FAA Commercial Pilot Airplane (CAX)Aircraft SystemsMedium
While in cruise flight with the propeller control set for 2,400 RPM, engine RPM begins to decrease below the selected value (an underspeed condition). How does the constant-speed propeller governor respond?
- AGovernor shuts off oil flow completely, causing the propeller to feather
- BGovernor increases fuel flow to the cylinders to compensate for RPM loss
- CGovernor increases oil pressure to the propeller hub, driving the blades toward a lower (finer) pitch angle
- DGovernor decreases oil pressure to the propeller hub, allowing counterweights to increase blade angle
Show answer & explanationAnswer & explanation
Correct answer: C. Governor increases oil pressure to the propeller hub, driving the blades toward a lower (finer) pitch angle
In an underspeed condition, the flyweights move inward, causing the pilot valve to direct governor oil to the propeller hub. This oil pressure drives the blades to a lower (finer) pitch, reducing resistance and allowing RPM to increase back to the selected value.
Why the other options are wrong
- A. Feathering is not a normal governor response to a minor RPM change.
- B. Fuel flow is controlled by the mixture/injection system, not the propeller governor.
- D. This describes an overspeed response, not underspeed.
Underspeed Governor Response
When RPM drops below the selected value, the governor increases oil pressure to the hub, moving blades to a lower pitch angle to restore RPM.
- Flyweights move in during underspeed
- Oil pressure increases to hub
- Blades move to fine (low) pitch, RPM rises
Memory trick: Underspeed = under pitch: low RPM gets low (fine) pitch to speed back up.