Definition
A hydraulic or pneumatic actuator that uses fluid pressure to move its piston in both directions. Pressure is applied to one side of the piston to extend it and to the opposite side to retract it, giving the system powered control over both strokes.
Plain English
A piston-and-cylinder unit that is pushed by fluid pressure both when extending and when retracting. Neither direction relies on a spring or gravity to move it back.
Context Anchor
Seen in aircraft maintenance descriptions of systems that need powered movement both ways, such as landing gear, flaps, doors, or flight-control parts.
Derivation
"Double-acting" simply means it acts (is powered) in two directions, as opposed to a single-acting actuator that is powered in only one direction and returned by a spring or external force. The distinction matters because it tells you how the actuator behaves if pressure is lost.
Why Pilots Care
Provides reliable bidirectional motion for critical systems such as retractable landing gear and flight control surfaces.
Analogy
It is like a powered drawer slide that can both push the drawer open and pull it closed, instead of only pushing it one way and needing something else to bring it back.
Intuition Check
Double-acting does not mean there are two separate actuators. It means one actuator is powered in both directions.
Example Sentence 1
The landing gear retraction system uses a double-acting actuator so hydraulic pressure can both raise and lower the gear.
Example Sentence 2
Hydraulic pressure extends and retracts the flaps through a double-acting actuator.