Definition
A hydraulic actuator that uses fluid pressure to move its piston in one direction only, relying on a spring, gravity, or some other external force to return the piston to its starting position.
Plain English
A hydraulic device that pushes one way under pressure and is brought back the other way by something else, like a spring.
Context Anchor
Seen in hydraulic system descriptions, especially where pressure is used to move a part but the return movement comes from a spring, weight, or outside force.
Derivation
Single-acting means the fluid acts on the piston from one side only. A servo, from the Latin servus meaning servant, is a device that does work on command. Together: a servant that is pushed by fluid in only one direction.
Why Pilots Care
Knowing how single-acting servos work helps pilots understand system response and diagnose issues during preflight or in-flight hydraulic anomalies.
Analogy
Think of a spring-loaded door closer. You push the door open (one direction of force), and a spring pulls it shut. The push is the hydraulic side; the spring is the return.
Intuition Check
Do not read “single-acting” as meaning the servo works only once. It means hydraulic pressure acts on it in only one direction; the return stroke depends on another force.
Example Sentence 1
The brake actuator on this trainer is a single-acting servo, so a return spring releases the brake when hydraulic pressure drops.
Example Sentence 2
During the preflight check the mechanic confirmed the single-acting servo returned to its neutral position via the spring.