ACS Engine Testing & Diagnostics 2 — Questions and Answers
Question 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)?
- Timing light
- Piston stop tool (Correct answer)
- Compression tester
- Feeler gauge
Correct 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.
Question 2: An aircraft engine exhibits low oil pressure during ground run-up but normal pressure in flight. What is the most likely cause?
- Worn main bearings
- Faulty oil pressure relief valve (Correct answer)
- Diluted oil from fuel contamination
- Clogged oil suction screen
Correct 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.
Question 3: What does a negative differential compression reading between cylinder top and side ports indicate?
- Exhaust valve leakage (Correct answer)
- Intake valve leakage
- Piston ring leakage
- Head gasket failure
Correct answer: Exhaust valve leakage
Air escaping into the exhaust stack during a differential compression test indicates a leaking exhaust valve.
Question 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?
- Fouled spark plugs (Correct answer)
- Enriched mixture setting
- Retarded ignition timing on both magnetos
- Carburetor air leak
Correct answer: Fouled spark plugs
Fouled or carbon-bridged spark plugs are the most common cause of excessive RPM drop on both magneto checks simultaneously.
Question 5: Engine oil temperature rises above limits during taxi but returns to normal during climb. What is the most probable explanation?
- Insufficient airflow at low taxi speeds with no forward motion cooling (Correct answer)
- Defective oil cooler thermostat stuck closed
- Oil pump cavitation at low RPM
- Incorrect oil viscosity grade
Correct 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.
Question 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?
- Magneto timing advanced beyond specification (Correct answer)
- Oil viscosity changed to a lighter grade
- Fuel injector nozzles cleaned
- Primer system reconnected
Correct 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.
Question 7: During engine run-up, a pilot reports intermittent engine roughness that disappears when carburetor heat is applied. What does this indicate?
- Ice formation in the carburetor venturi (Correct answer)
- Lean fuel-air mixture
- Fouled spark plugs
- Cracked intake manifold
Correct 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.
ACS Engine Testing & Diagnostics 2
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)?