What happens when the cockpit control level for a hydromatic constant-speed propeller is actuated?

• A. the pilot valve opens fully
• B. the blade angle increases instantly
• C. the oil temperature rises
• D. the speeder spring compresses

Answer: (D)

In a hydromatic constant-speed propeller, the cockpit control lever sets the commanded RPM by changing the hydraulic bias inside the propeller governor. When you move the control, the linkage acts on the governor’s speeder spring, compressing it. That compression shifts the pilot valve, altering the oil pressure sent to the propeller hub. The change in oil pressure then moves the blades to the desired pitch to achieve the commanded RPM. The speeder spring compression is the direct mechanical action tied to the lever movement.

The other possibilities don’t match how the system operates: the pilot valve doesn’t simply open fully as a result of just moving the lever; blade angle doesn’t change instantaneously and is limited by hydraulic response; and oil temperature isn’t affected in any meaningful way by the lever actuation itself.