FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A mechanic is troubleshooting a constant-speed propeller system. If the engine oil supply to the propeller dome is interrupted, what is the most likely immediate effect on propeller pitch?
- AThe propeller pitch will remain unchanged.
- BThe propeller will continuously cycle between fine and coarse pitch.
- CThe propeller will move to a finer pitch.
- DThe propeller will move to a coarser pitch.
Show answer & explanationAnswer & explanation
Correct answer: D. The propeller will move to a coarser pitch.
Constant-speed propellers are designed to fail-safe into a coarse pitch (high RPM) due to aerodynamic twisting forces and spring pressure in many designs, or counterweights, when oil pressure is lost. This allows for controlled descent or glide.
Why the other options are wrong
- A. Without oil pressure, the pitch will change as the system loses its controlling force.
- B. Continuous cycling is more indicative of a governor malfunction with fluctuating oil pressure, not a complete interruption.
- C. Moving to a finer pitch would cause overspeeding and is not the typical fail-safe design.
Constant-Speed Propeller Fail-Safe
A design feature causing a constant-speed propeller to move to a coarser pitch in the event of oil pressure loss, ensuring a safe, controllable condition.
- Achieved by counterweights, springs, or aerodynamic twisting forces.
- Coarse pitch reduces drag and RPM, preventing overspeed.
- Critical for engine-out procedures.
Memory trick: No oil, no control, prop goes coarse to 'cruise' safe.