ACS Engine Testing & Diagnostics 2

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)?

Accepted Answer

Piston stop tool

Answer

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?

Accepted Answer

Faulty oil pressure relief valve

Answer

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?

Accepted Answer

Exhaust valve leakage

Answer

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?

Accepted Answer

Fouled spark plugs

Answer

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?

Accepted Answer

Insufficient airflow at low taxi speeds with no forward motion cooling

Answer

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

(ACS) Airman Certification Standard Certified Mechanic Practice Test

(ACS) Airman Certification Standard Certified Mechanic Practice Test

ACS Engine Testing & Diagnostics 2