I have a Cessna variable-volume pump and it is always turning when the engine is running. You need a reservoir but not much fluid is circulated when the pump is idling but the case drain continuously returns a small amount of fluid and some is circulated for cooling. Actually, the amount of case drain fluid increases as the pump ages and reflects the condition of the pump. Anyway, my pump is 26 years old and works fine. You turn on a switch in the wheelhouse to open a solenoid to start to circulate the fluid through the systems and then you just control them at their location.
The bronze plate on the left is my fire pump.
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