be monitored during start and hover checks.

Freewheel unit (sprag or roller ramp) transmits torque engine-to-rotor and overruns when the rotor turns faster than the engine, allowing autorotation.

Freewheel engagement is one-way: during autorotation the engine can be at idle or off while the rotor drives the transmission, so no engine drag reaches the rotor.

A seized freewheel prevents autorotation and can cause engine/rotor overspeed mismatch; a slipping freewheel gives no drive above a certain torque.

Rotor rpm (NR) is governed by the engine fuel control in powered flight and by the pilot (collective) in autorotation; NR limits are airframe limits.

Clutch (on some light helicopters) engages the rotor progressively; slipping clutch shows as delayed rotor acceleration and smell/heat on start.

Rotor overspeed on engine failure is limited by the freewheel and by pilot collective lowering; the freewheel protects the engine, not the rotor.

Freewheel unit inspection includes wear on sprags/rollers, correct lubrication, and end-float; it is a life-limited or on-condition item per the manual.

Limits are published in the flight manual as torque, N1/N2, T4/T5 EGT, oil pressure/temperature and NR; exceedance requires a report and inspection.

EGT exceedance is cumulative: record the peak value and duration, then consult the maintenance manual for inspection thresholds.

Torque exceedance is treated as a transmission event; the MGB and drive train may need inspection even if the engine is unaffected.

Abnormal signs: high vibration, unusual noise, EGT rising with falling torque, oil pressure fluctuation, chip warning, smoke or fuel smell.

Compressor surge in flight gives bangs and power loss; action is to reduce collective/power and land as soon as practicable.

Engine fire indications call for fuel shut-off, ignition off, and, if fitted, fire bottle discharge; land immediately.

Autorotation entry after power loss: lower collective, maintain NR, select a landing area; the freewheel allows the rotor to keep turning.

Maintenance is controlled by the approved maintenance programme and the manufacturer's maintenance manual; tasks are recorded in the aircraft logbook and engine logbook.

Life-limited parts are tracked by hours or cycles from new or last overhaul; records must be traceable and the part must be removed before the limit.

Torque values, sequences and locking methods come from the manual; safety wire, tab washers and self-locking nuts are replaced per specification.

Borescope inspection of combustion and turbine sections is a scheduled task; findings are assessed against the manual's acceptance criteria.

Compressor wash and fuel nozzle cleaning are on-condition tasks used to restore performance; they must be recorded and followed by a performance check.

After any engine or transmission disturbance, ground run checks include idle, acceleration, oil pressure, chip detector and leak inspection.

Parts must be approved (OEM or approved PMA) and traceable; unapproved parts are a continuing airworthiness risk.

CAA NZ rules: Part 91 for general operating rules, Part 135 for air transport operations, Part 145 for maintenance organisations, and Part 43 for maintenance and inspection.

An airworthiness certificate is valid only while the aircraft conforms to its type design and is maintained per the approved programme.

Defect reporting: serious defects and incidents are reported to the CAA; the operator keeps a defect record and the maintenance organisation clears them.

Maintenance release is required before flight after maintenance; the certifying engineer holds the appropriate licence and rating.

Fuel, oil and parts must meet the type design specification; using unapproved fuel or oil can void airworthiness and damage the engine.

Engine and transmission logbooks must be carried or available as required, and entries must be made for all maintenance and life-limited part changes.

Pre-flight inspection covers engine intake, exhaust, oil and fuel levels, chip detector indications and transmission leaks; it is a legal requirement, not optional.