FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsHard
A transport category aircraft's primary flight controls are moved by hydraulic power control units. As the pilot moves the control column, an internal metering valve directs pressurized fluid to move a piston connected to the control surface. As the piston moves, a mechanical linkage repositions the metering valve back toward neutral. What is this self-correcting arrangement called?
- ASequence valve system
- BBootstrap reservoir system
- CPriority valve system
- DFollow-up (feedback) servo actuator
Show answer & explanationAnswer & explanation
Correct answer: D. Follow-up (feedback) servo actuator
A follow-up (feedback) servo actuator uses a mechanical linkage from the actuator piston back to the metering valve, so the actuator stops moving once it reaches the commanded position and the valve returns to neutral. This closed-loop design provides precise, proportional control surface positioning corresponding to pilot input.
Why the other options are wrong
- A. A sequence valve times the order of multiple actuators, not position feedback for one actuator.
- B. A bootstrap reservoir pressurizes the reservoir, unrelated to control surface feedback.
- C. A priority valve manages flow allocation during low pressure, unrelated to position feedback.
Follow-Up (Feedback) Servo Actuator
A hydraulic power control unit in which actuator piston movement is mechanically fed back to reposition the metering valve toward neutral, stopping motion at the commanded position.
- Provides proportional, precise control surface positioning
- Metering valve returns to neutral as actuator reaches commanded position
- Common in transport aircraft primary flight control systems
Memory trick: Follow-up servo 'checks its work' by feeding position back to the valve.