OVRD PUMPs on 744

[Tonic Please]

Simple question:

Transitioning to phase 3 of fuel logic on the 744 upon receiving FUEL TANK/ENG, when one must go to each engine being fed by its own MAIN tank and switch off (generally speaking, I've read online that some do not for various reasons, discuss amongst yourselves) the crossfeed valves, what happens if you do not press off the OVRD PUMP switches (FWD and AFT) for tank 2 and 3 (considering they were currently feeding 1, 2, 3 and 4)?

It appears that fuel is still fed from the MAIN 2 and 3 via the OVRD pumps rather than the main tank. Would this result in 2 and 3 flame out or just more strain on the overide? I can't quite work it out.

Thanks in advance!

[kingstonboy]

Assuming the correct tank to engine config(crossfeeds !!!) You are correct the ovrd would supply until low level,due to higher ouput pressure.Flame out should not occur as long as sufficient FOB and no real strain on pumps as check valves and runaround are built into the AC fuel supply system (output px 45 psi as opposed to 22 psi approx iirc)If you forget the crossfeeds then a whole different scenario can and will develop.

[SMOC]

The O/J pumps will run as normal, they are not constant displacement type pumps so they don't have any pressure issues.

They will simply provide pressure until they reach the standpipe level of approx 3200kg in the respective inboard tank and then the boost pumps will kick in.

[Tonic Please]

Appreciate both responses. It begs the question why it's necessary to turn off the OVRD pumps, does it not?

[kingstonboy]

A guess and thats all this is! to alleviate the potential consequences of missing closing the crossfeeds.

[Tonic Please]

Good guess. How did you arrive at this? Experience?

Perhaps some current pilots could shed some light on why it's necessary?

[MarkerInbound]

A guess and thats all this is! to alleviate the potential consequences of missing closing the crossfeeds.

But you get the TANK/ENG message because a crossfeed is open and the fuel is about even between 1 and 2 or 3 and 4. I was taught pumps first and then X-feed because if you did the x-feeds first the message would blank and you could forget to turn off the O/J pumps.

[By George]

When you get the msg >FUEL/TANK/ENG if you turn the over-rides off but leave the cross-feed open you'll get the msg >XFEED CONFIG.

If you fail to turn the over-rides off but close the x-feeds you'll not get >FUEL/TANK/ENG.

The soft-ware IDS-503 in my case but can vary. This is how I've always seen it. Just follow the EICAS and don't try too hard to out-guess it. It's a simple system, nothing different to a Piper Navajo.

If you have an over-ride pump fail the MEL simply says "switch the corresponding pump on the other side off". It really is an easy system and the nature of your original question makes me feel you are looking "too deep". I do not mean any disrespect.

[By George]

As much as it pains me, that is not entirely correct, the >XFEED CONFIG will display if xfeed valve 1 OR 4 closed and main tanks not equal. See I told you it was a simple system.