FAA Commercial Pilot Airplane (CAX)Aircraft SystemsMedium
A single-engine airplane is equipped with a counterweight-type constant-speed propeller, in which governor oil pressure drives the blades toward low pitch and counterweights (assisted by a spring) drive the blades toward high pitch. If the engine-driven oil pump fails in flight, what will most likely happen to the propeller?
- AThe propeller will move toward a higher blade angle, resulting in reduced RPM
- BThe propeller will move toward full feather, stopping the engine completely
- CThe propeller will move toward full low pitch, causing an engine overspeed
- DThe propeller blade angle will remain fixed at its last commanded position
Show answer & explanationAnswer & explanation
Correct answer: A. The propeller will move toward a higher blade angle, resulting in reduced RPM
With no oil pressure available to hold the blades at low pitch, the counterweights (and spring force) move the blades toward a higher (coarser) pitch angle, which decreases RPM. This design prevents a dangerous overspeed condition when oil pressure is lost.
Why the other options are wrong
- B. Most single-engine trainers are not full-feathering; the blades only move toward high pitch, not full feather.
- C. This is backwards; loss of oil pressure removes the force that holds low pitch, so it cannot cause overspeed.
- D. Without oil pressure holding it in place, the blade angle will change, not remain fixed.
Counterweight Propeller Oil Failure
In a counterweight-type propeller, loss of governor oil pressure allows counterweights and spring force to drive blades toward high pitch, decreasing RPM.
- Oil pressure = low pitch (fine)
- Counterweights/spring = high pitch (coarse)
- Oil loss results in reduced RPM, not overspeed
Memory trick: No oil, no fine pitch — counterweights coarsen the blades and calm the RPM.