Definition
A device that converts hydraulic fluid pressure into mechanical motion to move an aircraft component. Hydraulic actuators take pressurized fluid from the aircraft's hydraulic system and use it to push a piston inside a cylinder, producing either linear motion (a piston rod extending or retracting) or rotary motion (a shaft turning). They are used to operate flight controls, landing gear, flaps, brakes, cargo doors, and other systems that require strong, controllable force.
Plain English
A part that uses pressurized fluid to push or turn something on the aircraft. When fluid is pumped into one side, it forces a piston to move, and that movement does the work — like extending the landing gear or moving a flight control surface.
Context Anchor
Seen in aircraft systems that move landing gear, flaps, brakes, flight controls, doors, or other movable parts using hydraulic pressure.
Derivation
From Greek hydor (water) and aulos (pipe), giving 'hydraulic' — relating to liquid moving through pipes. 'Actuator' comes from Latin actus (a doing or driving), meaning the thing that causes action. Together: the part that makes something happen using pressurized fluid.
Why Pilots Care
Actuators provide the force needed for reliable operation of critical systems; loss of hydraulic pressure can disable controls or landing gear.
Analogy
Think of a car's hydraulic jack. You pump fluid into a cylinder, and the piston rises with enough force to lift the whole car. A hydraulic actuator works the same way, just with the fluid pressure supplied by the aircraft's hydraulic pump instead of your hand.
Intuition Check
Do not think of a hydraulic actuator as the source of hydraulic pressure. It is the part that uses the pressure to make movement.
Example Sentence 1
When the gear handle is moved to the down position, hydraulic fluid is directed to the actuators, which extend the landing gear.
Example Sentence 2
Mechanics replaced the leaking hydraulic actuator on the nose landing gear before the next flight.