CAA NZ Basic Turbine Knowledge (Aeroplane) common mistakes
CAA NZ Basic Turbine Knowledge (Aeroplane) · Understand the traps and how to avoid them.
Treating the Brayton cycle as a closed four-stroke-like sequence and describing 'combustion at constant volume' or 'power stroke' in turbine answers, or confusing compressor pressure ratio with overall pressure ratio.
Candidates carry over piston-engine mental models from light aeroplane study and never rebuild the cycle as continuous flow: intake/compression, constant-pressure heat addition, expansion, exhaust. The constant-pressure combustor is the single most-tested distinction and is easily glossed over.
What to do instead
Write out the cycle from memory with station numbers (1 inlet, 2 compressor face, 3 compressor exit, 4 turbine inlet, 5 turbine exit, 8 exhaust nozzle) and state pressure and temperature behaviour at each station. Practise explaining why constant-pressure heat addition is used and where the useful work is extracted. Test yourself by sketching T-s and P-V diagrams and labelling them without notes.
Describing axial compressor stall and surge as the same phenomenon, or stating that bleed valves open to increase surge margin without explaining the low-speed/low-pressure-ratio reason.
Both involve flow breakdown and disrupted pressure rise, so the labels blur. Candidates also memorise 'bleed valves open at low power' without linking it to front-stage incidence angles and rear-stage choking.
What to do instead
Build a two-column comparison: surge = complete breakdown of flow, loud bang, large axial flow reversal, EGT rise; stall = localised separation on some blades, rumble, reduced efficiency. Then explain the bleed-valve logic: at low corrected speed the front stages run at high incidence while rear stages are choked, so air is bled to re-match stage velocities. Practise with variable stator vane (VSV) scheduling as a second surge-margin device.
Naming a single oil type or grade for all turbine engines, or claiming oil is only for lubrication, ignoring its cooling and sealing roles and the consequences of overfilling or underfilling the tank.
Piston-engine habits (one familiar grade) and simplified training material lead to generic answers. The dual-purpose nature of turbine oil and the tank's expansion space are easy to overlook.
What to do instead
Learn the standard specification families (e.g. MIL-PRF-23699 and equivalent civil grades) and that the engine/aircraft manual, not memory, determines the approved oil. State the three roles: lubrication, heat removal from bearings and accessory gearbox, and sealing via pressurised bearing chambers. Explain why the tank needs expansion space and how overfilling leads to breather loss and low oil indication, while underfilling risks starvation during prolonged negative-g or high-altitude operation.
Confusing the functions of the fuel control unit, the fuel metering unit, and the FADEC/EEC, and describing FADEC as 'electronic fuel injection' or as a system that removes the need for a hydromechanical backup.
The control chain is layered and the terminology overlaps across engine families. Candidates also assume modern engines are fully electronic with no hydromechanical element, which is wrong on most engines where the EEC commands and a hydromechanical unit meters.
What to do instead
Draw the chain: pilot input and engine sensors → EEC/FADEC computes demand → fuel metering unit or hydromechanical control schedules fuel flow, with the hydro-mechanical unit providing governing and overspeed protection. Learn what each sensor contributes (N1, N2, EGT, inlet total temperature and pressure, bleed status) and what happens on loss of EEC power (reversion to a backup or hydromechanical mode, fixed or scheduled fuel flow, reduced thrust). Practise describing a start sequence in terms of fuel scheduling and ignition timing.
Describing ignition and starting as one event, or stating that igniters run continuously in all phases, or that a starter motor turns the engine to self-sustaining speed without mentioning light-off and acceleration monitoring.
Start is a sequence of overlapping events and candidates compress it into 'press start'. The distinction between ignition during start, continuous ignition for anti-ice or heavy rain, and relight envelope is seldom memorised precisely.
What to do instead
Memorise the sequence with the parameters to watch: starter cranks N2 to the required motoring speed, ignition energises, fuel is introduced, light-off is confirmed by EGT rise, the starter disengages at a set N2, and the engine accelerates to idle within a time limit. Learn the hot-start, hung-start, and wet-start signatures and the crew action for each (fuel off, continue motoring, investigate before further attempt). Note when continuous ignition is selected and why.
Reading EGT as a direct temperature of the turbine blades or treating an exceedance as a transient event with no lasting consequence, and failing to state what to record after an exceedance.
Indications are taught as dials rather than as limits with certification meaning. Candidates underestimate that EGT exceedance consumes blade life and must be recorded even if the engine appears normal.
What to do instead
Learn what each indication represents: EGT measured at turbine inlet or between turbine stages, N1 and N2 as percentage of design speed, fuel flow, torque or engine pressure ratio for thrust, vibration as an advisory. For any exceedance, state the action: reduce power, land or shut down as the manual directs, record peak value and duration, and report to maintenance. Practise interpreting a start with a slow EGT rise or a rapid rise past the limit.
Assuming reverse thrust is available at any speed and on any surface, or describing reversers as producing reverse thrust from the fan or propeller rather than redirecting existing exhaust or fan flow.
Reverser operation is taught as a lever movement rather than a system with interlocks. Candidates miss that reversers redirect flow, are interlocked with thrust lever position and airframe squat or weight-on-wheels, and are limited to low-speed use.
What to do instead
Learn the reverser types (cascade, bucket/target, cold-stream) and the mechanism: translating sleeves or buckets redirect fan or exhaust flow forward. State the interlocks: thrust levers at idle, weight-on-wheels or squat switch, and the deploy/stow sequence with an EICAS or annunciator indication. Explain the prohibition on use below a specified speed, the FOD risk on unprepared surfaces, and the handling after stow before applying forward thrust.
Treating derate and assumed temperature as the same thing, or claiming a derated take-off reduces the certified maximum thrust the engine can produce, or that performance is unaffected by altitude and temperature.
Both reduce thrust but act differently: a derate selects a lower rated thrust level, while an assumed temperature method uses a higher fictitious temperature to reduce thrust within the rated level. Candidates also forget that thrust falls with density altitude and that flat-rated engines hold thrust only to a break temperature.
What to do instead
Build a comparison: derate = fixed lower rating selected before take-off; assumed temperature = thrust reduction calculated from a higher assumed OAT. Explain flat rating: thrust is constant up to the break temperature and then decreases. Work through why high altitude and high temperature reduce available thrust and increase take-off distance, and why a derate must be recalculated if conditions change. Learn the meaning of maximum continuous, take-off, and transient ratings and their time limits.
Confusing APU bleed air with main engine bleed air in the anti-ice and air-conditioning context, or stating that bleed air is taken only from the high-pressure compressor without explaining the pressure-regulating and shut-off valve arrangement.
The pneumatic system is a network of sources and consumers, and the APU, engines, and ground cart all feed the same manifold. Candidates also overlook that bleed extraction reduces engine thrust and increases EGT.
What to do instead
Trace the pneumatic system: engine HP and LP bleed ports, bleed valves and pressure regulators, APU load compressor or bleed, ground connection, and the consumers (air-conditioning packs, wing and engine anti-ice, starting). Learn the auto-switching between HP and LP ports with power setting, and the effect of bleed extraction on performance. Practise explaining what happens to available thrust and EGT when wing anti-ice is selected on take-off.
Answering maintenance and defect-reporting questions with generic 'inform maintenance' statements, omitting the specific defect entry, the difference between a defect and a permitted deferral, and the role of the approved maintenance organisation and the technical log.
Candidates treat airworthiness as paperwork rather than a decision chain. They also mix regulatory roles: pilot reporting, maintenance release, and the authority's rules on deferrable defects.
What to do instead
Learn the chain: defect observed → entry in the technical log or defect record with the symptom, conditions, and any exceedance data → assessment against the minimum equipment list or configuration deviation list → maintenance release by an approved organisation → return to service. Distinguish a defect that grounds the aircraft from one deferrable under the MEL with operational conditions. Practise writing a defect description that a maintainer could act on, including N1, N2, EGT, fuel flow, and vibration values at the time.