747-400 and A340 fuel system differences

[SkyCruiser]

Does anyone know of any differences in the 747-400 and the A340 fuel systems.

Cheers.:confused:

[QAVION]

Yes.... er... No.

Is this a trick question?

A better question might be "Are there any similarities?"

There are probably tens of thousands of differences between the two aircraft, depending on how deeply you want to go into the matter. Would you have expected otherwise, coming from two different manufacturers?

What exactly are we talking about here? Fuel feed, fuel capacity, tank configuration, fuel feed logic, fuel jettison logic/control, fuel pump/valve control, type/number/location of fuel quantity sensing devices, type of valves, fuel system cockpit displays, type of cockpit warnings relating to fuel system anomalies, etc.

BTW, I have a 300 page book on the 747-400 fuel system (simplified version)... and that's only for one type of 747-400. There are many 747-400 fuel config differences depending on engine type and airplane configuration (Does the 747-400 you have in mind have a Horizontal Stabilizer Tank or Auxilliary Tank?).

I think most of us wouldn't even know where to begin answering your question. Perhaps you could help us by telling us why you are asking? (Is the question desktop simulator related?).

Q.

[SkyCruiser]

The reason I ask is that I have an interview coming up soon, this question has been asked during the interview.
I guess they are looking for a basic answer of the differences.

Thanks for any help.

[SMOC]

Then the answer you’re looking for is the Airbus has a Trim Tank Transfer system which controls the A/C C of G.
At FL255 on the way up fuel is transferred aft (depending on a few conditions) then as fuel is burnt from the fwd tanks fuel will be transferred fwd depending on the C of G.
If any fuel remains in the aft/trim tank on the way down it'll be transferred fwd at FL245.
Now the -400 just has a specific order as to what fuel is to be used from what tanks, Assuming full tanks the order goes CTR tank to a certain amount then the STAB tank, then back to the CTR until it's empty [or the low Press lights come on, usually around 900kgs (which the scavenge or jet pumps can finish off)]
Then the I/B tanks, plus at a point the reserves transfer into the I/B tanks so this fuel is burnt until the I/B equal the O/B at which point you select Tank to Engine.

So the simple answer is the Airbus uses Fuel for C of G control and the -400 doesn’t.

Hope this helps.

[SkyCruiser]

SMOC, you sir, are a star.

Many thanks.:)

[mutt]

If you havent flown either of these two aircraft, whats the point in asking you about their fuel systems??? :confused: :confused:

Mutt.

[Flight Safety]

Hmm...I wonder why Airbus chose to use fuel transfer for CG control on the A340? Could trim drag reduction be one reason?

If so, I wonder how much range advantage one can get, using fuel transer to optimize the CG? Would a fuel transfer system for CG control also help the range of the 744?

[SMOC]

As you'd expect the A330 uses it as well, I'd say it would definately help the -400.

[jettison valve]

Never forget that the A330/A340s are a couple of years younger than the "old lady" (and that the folks in SEA seem to be far more conservative than the chaps in TLS)...

An addition from my side (I am more familiar with the A340 than the 747s):
The A340 trim tank xfer is a brilliant thing in general - however, it also gives some headache as sometimes the automatic (!) fwd transfer won`t start automatically (!).
Yes, it`s for drag reduction (reduces required downforce of the THS); the MEL says you have to consider 1.5% more fuel burn when the TT is inop (for whatever reason). Some guys were looking into flight planning data and came up with a worst case scenario of about 0.8% percent if I remember correctly.

Apart from the CG control, there are tons of differences as already indicated by QAVION - some of them:
No "real" tank-to-engine-feed during normal operations on A340s ever. Only when you shut the emergency-isolation- or split-valves (fuel leak suspected!), you seperate the two feeding inner tanks into a fwd and an aft section each to feed their respective engine (fwd tank feeds inner engine). This has been a playing ground for a number of FCOM revisions lately...
Only one fuel line between wing area and APU/TT on the `buses - one of the big trouble makers on the classic A340s (-500s/-600s will get two lines to feed the APU and operate the TT seperately).
All "main" (wing and the two center tank) pumps are of the same type - there is nothing like main boost pumps and override/jettison pumps as on B747s and B777s.

BTW:
A330s are fitted with the same FCMCs (Fuel control and monitoring computers) and have the same CG control systerm.

Cheers,
Jettison valve

[AhhhVC813]

The 340-500/600 seems to address those limitations of the 300, as it has four inner tanks, allowing tank to engine operation; an independent APU feed line; and transfer/jettison pumps.

[norodnik]

Not quite on topic.

Why is the Centre Tank (747) always the last to be filled, and the first to be used up if filled ?

I assume its a balance issue

[Justforkix]

norodnik

I have not flown the 747, but I suspect its because of structual limitations. Imagine filling up the 747 center tank and fill up the AC with cargo and pax ,what a stress on the wingroots without fuel in the wingtanks. Just a guess, I'm ready to be ridiculed by the more knowledgeable. ;)

[Tinstaafl]

Norodnik, it aids wing bending relief.

If ALL the weight of the a/c ie fuel, payload & airframe was kept in the fuselage then there will be a large amount of stress on the wing root (Lift from the wings trying to make them go up, Gravity trying to make the fuselage go down. Where they meet has a conflict of ideas...)

By burning what fuel there is in the fuselage first, and keeping as much as possible stored in the wings, SOME of the stress on the wing root is reduced.

[jettison valve]

what you said about fuel management (wing bending relief etc.) is certainly true.

unless... well, unless we are talking about current operations of for example B744s... :mad: ... have a look at

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/WebNewEmergencyAD/A633C3C40E22D17E86256C7B007DB077?OpenDocument

will they ever get the pump story fixed...??? :confused:

cheers,
j.v.

[john_tullamarine]

Two comments.

(a) in regard to the 744 aft tankage, the tank does provide a longitudinal balance input but the standard Boeing fuel use procedure does not optimise CG control to the Nth degree as it is not critical other than for payload-limited extreme range flights where every fuel burn kilo saved is a bunch of dollars generated in increased payload availability.

Qantas, which has just this problem going over the pond to the west coast, did a study several years ago and came up with a variation on the basic fuel use protocol and saved (if my recollection is correct) something in the order of 100-200 kg of fuel per sector ... which, of course, permits the carriage of MONEY-generating payload.

Interestingly, the program was generated due to internal lobbying by a pilot who had a bee in his bonnet over trim drag fuel penalties on such operations. Apparently, the ops eng people were a little surprised at the magnitude of the profit benefit.

I don't know whether the procedure was adopted in line ops after the study ... perhaps one of the QF chaps might comment. I had a jumpseat ride for part of cruise on a QF 744 mid-01 and clean forgot about this procedure and forgot to ask the crew on that occasion.

(b) in regard to filling centre wing tanks (and fuselage aux tanks) last ... this is a generally applied technique to maximise ZFW availability due to wing bending relief as indicated by Tinny. As a for instance, over the years aircraft, especially freighters, have been recertificated to higher MZFW limits by rescheduling fuel usage protocols to include a requirement for minimum outboard tank quantities to exploit the bending relief benefits. Ansett, for instance, did just this when the Golden Goose (L188 Electra) fleet was modified from pax to freight .... memories ....