FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesEasy
During a pre-flight inspection, a pilot notices a small amount of oil leaking from the propeller hub on a constant-speed propeller. This oil is most likely originating from which component?
- APropeller governor.
- BEngine oil sump.
- CEngine crankcase breather.
- DPropeller dome seal.
Show answer & explanationAnswer & explanation
Correct answer: D. Propeller dome seal.
Constant-speed propellers use engine oil under pressure to change propeller blade angle. A leak from the propeller hub indicates a failure of one of the seals within the propeller assembly itself, most commonly the propeller dome seal.
Why the other options are wrong
- A. The propeller governor controls oil flow to the propeller, but a leak from the governor itself would be at the engine-mounted unit, not the propeller hub.
- B. The engine oil sump is where engine oil is stored; a leak from there would be on the engine, not the propeller hub.
- C. The engine crankcase breather vents crankcase pressure and would not typically leak oil directly from the propeller hub.
Propeller Dome Seal
A critical seal within the propeller hub assembly that prevents engine oil, used for pitch change, from leaking out.
- Located in constant-speed propeller hub.
- Seals oil for pitch control.
- Failure causes oil leaks from hub.
Memory trick: The dome holds the oil, so a leak means the dome's seal is failing its role.