Definition
A hydraulic actuator in which fluid pressure can be applied to either side of the piston, allowing the servo to produce powered movement in both directions. Hydraulic fluid is directed to one side to extend the piston and to the other side to retract it, with the opposite side venting to return.
Plain English
A hydraulic ram that is pushed by fluid pressure both ways. Pressure on one side moves it out, pressure on the other side moves it back in.
Context Anchor
Seen in hydraulic system discussions, especially where pressurized fluid is used to move or position aircraft parts such as landing gear, brakes, flaps, or control surfaces.
Derivation
‘Servo’ comes from the Latin servus, meaning ‘servant’ — a device that does work on command. ‘Double-acting’ simply means it acts (is powered) in both directions, as opposed to a single-acting servo where pressure moves the piston one way and a spring or gravity returns it.
Why Pilots Care
Ensures precise, reliable control authority in both directions for primary flight surfaces, directly affecting handling and safety.
Analogy
Think of a sliding door that can be powered open and powered closed. A single-acting device would power it one way and depend on something else to bring it back; a double-acting servo powers both directions.
Intuition Check
Do not read “double-acting” as two separate servos. It means one servo can produce force in both directions.
Example Sentence 1
The landing gear is raised and lowered by a double-acting servo, with hydraulic pressure driving the piston in both directions.
Example Sentence 2
During the preflight check the mechanic confirmed that the double-acting servo retracted the aileron without binding.