Definition
A hydraulic pump whose output volume per revolution can be changed automatically while it is running, allowing it to deliver more or less fluid as the system demands without changing pump speed.
Plain English
A pump that adjusts how much fluid it pushes out on each turn, so it can give the system exactly as much as it needs at any moment.
Context Anchor
Seen in aircraft hydraulic system descriptions, especially where one pump must supply changing demand from items such as landing gear, brakes, or flight control systems.
Derivation
‘Displacement’ here means the amount of fluid moved per revolution. ‘Variable’ means that amount can be changed. So the name describes a pump where the volume moved per turn is not fixed.
Why Pilots Care
Matches hydraulic flow to actual demand, which reduces heat, wear, and power loss compared with a fixed-output pump.
Analogy
It is like a water pump that can automatically ease off when little water is needed and work harder when more water is needed, instead of running at full output all the time.
Intuition Check
Variable does not mean unreliable, and displacement does not mean the pump changes location. Here, variable displacement means the pump changes how much fluid it moves.
Example Sentence 1
The aircraft uses a variable displacement pump so hydraulic pressure stays constant whether the gear is cycling or the system is idle.
Example Sentence 2
During troubleshooting the mechanic confirmed the variable displacement pump was adjusting flow correctly without overheating.