Hydraulic Pressure Fluctuation and High Case Drain
On a 3,000-psi hydraulic system with an engine-driven pump and a 28-V electric standby pump, the crew reports pressure fluctuating between 2,600 and 3,000 psi and slow landing-gear retraction. When the electric standby pump is selected ON, it runs continuously. Reservoir level is normal and there are no external leaks. On the ground, the engine-driven pump outlet reads 2,950 psi, system pressure at the manifold reads 2,650 psi, case-drain flow is 1.8 gpm (limit 0.5 gpm), and filter delta pressure is 45 psi (limit 60 psi). The aircraft is at an outstation with no replacement pump available.
1. Identify the most likely fault and support the conclusion with the measured values.
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The most likely fault is internal leakage or wear in the engine-driven pump. The case-drain flow of 1.8 gpm is well above the 0.5 gpm limit, which indicates excessive internal bypass past the pump's rotating group or control components. The pump outlet is 2,950 psi while manifold pressure is 2,650 psi, a 300-psi drop that is consistent with the pump not delivering full flow under load. The filter delta pressure of 45 psi is below the 60-psi limit, so a clogged filter is not the primary cause. The standby pump running continuously confirms that the system cannot maintain pressure with the engine-driven pump alone.
2. Can the aircraft be released for the planned flight? What operating or ferry conditions apply?
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No normal release is appropriate. A hydraulic system that cannot maintain pressure and requires continuous standby-pump operation affects landing-gear retraction and possibly other systems. The high case drain also indicates a progressing pump failure.
If the operator wants to move the aircraft, a special flight permit or other approved ferry authorization would be needed, and the aircraft should be configured to minimize hydraulic demand. The standby pump should not be treated as a normal substitute for a worn engine-driven pump. The safe course is to repair or replace the pump at the outstation before further flight.
3. What confirmatory test or step should be done before replacing the pump?
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Before condemning the pump, confirm that the high case drain is coming from the pump and not from another component sharing the case-drain return. Isolate the pump case-drain line and measure flow directly. Also verify that the system relief valve is not stuck open or bypassing, and check the pump compensator or pressure-control setting. If the isolated pump case drain remains above the 0.5 gpm limit and the relief valve checks good, the pump is the fault and should be replaced or repaired.