1.) Why exactly can hydraulic fluid transfer if the 4th system isn't pressurized first?

2.) Can hydraulics be pressurized in any order after the 4th? 

-Angelo Busato

It's because brakes can be pressurised from alternative hydraulic systems, so those systems are indirectly connected through the brake transfer valves. You can therefore pressurise the brakes from one system and release that pressure into a different system, thus transfering fluid. If you don't operate the brakes while pressurising the hydraulics then it shouldn't matter what order they are activated.

Edited September 2, 20187 yr by kevinh

Reference Normal Procedures, Amplified in the FCOM NP21.25, the order after pump 4 is set to AUX is to set AUTO to 1, 2 then 3 but I cannot think of any harm in the 3, 2, 1 order.  If the aircraft has an AUX pump for pump 1 then it is also set before setting 2 and 3 to AUTO I believe... nothing to reference here specifically.

I'm not a systems expert, but I believe the hydr 4 first is because a control surface (rudder I think) can be forced to end travel if the 4th isn't pressurized first.  There are many more qualified than I to answer this directly.

EDIT:  Keven was ahead of me.. there you go.

Edited September 2, 20187 yr by downscc