When would you disconnect?

[Guest PittsburghKSEA]

I recently found my old Quick Reference Checklist from the murky darkness of my drawers. I noticed that going through the check list after a long absense :-( I don't see any mention of engine hydraulic pumps, only for the electric pumps. Further down in the check list, I see that System A and System B switches are turned on during the overhead panel setup. I am assuming we are talking about the electric pumps here. And finally, in the shutdown/secure part of the check list the electric hydraulics are mentioned explicitly to be shut down. Are the engine pumps only turned off in case of a malfunction?Thanks,Boaz

[Guest benhoffman]

The 737 has two hydraulic systems; system A and system B either of which can power the other one.Each system has an engine driven pump and an electrical driven pump to supply hydraulic pressure and move fluid.I'm pretty sure that during times of heavy hydraulic pressure loads e.g. gear or flaps extended, all four pumps are active whilst at other times, only the engine driven pumps are supplying hydraulic demand.On the ground when the engines are not running and therefore, the engine electrical generators are not providing power, you leave the pumps off because there is no power to them.When the engines are working and supplying the electrical system, you need to turn these pumps on so they can power the hydraulic system.

[kevinh 409]

Normally, the engine driven pump switches are left on, even after shutdown. The electric pumps must be switched off so that when power is re-established they don't start running immediately, pressurising the system too early.There is no reference to switching off the engine driven pump output for any normal or abnormal procedure. In the event of a fire the hydraulic system is isolated by the fire handle. So these switches are normally left on all the time.Kevin

[markadeane 272]

Engine Driven Pumps are left in the ON position, save for any abnormal situation where you specifically wanted to stop the pump from operating.EDIT: Sorry Kevin I see you had answered this, don't know how I missed it!

[Guest PittsburghII]

Thanks, Kevin! This is what I suspected. Thanks for the info.:-waveBoaz

[zubart 5]

The engine driven HYD pumps should always be left in the ON positon during a final shutdown. Floyd