CAA NZ AME Piston Engines common mistakes
CAA NZ AME Piston Engines · Understand the traps and how to avoid them.
Treating detonation and pre-ignition as the same phenomenon, so the candidate describes both as 'fuel igniting too early' and gives the same cause and cure for each.
Both are abnormal combustion with similar audible and thermal symptoms, and study material often presents them together in one paragraph, so the distinction between an ignition-timed second flame front (detonation) and ignition before the spark (pre-ignition) blurs.
What to do instead
Write a two-column comparison from memory: for detonation give the cause (end-gas auto-ignition from excessive pressure/temperature, lean mixture at high power, low octane, over-advanced timing, high inlet temperature, high load/low rpm); for pre-ignition give the cause (a hot spot such as a glowing plug electrode, carbon deposit, or overheated exhaust valve igniting the charge before the spark). State the diagnostic difference (detonation stops when mixture is enriched or timing retarded; pre-ignition continues and often survives mag-switch checks) and the consequence (detonation raises temperatures and can lead to pre-ignition; pre-ignition can destroy a piston in seconds).
Calculating brake mean effective pressure, indicated power and mechanical efficiency without keeping brake and indicated quantities separate, and using the wrong stroke count (treating a four-stroke cycle as one power stroke per revolution).
The formulas look interchangeable on a formula sheet, and candidates plug numbers in without asking whether the value is measured at the propeller shaft (brake) or derived from the indicator diagram (indicated).
What to do instead
Practise with the definitions fixed in words first: indicated power is the power developed in the cylinders; brake power is what reaches the output shaft; friction power is the difference; mechanical efficiency = brake/indicated. For a four-stroke engine use one power stroke per two revolutions, so state the factor of 1/2 explicitly in any BMEP or power calculation, and check units (N/m2, bar, kPa) before substituting. Work two numerical examples end to end and verify the answer is plausible (BMEP for a naturally aspirated piston engine is typically in the region of 8-12 bar at full power).
Describing the crankshaft, connecting rod and bearing arrangement without stating the firing order consequences and the direction of loads, for example claiming the connecting rod is only in tension or that all cylinders fire evenly regardless of arrangement.
Candidates memorise component names and forget the dynamic purpose: the crank throws, firing order and balance masses exist to spread firing impulses and control torsional and bending loads, and the rod alternates between compression and tension each cycle.
What to do instead
For each configuration (inline, horizontally opposed, radial, V) sketch the crank throws and write the firing order, then explain how the order and throw layout give even firing intervals and reduce torsional vibration. State that the connecting rod is loaded in compression on the power stroke and in tension on the induction and exhaust strokes, and that the big-end bearing carries the firing load through the oil film. Link this to the propeller: explain how firing impulses and crankshaft torsional vibration interact with propeller balance and why a damper or flexible coupling may be fitted.
Explaining piston ring function as a single 'seal' and missing the separate jobs of compression rings and oil control ring, the importance of ring end gap, and the effect of ring flutter or a broken ring on oil consumption and crankcase pressure.
Rings are visually simple, so candidates give a one-line answer and lose marks on the detail examiners want: gas loading, ring land clearance, gap measurement in the bore, and the oil ring's scraper action.
What to do instead
Build a table with three rows: compression ring (primary gas seal, gas pressure forces it against the land and bore), second/scraper ring (supplementary sealing and oil scraping), oil control ring (meters oil film, returns oil via drain holes). Add the checks: end gap measured square in the bore with a feeler gauge, side clearance in the groove, correct orientation of any marked face, and the symptoms of wear (blow-by raising crankcase pressure, oil consumption, glazed bores). Explain that ring end gap must exist to allow for thermal expansion and that too small a gap causes ring breakage.
Setting valve clearances and timing without stating when the measurement is taken (engine cold, cam on base circle, piston at the specified position) and without linking valve lead, lag and overlap to volumetric efficiency.
Candidates quote a clearance figure but omit the conditions, and treat valve timing as a set of numbers to memorise rather than a means of using gas inertia to improve cylinder filling.
What to do instead
State the procedure in order: engine cold unless the manual says otherwise, rotate to the specified position, confirm the follower is on the base circle, measure with the correct feeler gauge, adjust to the manual figure and recheck. Then explain the purpose: inlet valve lead and exhaust valve lag use charge inertia and pressure waves to improve filling and scavenging, overlap is limited by backfiring and rough idle risk, and excessive clearance causes noise, reduced lift and burnt valves while insufficient clearance risks a valve not seating. Tie the numbers to volumetric efficiency rather than reciting them.
Explaining float-type carburettor mixture control purely as 'the mixture lever moves a needle', without describing the balance of air pressure, venturi depression, float chamber venting and the idle and main metering systems, and without explaining icing and its cure.
The carburettor is taught as a diagram with labels, so candidates learn the parts but cannot explain why mixture leans or richens with altitude, throttle movement or air filter blockage.
What to do instead
Explain the chain in words: airflow through the venturi creates a depression that draws fuel through the main jet; the float chamber maintains a near-constant fuel level and must be vented to a pressure reference (impact pressure or the air intake); the mixture needle controls fuel flow; the idle system supplies fuel at closed throttle. Then work the failure cases: blocked air filter or partially closed throttle increases depression and richens the mixture; altitude reduces air density and richens unless leaned; throttle ice forms in the venturi and around the butterfly from fuel vaporisation and pressure drop, and carburettor heat is the preventive and recovery action. Link each symptom to the physical cause rather than a memorised rule.
Describing magneto operation without explaining the self-contained generation of the spark, the function of the impulse coupling, the reason for two magnetos, and the correct mag-check procedure and its limitations.
Candidates learn 'magneto makes the spark' and then answer timing and mag-check questions from habit, missing that a magneto needs no battery, that the impulse coupling retards and accelerates the magnet for easy starting, and that a mag check at low power may not reveal a fault.
What to do instead
Explain the sequence: rotating magnet induces current in the primary, the contact breaker opens the primary circuit, the rapid collapse induces a high voltage in the secondary, the distributor directs it to the correct plug. State why two independent magnetos and two plugs per cylinder are used (redundancy, faster and more complete burn, slight power gain) and how timing is set (to the specified degrees before top dead centre, checked with a timing device, not by ear). Describe the mag check: run at the specified rpm, switch to each mag in turn, note the rpm drop and roughness within limits, then back to both; explain that the check confirms each magneto can carry the load at that power setting and that a dead mag may be masked at low power or by the good mag, so a check at higher power and a check for a live mag on shutdown are also required.
Explaining the propeller governor as a simple 'set rpm' control, without describing the pilot valve, flyweights, speeder spring, oil pressure to the hub and the on-speed condition, and without distinguishing underspeed, overspeed and feathering behaviour.
Governors are treated as a black box, so candidates cannot explain what happens when load changes in a climb or descent, or why oil pressure and counterweights matter.
What to do instead
Describe the on-speed balance: flyweight force opposes speeder spring force; when rpm falls, the spring moves the pilot valve to direct oil to decrease blade angle and restore rpm; when rpm rises, flyweights move the valve to increase blade angle. State the failure modes: loss of oil pressure drives the blades to fine pitch (high rpm) on a single-acting system, hence the need for a feathering accumulator or counterweight design, and governor failure means the propeller goes to fine or coarse depending on design. Link this to the engine: manifold pressure and rpm limits, avoiding high power with low rpm (detonation risk), and the correct order of power changes.
Answering instrumentation and fire protection questions with generalities, for example 'the gauge shows oil pressure' without stating what the sensor measures, where it is tapped, what a given indication means, and how the fire detection and extinguishing system is tested and used.
These topics sit at the edge of the engine syllabus, so candidates skim them and cannot convert an indication into a diagnosis or describe a fire drill in sequence.
What to do instead
For each gauge state the parameter, the sensing point and the meaning of an abnormal reading: oil pressure tapped downstream of the pump and filter (low pressure means pump wear, relief valve stuck open, blockage or low level), oil temperature (high temperature means cooling or level problem), cylinder head temperature (high CHT means lean mixture, high power, baffle or cooling problem), exhaust gas temperature (used for mixture setting and trend monitoring), tachometer and manifold pressure (used together to set power). For fire protection, give the detection types (continuous loop, spot detector), the warning, the drill (close fuel and ignition, use the extinguisher, land as soon as possible) and the ground test procedure for the detector and extinguisher circuit.
Describing ground running and preservation as a checklist without explaining why each step is done, such as why the engine is warmed before a power check, why a magneto check is done at specified rpm, why a run-up includes a propeller cycle, and what preservation oil or inhibited oil actually protects against.
Candidates memorise a run-up card and cannot answer follow-up questions about order, limits or the consequence of skipping a step, and they confuse short-term storage with long-term preservation.
What to do instead
Write the run-up as a sequence with a reason for each item: warm-up to reach operating temperatures and clear oil; magneto check at specified rpm to load each ignition system; propeller cycle to exercise the governor and check response; idle and acceleration check for mixture and throttle response; suction and pressure checks for induction and pumps. State the limits and what an out-of-limit result means. For preservation, distinguish short-term (run to temperature, inhibit with the specified oil, seal and tag) from long-term (drain, inhibit, dehydrate, rotate or store per the manual), and explain that inhibited oil leaves a protective film on internal surfaces to prevent corrosion, which is the main threat to an idle engine.