Definition
A manually operated pump that moves fluid only on one stroke of the handle — typically the push or the pull — while the opposite stroke simply resets the pump for the next working stroke. Used in aircraft systems for tasks such as emergency hydraulic actuation, fuel priming, or transferring fluid in low-volume situations.
Plain English
A hand pump that only pushes fluid one way each time you work the handle. One direction does the work; the other direction just gets the pump ready to do it again.
Context Anchor
Seen in aircraft hydraulic system descriptions, emergency backup systems, and maintenance procedures where pressure may need to be made by hand.
Derivation
‘Single-action’ means one working stroke per cycle, as opposed to ‘double-action’ where both strokes move fluid. Knowing this contrast makes the term immediately clear: single = one productive stroke, double = two.
Why Pilots Care
Provides a backup method to operate critical systems when engine-driven or electric pumps are unavailable.
Analogy
Like a basic bicycle tire pump: air only goes into the tire when you push down. Pulling up does nothing useful except set you up for the next push.
Intuition Check
Single-action does not mean you move the handle only once. It means only one direction of handle movement actually pumps the fluid.
Example Sentence 1
The technician used the single-action hand pump to build hydraulic pressure in the system before checking for leaks.
Example Sentence 2
The technician tested the single-action hand pump as part of the emergency gear extension check.