why do we actually pressurize first the R Hyd system?it says to prevent hyd fluid transfer.From which side to which side?
Its to do with the parking brake accumulator.
When a B757 is parked, the brake accumulator will slowly lose pressure during time. After a nightstop, the pressure will be down at 1000psi. As this happens, hyd fluid is returned to the R hyd tank. If you then turn on the L hyd pump first, fluid from the L hyd tank will fill up the hyd brake accumulator. At the next nightstop, this will drain back into the R hyd tank. No fluid is lost, it just transfers from L to R.
Us engineers fix this by transferring it back, by turning the pumps on and off and putting the park brakes on and off in the right order, the fluid will go back.
So if the crew always start by turning on the R hyd pump, this won't happen.
This happens on most Boeings, as they have one set of brakes, but two sources of fluid.
On Airbusses there are two sets of brake cylinders, so fluid transfer through the brakes doesn't occur. Airbusses also have a different system of setting the park brake so that pressure is not lost overnight.
why then R Eng,C1,then C2 then L Eng?Why not R eng,C2,C1,L eng,so just from R to L?
On the ground, only one of the C pumps will run. If both are on, C1 will run.
So if you select C2 first, then C1, C2 will start to run, but stop to let C1 run. Starting these electric pumps takes an awful lot of amps and starting two quickly could overload the electrical system.
For this reason, it is a good policy to not turn on more than one electric pump when you are on ground power. Wait until the APU is supplying before you select a second pump.