FAA Aviation Mechanic Powerplant (AMP)PropellersHard
A hydraulic constant-speed propeller on a turboprop engine includes a pitch-lock mechanism. Under which condition does this mechanism engage to prevent the blades from moving toward a lower (finer) pitch angle?
- AWhen the synchrophaser detects a phase mismatch between engines
- BWhen oil pressure to the propeller dome is lost and the propeller begins to overspeed
- CWhen the throttle is retarded to idle during descent
- DWhen the aircraft touches down and reverse thrust is selected
Show answer & explanationAnswer & explanation
Correct answer: B. When oil pressure to the propeller dome is lost and the propeller begins to overspeed
The pitch-lock mechanism engages automatically when governing oil pressure is lost and the propeller starts to overspeed toward low pitch, mechanically locking the blades at their current angle to prevent an uncontrolled overspeed. Reverse thrust, synchrophasing, and throttle position are unrelated triggers.
Why the other options are wrong
- A. Synchrophasing is unrelated to pitch-lock engagement.
- C. Normal throttle retardation does not trigger the safety lock.
- D. Reverse thrust uses the Beta system, not the pitch lock.
Pitch-Lock Mechanism
A safety device on some hydraulic constant-speed propellers that mechanically locks blade angle if governor oil pressure is lost and the propeller begins to overspeed, preventing further pitch decrease.
- Engages on overspeed with oil pressure loss
- Locks blades at current angle
- Prevents uncontrolled fine-pitch overspeed
- Found on some turboprop hydraulic propellers
Memory trick: No oil, no overspeed — the pitch lock freezes the blades in place.