FAA Aviation Mechanic Powerplant (AMP)PropellersHard
A technician is troubleshooting a counterweight-type constant-speed propeller. If governor oil pressure to the propeller is lost, what happens to the blade pitch?
- AThe blades remain fixed at their last commanded pitch until oil pressure is restored
- BSpring force alone drives the blades to full feather immediately
- CThe counterweights move the blades toward high (increase) pitch, feathering the propeller
- DThe counterweights move the blades toward low (decrease) pitch, causing an overspeed condition
Show answer & explanationAnswer & explanation
Correct answer: D. The counterweights move the blades toward low (decrease) pitch, causing an overspeed condition
In counterweight-type constant-speed propellers, governor oil pressure moves the blades toward high (increase) pitch, while counterweights use centrifugal twisting moment to drive blades toward low pitch. A loss of oil pressure allows the counterweights to swing the blades toward low pitch, which can lead to an overspeed condition.
Why the other options are wrong
- A. Blades do not stay fixed; counterweights actively move them without oil pressure.
- B. This design does not feather on oil pressure loss; it moves toward low pitch instead.
- C. This describes non-counterweight (oil-to-increase-rpm) designs, not counterweight-type props.
Counterweight Propeller Design
A constant-speed propeller type where oil pressure drives blades to high pitch and counterweights, via centrifugal twisting moment, drive blades to low pitch.
- Loss of oil pressure = blades move to low pitch = possible overspeed
- Opposite of some non-counterweight designs where oil pressure decreases pitch
- Counterweights use centrifugal force acting on offset weights attached to blade shanks
Memory trick: No oil, no push to HIGH pitch — counterweights fling blades LOW, RPM runs away