If a pilot reports normal pressure when a hydraulic pump is running, but no hydraulic pressure when it stops, what might this indicate?

When a pilot reports normal pressure from a hydraulic pump while it's operational, but no pressure when the pump is off, this situation is indicative of a potential issue with the accumulator, specifically related to the air valve. An accumulator stores hydraulic fluid under pressure, and when the pump is running, it helps maintain system pressure.

If the air valve of the accumulator is leaking, it could allow the compressed air to escape, preventing the accumulator from holding pressure when the pump stops. This results in a lack of hydraulic pressure available in the system when the pump is not actively supplying it, leading to the observed scenario.

In contrast, a malfunctioning hydraulic pump would likely not produce normal pressure when operating, a clogged hydraulic filter would restrict flow and pressure during operation rather than differential pressure observed when stopped, and a faulty pressure gauge would not provide true pressure readings while the pump is operational or not. Thus, the lack of pressure when stopped aligns well with the scenarios associated with a leaking accumulator air valve.