CAA NZ AME Licence Exam - Turbine Engines (Subject 6) common mistakes
CAA NZ AME Licence Exam - Turbine Engines (Subject 6) · Understand the traps and how to avoid them.
Confusing the roles of the compressor, combustor and turbine when asked to trace the energy conversions through the core, e.g. stating that pressure rises across the turbine or that the combustor produces shaft work.
Candidates memorise component names in sequence but never build a mental model of the Brayton cycle, so static pressure rise (compressor), constant-pressure heat addition (combustor) and pressure drop with work extraction (turbine) get shuffled under exam pressure.
What to do instead
Draw the cycle from memory once a day for a week: mark pressure, temperature and velocity at each station (intake, compressor, diffuser, combustor, NGV, turbine, exhaust) and state the energy conversion at each. Then answer past-style questions by writing the station numbers beside the stem before choosing an option.
Treating turbine entry temperature and compressor delivery temperature as interchangeable when discussing thermal efficiency, or claiming efficiency always rises with overall pressure ratio regardless of TET.
The relationship between pressure ratio, TET and specific fuel consumption is genuinely non-linear, and revision notes often compress it into a single sentence that candidates then over-apply.
What to do instead
Work three numerical examples by hand using ideal cycle relations: fix TET and vary pressure ratio, then fix pressure ratio and vary TET. Note where specific work peaks and where it falls. Learn the qualitative rule that higher pressure ratio raises thermal efficiency but useful work depends on TET and component efficiencies.
Mixing up compressor surge and rotating stall, or describing surge as a purely local blade phenomenon and stall as a whole-engine flow reversal.
Both involve flow breakdown and both appear in the same chapters, so candidates learn the labels without the distinguishing mechanism (surge = axisymmetric, whole-annulus oscillation with flow reversal; rotating stall = local cell travelling around the annulus at part speed).
What to do instead
Write a two-column comparison covering: extent of flow disturbance, whether flow reverses, typical speed range, audible/indicated symptoms, and recovery action. Attach each to a real scenario (e.g. hot-day start, rapid throttle slam, dirty compressor) so the mechanism, not the label, drives the answer.
Describing compressor blade twist and camber as uniform from root to tip, or failing to explain why blade angles change along the span.
Blade design is taught with diagrams that candidates skim; the link between tangential velocity varying with radius and the need for reduced incidence at the tip is easily missed.
What to do instead
Derive it once: at constant angular velocity, tangential speed rises with radius, so relative flow angle changes along the span. Explain that twist keeps incidence near optimum at all radii and reduces tip losses. Practise explaining this in three sentences without a diagram.
Confusing the functions of the flame tube primary, intermediate and dilution zones, or stating that dilution air cools the flame to stop combustion.
The three zones are listed together in most texts and candidates memorise names without the air-mass split and the purpose of each (primary: stabilise and anchor the flame; intermediate: complete combustion of CO and soot precursors; dilution: reduce gas temperature to a value the turbine nozzles can tolerate).
What to do instead
Build a table with zone, approximate percentage of total air, purpose and consequence of getting it wrong (e.g. too little dilution air gives high TET and nozzle distress; too much primary air gives weak, unstable flame and poor relight). Recall the table before answering any combustor question.
Answering turbine cooling questions by saying cooling air is taken from the turbine or from the exhaust, instead of identifying compressor delivery air and its effect on cycle efficiency.
Candidates know cooling air exists but not its source path or its penalty, so they guess. The trade-off (cooling permits higher TET but bypasses the combustor and costs work) is rarely stated explicitly in revision notes.
What to do instead
Trace the path from HP compressor delivery through internal passages in NGVs and blades to the gas path, and note that this air does no combustor work. Learn the two consequences: higher achievable TET raises specific work, but the cooling bleed reduces overall efficiency. Practise a question that asks you to weigh both.
Mixing up the roles of the oil pressure pump, scavenge pump and oil/fuel heat exchanger, or assuming a low oil pressure indication always means pump failure.
Lubrication is taught as a list of components. Candidates do not learn the system as a flow path with pressure and temperature relationships, so they cannot reason from a symptom to a cause.
What to do instead
Draw the oil system as a loop: tank, pressure pump, filter, cooler, bearings, scavenge, return. Annotate what each component does to pressure and temperature. Then list causes of low indicated pressure in order of likelihood (low level, blocked filter bypass, air ingress, sensor fault, pump wear) and check each against the loop.
Describing the ignition exciter as a low-energy device or confusing the continuous ignition versus starting ignition duty cycles, and mis-stating when ignition is selected automatically.
Ignition is a short chapter and candidates skim it, missing that the exciter is a high-energy capacitor-discharge unit and that continuous ignition is used for relight protection in rain, turbulence or approach idle.
What to do instead
Learn the exciter as a capacitor discharge unit producing a high-voltage pulse at the plug, and list the conditions that select continuous ignition (engine anti-ice, heavy rain, turbulence, low power settings on approach). State the difference between the starting exciter duty and continuous duty in one sentence each.
Treating FADEC as a passive monitor rather than the primary control authority, or failing to explain reversionary modes and their limitations.
Candidates learn FADEC as an acronym and a block diagram, but not the hierarchy of control (full authority digital control, hydromechanical backup, manual reversion) or what is lost in each mode.
What to do instead
Write out the control chain: inputs (N1, N2, EGT, fuel flow, TLA, bleed, altitude), the electronic engine control unit, the fuel metering unit and the actuation. Then list what degrades in each reversionary mode (e.g. loss of automatic bleed handling, fixed schedule, reduced thrust margin). Answer questions by naming the mode and its consequence.
Misreading EGT indications by confusing probe location and the effect of thermocouple placement on indicated temperature, or assuming all EGT probes read the same value.
Indicating systems are often learned as instrument names rather than as measurement chains. Candidates do not appreciate that EGT is an average of several probes and that probe position relative to the gas path affects the reading.
What to do instead
Learn the measurement chain for each parameter: sensor type, location, signal conditioning, display and the meaning of limits (start limit, take-off limit, transient limit). For EGT specifically, note that multiple probes are averaged, that a single failed probe can bias the average, and that indicated EGT depends on probe location. Practise interpreting a start EGT trend.