FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
During a ground run following propeller governor reinstallation, a mechanic notices that the engine RPM exceeds the maximum allowable limit even though the cockpit propeller control is at the full increase-RPM position and the governor is functioning normally in flight-range control. What is the most likely cause?
- AThe oil pressure to the propeller dome is excessive
- BThe governor's speeder spring tension is set too low
- CThe propeller's low-pitch (fixed-pitch) stop is allowing the blades to go below minimum blade angle
- DThe magneto timing is advanced beyond specification
Show answer & explanationAnswer & explanation
Correct answer: C. The propeller's low-pitch (fixed-pitch) stop is allowing the blades to go below minimum blade angle
On a constant-speed propeller, if the mechanical low-pitch stop does not hold the blades at the correct minimum blade angle, the blades can flatten further than designed, reducing load on the engine and allowing RPM to overspeed even with the governor commanding maximum RPM correctly.
Why the other options are wrong
- A. Excessive oil pressure to the dome would move blades toward higher pitch, not cause overspeed.
- B. Low speeder spring tension would cause a lower governed RPM, not an overspeed beyond the stop.
- D. Magneto timing affects combustion timing, not propeller blade angle or RPM governing.
Propeller Low-Pitch Stop
A mechanical stop that prevents constant-speed propeller blades from moving below the minimum design blade angle, protecting against ground/engine overspeed.
- Works with the governor to limit blade travel range
- Failure allows blades to flatten too much, causing overspeed
- Distinct from the governor's speeder spring, which sets governed RPM
Memory trick: If the stop fails, the blade goes flat and the engine screams past its limit.