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Hachiouji-shi
15th May 2006, 06:52
If aft CG is so much desired, why does the 744 uses its Stab tank fuel immediately after its Centre tanks are depleted?

Airbus on the other hand transfers fuel to the Trim tanks if it's not full passing FL 255. Thereafter, as fuel is consumed it transfer Trim fuel bit by bit Forwards to maintain that Aft CG target minus 0.5%.

javelin
15th May 2006, 09:32
I think you answered your own question !

330/340 are fly by wire and can tolerate a c of g further back. They also want it back there for fuel efficiency - lack of trim drag. 744 is probably being more conservative.

On the 330, if the c of g gets forward due to poor loading or lots of freight, the burn goes up by about 750kg over PLOG on a transatlantic sector due to trim drag.

Captain Airclues
15th May 2006, 09:46
The Stab fuel is used long before the Centre Wing Tank fuel is depleted. It's a few years since I flew a 744 with a stab tank (freighters don't have them), but from memory, the stab fuel transfers to the CWT during the climb.
If the stab fuel fails to transfer then the CWT pumps must be switched off and fuel burnt (not dumped) from the other wing tanks until down to MLW. Failure to do this will result in the CofG exceeding the aft limit.

Airclues

Bellerophon
15th May 2006, 10:21
Hachiouji-shi

...why does the 744 uses its Stab tank fuel immediately after its Centre tanks are depleted? ...

If by depleted you meant empty, it doesn't.

The B744 uses the fuel in the stabiliser tank much earlier than that, and starts to transfer fuel from the stabiliser tank, into the CWT, when the CWT contents drop to around 36,500 kg.

I suspect that one reason for the Boeing procedure may be that it is designed to minimise the effect of a total failure of the stabiliser fuel transfer system by detecting any such failure relatively early in the flight.

Given that the worst case scenario of a total failure of the stabiliser tank transfer system can result in around 67,500 kg of fuel becoming unuseable, it would seem better to discover this failure early in the flight, whilst there is still a reasonable amount of fuel in main tanks 1-4, rather than later, when the CWT is empty and the CG likely to go out of limits.

Regards

Bellerophon

Hachiouji-shi
15th May 2006, 10:22
So why does it seem that the 744 is in a "hurry" to get rid of its trim fuel? I mean don't they want as much Aft CG as possible within limits? In the Airbus, there is an AFT CG target and as fuel is consumed during cruise, the CG moves aft a little and the FCMC then transfers trim tank fuel forwards bit by bit to get it back to the Aft CG target (minus half a percent).

As I write this, I begin to come to my own conclusion, maybe the 744 does not have a CG control system using fuel transfers in its designing stage while the Airbus has.

Bellerophon, sorry, my post is about 1 minute after yours ( I was typing still while you posted it) and to some who didnt notice that, it might seem i have not taken into account your informative post. :-)