Practice Test Geeks(ACS) Airman Certification Standard Certified Mechanic Practice Test
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Engine Testing & Diagnostics Flashcards

7 cards from real ACS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Engine Testing & Diagnostics flashcards as text

  1. During a magneto timing check, the engine is rotated to find the firing position. What tool is commonly used to locate top dead center (TDC)?

    Answer: Piston stop tool

    A piston stop tool is threaded into the spark plug hole to precisely locate TDC by stopping piston travel at a known point.

  2. An aircraft engine exhibits low oil pressure during ground run-up but normal pressure in flight. What is the most likely cause?

    Answer: Faulty oil pressure relief valve

    A stuck-open oil pressure relief valve can allow excess oil to bypass at low RPM, causing low pressure at idle that normalizes at higher power settings.

  3. What does a negative differential compression reading between cylinder top and side ports indicate?

    Answer: Exhaust valve leakage

    Air escaping into the exhaust stack during a differential compression test indicates a leaking exhaust valve.

  4. When performing a static RPM run-up, the engine reaches maximum RPM but both magneto checks show a greater-than-normal drop. What should the mechanic suspect first?

    Answer: Fouled spark plugs

    Fouled or carbon-bridged spark plugs are the most common cause of excessive RPM drop on both magneto checks simultaneously.

  5. Engine oil temperature rises above limits during taxi but returns to normal during climb. What is the most probable explanation?

    Answer: Insufficient airflow at low taxi speeds with no forward motion cooling

    Air-cooled engines rely heavily on forward airspeed for cooling; taxiing at low speed reduces airflow over cylinders and oil cooler significantly.

  6. A reciprocating engine's tachometer shows 50 RPM higher than normal at a fixed throttle setting after maintenance. What adjustment was most likely made incorrectly?

    Answer: Magneto timing advanced beyond specification

    Advancing magneto timing beyond specification causes the engine to produce more power at a given throttle setting, resulting in higher RPM.

  7. During engine run-up, a pilot reports intermittent engine roughness that disappears when carburetor heat is applied. What does this indicate?

    Answer: Ice formation in the carburetor venturi

    Carburetor ice causes rough running and partial power loss; applying carb heat melts the ice and restores smooth operation.