Definition
A hydraulic actuator that uses fluid pressure to produce powered movement in both directions. Hydraulic fluid is directed to one side of the piston to extend it and to the other side to retract it, with the opposite side venting back to the system return line during each stroke.
Plain English
A hydraulic ram that is pushed by fluid in both directions — fluid drives it out, and fluid drives it back in. It does not rely on a spring or gravity for the return stroke.
Context Anchor
Seen in aircraft hydraulic system descriptions, especially for parts that must move both directions under control, such as some landing gear, flap, door, or flight control mechanisms.
Derivation
‘Double-acting’ simply means the actuator is acted on by hydraulic pressure on two sides, not just one. The contrast is with a ‘single-acting’ actuator, where pressure moves the piston one way and a spring or external force returns it.
Why Pilots Care
Enables reliable bidirectional movement of critical components such as landing gear and control surfaces.
Grounding Statement
Picture hydraulic pressure pushing the same moving part one way, then being sent to the other side to push it back.
Intuition Check
Double-acting does not mean the actuator does two unrelated jobs. It means hydraulic pressure is used for both directions of movement.
Example Sentence 1
The landing gear uses a double-acting actuator so that hydraulic pressure can both extend the gear down and retract it back up into the wheel well.
Example Sentence 2
During troubleshooting the mechanic confirmed the double-acting actuator retracted the gear normally under system pressure.