Hydraulic Pressure Fluctuation with a Foamy Reservoir
After a 12-minute climb in a twin-engine business jet, the Green hydraulic system pressure begins fluctuating between 2,100 and 2,900 psi. The Green reservoir sight glass is full of foam, although the quantity gauge still indicates 7.8 L out of 8.0 L. The Green electric backup pump current is erratic and its pressure rise is poor. The engine-driven pump case-drain line is cool to the touch and its filter indicator is not popped. There is no visible leak in the wheel well or engine pylon. The Blue system is steady at 2,980 psi and its reservoir is clear. The Green reservoir air-pressure gauge reads 12 psi; the maintenance manual states the reservoir is normally pressurised to 45 ± 5 psi from the bleed-air system. The aircraft is at FL180, 25 minutes from a suitable airport and 70 minutes from destination. Weather is fine. The landing gear and flaps are Green-system consumers.
1. What is the most likely cause of the fluctuating pressure and foamy reservoir, and why does the electric backup pump not restore normal pressure?
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The cool case-drain line and unpopped indicator argue against an engine-driven-pump seal failure. A foamy reservoir with normal indicated quantity but only 12 psi air pressure points to loss of reservoir pressurisation. The pump inlet then has insufficient head, so the pump cavitates; pressure fluctuates and the electric pump current is erratic. The electric backup pump cannot cure this because it draws from the same unpressurised reservoir, and running it may make the foaming worse. The quantity gauge is unreliable while the fluid is aerated. Check the reservoir air regulator, bleed supply, relief valve and gauge. If pressure can be restored, run the electric pump briefly to stabilise the system, then monitor. If not, treat the Green system as degraded.
2. State your immediate actions and the operational risk if you continue to destination.
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Reduce Green demand immediately: avoid unnecessary gear and flap cycles, keep the configuration stable, and plan to land at the 25-minute airport rather than the destination. Restoring reservoir pressure is the only real fix. If it cannot be done in flight, the risk is pump damage and loss of Green pressure during gear extension or flap movement. You should brief for alternate gear extension and advise ATC. Continuing for 70 minutes increases cavitation damage and leaves no margin for a go-around or a second system failure. A precautionary landing at the nearer airport is the safer judgement call.