747-400 Fuel System Query

[craig_read 43]

Hi,

It may be a mistake I am making and perhaps a misunderstanding but I have a query regarding the fuel system.  I am currently taxiing out to a runway at EHAM in PH-BFA and I have 93.4Kgs (x1000) on board.  Tanks show:

MAIN 1; 13.3

MAIN 2; 29.4

MAIN 3; 29.4

MAIN 4; 13.3

CENTRE; 0.0

RES 1; 4.0

RES 2; 4.0

STAB; 0.0

In terms of my overhead

- FUEL X FEED (1) shows ON i.e. button pushed white bar showing.

- FUEL X FEED (2) shows ON i.e. button pushed white bar showing (guarded switch closed).

- FUEL X FEED (3) shows ON i.e. button pushed white bar showing (guarded switch closed).

- FUEL X FEED (4) shows ON i.e. button pushed white bar showing.

In terms of fuel pumps, all on (including OVRD pumps) except CTR L, CTR R, STAB L and STAB R.

 

My query is, on my status page for the fuel system FUEL X FEED 2 and 3 show as closed i.e. off centre, no green bar through the valve.  If I then change the switches manually for X FEED 2 and 3, this has no effect.  I tried to cycle these switches through OFF and ON (opening the guard etc) and no effect.  I then wondered if this part of the automatics, so figured if I turned off MAIN 2 FWD and AFT pumps the valve would open as there was no fuel pumping to engine 2, but it didn't.  Can someone explain this behaviour to me?

Regards

Craig Read

 

 

[Captain Kevin 586]

You're on the ground, so when you have the flaps set for take-off, crossfeeds 2 and 3 will close automatically. This behavior is correct. Once you're airborne and you bring the flaps up, they should open. More information can be found in the FCOM, specifically section 12.20.13.

[craig_read 43]

36 minutes ago, Captain Kevin said:

You're on the ground, so when you have the flaps set for take-off, crossfeeds 2 and 3 will close automatically. This behavior is correct. Once you're airborne and you bring the flaps up, they should open. More information can be found in the FCOM, specifically section 12.20.13.

Ah thank you for that!  I've learned something, I didn't realise it did that!  I'd not noticed it before I will go and study the section.  The richness of the simulation is impressive.

Regards

Craig Read

[Captain Kevin 586]

I'm more curious as to why it does that. The FCOM doesn't appear to tell you why it does this, only that it does it.

[77west 139]

1 hour ago, Captain Kevin said:

I'm more curious as to why it does that. The FCOM doesn't appear to tell you why it does this, only that it does it.

Could be additional redundancy during takeoff, a pump failure would only affect a single engine, and would allow the crew to complete the takeoff and deal with the problem once safely airborne. Just my guess. Similar to how certain electrical buses are isolated during an autoland / CAT3 approach.