Definition
A pump that moves a fixed, measured volume of fluid with each cycle of its mechanism, regardless of the pressure in the system downstream. Each rotation or stroke traps a specific quantity of fluid in a chamber and forces it out the discharge side. Common types include gear, vane, piston, and gerotor pumps. Because the output volume is set by the pump's geometry rather than by system pressure, a positive-displacement pump must be protected by a pressure relief valve to prevent damage if the discharge line is blocked.
Plain English
A pump that pushes the same exact amount of fluid out with every turn, no matter how much pressure is fighting back against it. It will keep building pressure until something gives, which is why it always needs a relief valve.
Context Anchor
Seen in aircraft hydraulic, oil, and fuel system discussions, especially when explaining how system pressure is produced and controlled.
Derivation
Positive' here means 'definite' or 'fixed' — a set amount, not 'good.' 'Displacement' means moving fluid out of a space by taking that space up with something else (like the pump's gears or pistons). Together: a pump that moves a definite, fixed amount of fluid each cycle.
Why Pilots Care
Delivers reliable fuel or hydraulic fluid flow under varying pressures, preventing engine starvation or loss of flight controls.
Analogy
A hand syringe is a simple positive-displacement device. Each push of the plunger forces a definite amount of liquid out, and if the tip is blocked, pressure rises quickly.
Intuition Check
“Positive” does not mean good, approved, or electrically positive here. It means the pump positively, or definitely, moves a trapped amount of fluid each cycle instead of just stirring or spinning the fluid.
Example Sentence 1
The aircraft's hydraulic system uses a positive-displacement pump driven by the engine to maintain operating pressure.
Example Sentence 2
Hydraulic systems use positive-displacement pumps to extend the landing gear at a consistent rate.