Definition
A pump that moves a fixed quantity of fluid for each revolution or cycle of its operation, regardless of the pressure in the system. Output volume is determined by pump speed, not by downstream demand. Because the output is fixed, a constant-displacement pump must be paired with a pressure regulator or relief valve to prevent excessive system pressure when demand is low.
Plain English
A pump that pushes the same amount of fluid every turn, no matter how much the system actually needs. Because it always delivers that fixed amount, the system needs a valve to bleed off the extra when nothing is calling for it.
Context Anchor
Seen in aircraft hydraulic system descriptions, especially when comparing fixed-output pumps with pumps that can change their output.
Derivation
Displacement here means the volume of fluid the pump moves, or 'displaces,' in one cycle. Constant-displacement means that volume per cycle never changes. This contrasts with a variable-displacement pump, which can adjust how much it moves per cycle based on demand.
Why Pilots Care
Delivers predictable flow so actuators move at consistent speeds, keeping critical systems reliable without constant adjustment.
Analogy
Think of a hand soap pump that gives about the same amount of soap each time you press it. Press it again, and it gives another set amount whether you needed that much or not.
Intuition Check
Constant does not mean the pump always gives the same total flow in every situation. It means the pump moves the same amount per turn or stroke; if the pump turns faster, total flow can increase.
Example Sentence 1
The hydraulic system uses an engine-driven constant-displacement pump, with a regulator to unload the pump once system pressure is reached.
Example Sentence 2
During preflight the mechanic verifies constant-displacement pump output to confirm adequate pressure will be available for the flight controls.