FAA Aviation Mechanic - Powerplant (AMP) common mistakes
FAA Aviation Mechanic - Powerplant (AMP) · Understand the traps and how to avoid them.
On items about abnormal combustion, the candidate selects a glowing spark plug electrode, carbon deposit, or hot exhaust valve as the cause of detonation, or labels any pre-ignition event as detonation.
The two terms are studied together and both involve high cylinder temperatures, so the candidate collapses them into one phenomenon and then picks the 'hot spot' answer for whichever term appears in the stem.
What to do instead
Separate them by ignition source and timing: detonation is the explosive, uncontrolled burning of the end gas after normal spark ignition, driven by excessive temperature and pressure (lean mixture, high manifold pressure, high inlet temperature, low octane); pre-ignition is ignition of the charge before the spark, caused by a hot spot such as a glowing plug, carbon, or overheated valve. Read the stem for which one it names before looking at the options.
Treats exhaust gas temperature, turbine inlet temperature, and interturbine temperature as one quantity and applies whichever limit in the manual happens to be highest, or answers a limit question without asking where the probe is installed.
All three appear in the same turbine engine section as 'temperature limits,' and candidates memorize a number without tying it to the probe location that produces it.
What to do instead
Anchor each term to its probe station: exhaust gas temperature is measured downstream in the exhaust, turbine inlet temperature at the turbine inlet (the highest of the three), and interturbine temperature between turbine stages. The applicable limit belongs to the specific gauge and probe installation in that engine's manual, so match the stem's gauge name to the limit rather than picking the largest number.
Marks a manufacturer service bulletin as 'must be accomplished before further flight' or otherwise treats it as legally mandatory, or conversely treats an airworthiness directive as a recommendation.
Both documents arrive as numbered, model-specific maintenance instructions and both are commonly complied with in practice, so the regulatory distinction gets lost.
What to do instead
Fix the rule in one line: an AD is issued or adopted by the FAA and is enforceable by regulation; a service bulletin is the manufacturer's recommendation and becomes mandatory only when an AD incorporates it by reference. In the stem, look for who issued it and whether an AD number is cited.
Assumes that increasing sender resistance always drives the gauge reading up (or always down) regardless of the instrument circuit, and answers troubleshooting questions on that single rule.
The candidate learns one troubleshooting example, usually a simple series circuit, and generalizes it to every temperature, pressure, and quantity gauge on the aircraft.
What to do instead
Classify the circuit first. In a simple series circuit, more sender resistance means less current and a lower indication. In a ratio or bridge-type instrument, the sender is one leg balanced against a reference winding and the pointer follows the ratio of the two currents. State which circuit the stem describes before deciding the direction of pointer travel.
After a satisfactory cockpit fire-warning test, signs the system off as fully functional without separately verifying detector loop continuity, bottle weight or pressure, and squib circuit condition.
The test switch produces the same warning light and horn the crew would see in a real fire, so it feels like a complete system check.
What to do instead
Treat the test as proof of the warning circuit only. Finish the inspection with the separate checks the manual calls for: detector loop continuity and routing, bottle weight or pressure within limits, discharge cartridge or squib condition, and wiring continuity to the bottle. Write the test and the physical checks as two distinct sign-offs.
Confuses the oil pressure relief valve with the oil cooler bypass valve, for example stating that the relief valve opens on a cold start to route oil around the cooler or that the bypass valve limits system pressure.
Both are spring-loaded valves in the same lubrication schematic and both 'open under some condition,' so the candidate assigns the wrong condition to each.
What to do instead
Assign one job to each: the pressure relief valve regulates system pressure by dumping excess oil back to the sump or inlet side; the cooler bypass valve (often thermostatic) routes oil around the cooler when the oil is cold and thick or when the cooler is restricted, protecting the cooler from overpressure. Match the stem's stated condition (cold oil, high pressure, blocked cooler) to the correct valve.
States that the impulse coupling advances ignition timing, or that it is active throughout the engine's operating speed range.
The impulse coupling is described as a 'starting aid' that improves spark, and candidates assume improved starting means advanced timing.
What to do instead
Recall the actual behaviour: at low cranking speed the coupling retards the spark to near top dead center and simultaneously winds up a spring to snap the magneto past the E-gap for a hot spark; centrifugal action locks it out above a set speed so normal timing returns. Any option containing 'advance' for the impulse coupling is wrong.
Answers that a float-type carburetor meters a progressively leaner mixture as altitude increases, or that leaning is unnecessary at altitude because the carburetor compensates.
The reasoning 'less air up high means less fuel needed' feels correct and ignores how the venturi metering force actually changes with density.
What to do instead
Work it from the metering force: lower air density reduces the pressure drop at the venturi, but fuel flow does not fall in the same proportion, so the mixture becomes richer as altitude increases and must be leaned by the pilot. Remember the float carburetor has no automatic altitude compensation; only a pressure carburetor or fuel control with a bellows or aneroid provides that.
Chooses opening the cowl flaps during cruise for maximum cooling, or leaves them open after level-off, rather than closing them.
The candidate associates 'more cooling is always safer' with cylinder head temperature and forgets the drag and even-cooling penalties.
What to do instead
Tie cowl flap position to flight phase: open for ground operation, takeoff, and climb where power is high and airflow is low; closed in cruise and descent to reduce drag and to keep cylinder head temperatures in the normal range, since wide-open flaps in cruise over-cool the rear cylinders and can cause uneven temperatures. The correct answer is the one that holds CHT within limits, not the one with the most airflow.
Reverses the propeller pitch-to-rpm relationship, answering that the governor increases blade angle to raise rpm or that a high-pitch setting produces a high rpm.
Pitch is intuitively tied to 'speed,' and candidates carry that intuition over from aircraft airspeed instead of from blade load on the engine.
What to do instead
Fix one sentence: more blade angle means more load on the engine, so rpm falls; less blade angle means less load, so rpm rises. The governor therefore increases pitch to bring an overspeeding engine back down to the selected rpm and decreases pitch to raise rpm when the engine is below the setting. On-speed means the governor is holding, with no need to change pitch.