Definition
A hydraulic or pneumatic actuator that uses fluid pressure to move its piston in one direction only. The return stroke is produced by another means — usually a spring, gravity, or air loads — rather than by fluid pressure on the opposite side of the piston.
Plain English
An actuator that is powered in only one direction. Something else (a spring, weight, or airflow) pushes it back the other way.
Context Anchor
Seen in aircraft hydraulic and pneumatic system descriptions, especially when explaining how cylinders move doors, locks, brakes, or other mechanical parts.
Derivation
‘Single-acting’ literally means it acts (applies force) in only one direction. ‘Actuator’ comes from Latin actus, meaning ‘a doing’ or ‘driving’ — a device that produces motion. Together: a device that drives motion one way only.
Why Pilots Care
Knowing an actuator is single-acting tells a technician (and a pilot reading a system description) what brings the component back to its rest position. If the spring or return mechanism fails, the actuator may not retract or reset, even though hydraulic pressure is still available.
Analogy
Like a spring-loaded door closer: pushing the door open takes effort, but a spring closes it again automatically once you let go.
Intuition Check
Do not read “single-acting” as meaning the unit works only one time. It means pressure drives it in only one direction; the return direction depends on another force.
Example Sentence 1
The brake master cylinder uses a single-acting actuator, with a return spring bringing the piston back when the pedal is released.
Example Sentence 2
Technicians bleed air from the single-acting actuator before testing spring return force on the ground.