classic Hyd
Brakes can be selected to system 1 on the F/E panel if needed.
But you need to make sure that you have accumulator pressure showing in the brake pressure gauge.
If not and you shift system 4 to system 1 then you will lose system 1 also as it will be dumped out through system 4.
This will then give you a dual hydraulic failure.
On a normal operation system 4 powers the brakes and everything on the outside, meaning brakes, wing gear and outboard flaps.
System 1 everything on what we call the inside, body gear, nose wheel steering and the inboard flaps.
If 1 and 4 are gone then the gear can be lowered manually with the switches on the overhead panel, just the gear doors remain open as no hyd to close them.
Brakes will then be the reserve brakes which operates off system 2.
But once parked pressure has to be maintained to system 2 as there is no accumulator and once the engines are shut down then the parking brake becomes another problem, this can be accomplished with the apu air and the system 2 ADP until chocked.
Also some other anti skid sensing issues when using system 2 brakes.
There are also some 747 classics that can use the apu air source in flight, this is usually shown with a placard above the apu panel.
But I have never seen one that allowed you to use the APU electrics as for one it would never parallel with the engine driven generators.
Two it is in the manuals as forbidden.
Now if you lost all engine driven generators would it work, maybe.
But no procedure for it.
Someone want to expand on this feel free.
Starting to sound like a recurrent here, sorry.
Hope this helps.
Last edited by Earl; 1st Oct 2008 at 04:40.