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_Viking_
1st Oct 2018, 20:29
Hello all,

In the QRH for HYDRAULIC PUMP OVERHEAT on the max it states that the electric hydraulic pump (associated with the overheat light) should be turned off, as expected. Then, hydraulic pressure should be checked and if above 3300 psi, the engine driven hydraulic pump should be shut off as well leaving the crew with a Loss of System A or B situation.

Does anyone know the reason for the later step regarding turning the engine driven pump off above 3300psi? Hard to find any info on this in the FCOM...

Goldenrivett
2nd Oct 2018, 09:07
Hi Viking,

>3300 psi sounds like the EDP pump isn't regulating pressure automatically and is giving maximum delivery and pressure output. I guess a separate overpressure relief valve has taken over and is returning the excess fluid back to the reservoir. If allowed to continue, the temperature of the fluid would continue to rise.

Once both pumps have been turned off and the fluid temperature allowed to cool to within normal limits, does the QRH say to pressurise the system again using only the Electric pump?

porch monkey
2nd Oct 2018, 11:24
Not just the Max. The current NG QRH has the same actions. Was changed some time ago. Don't know why, but Rivett might be onto something.

IFixPlanes
2nd Oct 2018, 17:30
Pressure above 3300 psi is in general a sign for increased case pressure which is used to adjust the outlet pressure of an hydraulic pump. If a bearing in pump is beginning to die the case drain is collecting its chips and starts to clock. This increase the case pressure and is therefore a sign for a defective pump.