B777 Hydraulic System
 

New to T7. In FCOM NP, to pressurize hydraulic systems before engine start ( and to depressurize after engine shutdown) there's a sequence to operate the hydraulic pumps to prevent fluid transfer between systems.

I'm looking for more technical details on how fluid transfer between systems may occur if the right system is not pressurized and the other systems are. Gracias

Shuttle valve in the braking system is the mode of transfer on the B747/B744, so I assume 777 is the same.

It's usually through the brake system, the 747 has the same procedure to pressurize the primary brake system first to prevent fluid transfer.

It's only a small amout but if done during every start will eventually require maintenance to rectify.

The reason is multiple sources are available to supply the brakes but ALL return fluid from the brakes goes to system 4 on the 747, I assume it is the RIGHT system in the T7.

So if we pressurize sys 1 on the 744 first a certain amount will go to the brakes and will end up via the return system in sys 4.

Caution: obey the sequence in which you pressurize or remove the pressure from the hydraulic systems. If you do not obey the sequence, fluid can move between the center and right systems. This can cause a high fluid level in one system and a low fluid level in the other system. Damage to the hydraulic pump and aircraft systems that use hydraulic fluid can occur

And if you wonder why it happens on all Boeings (except 787!) and no Airbus the reason is in the brake design. On the Boeing there is a shuttle valve that supplies hydraulic fluid from main or alternate hydraulic system. But on an Airbus brake there are two sets of pistons, One for main and one for alternate.
Go look at an A320 brake, then go to a B737 and it is obvious.

when the T7 entered service there was also a logic problem if they werent selected off and on in the correct sequence which could give you fault messages which the crew couldnt clear. This eventually got modded out so not normally an issue. The fluid transfer is small but on an aircraft that has no leaks will eventually cause level issue's . Not a problem as there is a tolerance and the engineer should be looking at it before you do, and then you check before you pressurise anything. Boeing have the 'OF' and 'RF' prompt if overfull or low. Basically good practise but not critical.

Same on the 767 ...

One trick taught during my IOE was to remember the "&" sign, starting on the lower right, i.e. R ELEC, C ELEC 1 C ELEC 2, L ELEC and C AIR. When shutting down, the opposite way: C AIR, L ELEC, C ELEC 2 C ELEC 1 and R ELEC


Have fun on the 777

B777 Hydraulic System
 

Thank you all . Really appreciate all the response. Cheers