Wing tank fuel imbalance - Causes

[Superpilot]

Occasionally a wing tank fuel imbalance of 500 to 600Kg occurs. I agree some imbalances can be attributed to lazy fuel pumps or transitory fuel transfer issues but 600Kg is a little extreme. Can anyone shed some light as to what might be causing this? We fly A321s on long sectors 5+ hours. Anything to do with the side the sun is shining on the most :E, or maybe something to do with large crosswinds when at high altitude? Interested to hear your thoughts.

[Starbear]

SP, a couple of thoughts in addition to your comment on fuel pumps which should not be lightly ignored on this topic.

Is it all of your A321 a/c or an individual one?
Is the imbalance always in the same direction?
Is the centre tank still feeding or had been feeding asymetrically for any period. Guessing on those sector lengths, you always carry centre tank fuel?
APU usage on ground for how long on affected flights?

With the T/levers at clb gate and EPR or NI aligned are FF exactly the same?
If you align FFs ie manual thrust what is difference in EPR or N1?
If a single a/c affected its not unknown for a tank internal pipe or gallery to rupture and allow cross transfer between actual tanks-something which was in the hands of the crew but generally done away with many years ago.

Back to fuel pumps; All fuel pumps as I am sure you know have a +/- tolerance on their output pressures and this can be significant (its actually why some manufacturers dissuaded the training habot of some trainers to encourage automatically of opening the xfeed valve after an engine failure on T/O (in addition to possibility of a fuel leak) The point was made that ALL fuel pump output pressures in a simulator are perfect for the simple reason thaere are no pumps at all of course, just wiggly amps.

[Uplinker]

Yeah, agreed.

It is sometimes possible that:-

APU useage on the ground,
mismatched engines, (older engine using more fuel),
misbalance of fuel in tanks at refuelling shut-off,
etc.,

...all cause a slight imbalance but perhaps not enough to notice each individual difference once pushed back, because you are busy with other stuff!

Later, in the cruise during a fuel check, if all the above factors have stacked up in the same direction, you can get the sort of imbalance you have noticed. I have seen this myself but it is rare to have that much. Up to a few hundred kilos is fairly common on our fleets.

[Black Pudding]

Interesting point with regards this.

If you leave the imbalance of 600kgs and for some reason you end up planning to land with only final reserve left, be aware of a possible flame out on finals at your alternate or destination field.

[Goldenrivett]

Hi Black Pudding,
be aware of a possible flame out on finals at your alternate or destination field
Care to explain why you think so?

[Citation2]

Final reserve is approximately 1200 kg so 600 kg per wing tank.

[Machinbird]

Assuming a problem with only one particular aircraft, what about the possibility of a fuel quantity system error in one fuel tank?

In other words, is the fuel differential that you are seeing real?

[Cough]

BP/Citation, that's just not possible, if you follow SOP's for the ickle bus.

Once either inner wing tank gets below 750??kg, a procedure pops up, getting you to open the cross feed. At that point, both tanks feed both engines...

Ps, edit to add, one of ours did this. A simple look at fuel flow gave the game away and a subsequent chat with one of our power plant experts showed me it was all within tolerance (one older eng, one newer one)

[Black Pudding]

Cough

Thanks for mentioning that.

I don't recall reading this in our SOPs or FCOMs, I'll have a look for it on my next flight. Do you have a FCOM reference for this ?

Regards BP

[Citation2]

Both tanks feed both engines , that is not correct but one tank is feeding both engines.
Procedure says

FUEL L(R) WING TK LO LVL :
IF NO FUEL LEAK AND FUEL IMBALANCe
FUEL XFEED....................................ON
TK PUMP 1 (ON SIDE WITH LO LVL)....OFF
TK PUMP 2 (ON SIDE WITH LO LVL) ...OFF

So one side ( full side ) will be depleting at a faster rate , can you still continue to destination?

[Cough]

Which will continue until you get..

Fuel L+R wing tk lo lvl.

Which makes all fuel useable. As for continuing to destination - Thats for your SOP's and OM to decide.

[Black Pudding]

Cough

I should have made my question clearer.

You said "Once either inner wing tank gets below 750??kg, a procedure pops up"

Do you have any reference to where this maybe written in FCOMs

[Check Airman]

I assume he's referring to this message

FUEL L (R) WING TK LO LVL

In our book, it says the msg pops up at 1650lb, which I think is how you say 750kg in Metric. ;)

[RAT 5]

In our book, it says the msg pops up at 1650lb, which I think is how you say 750kg in Metric.

How, in the 21st century, did 'lbs' find its way into any Airbus manuals? Ah; now I see, USA. One could open a betting book abut the date when USA will join the metric world. It is a good chance to win as snow in London on Christmas day.

[Check Airman]

The zone temps on the COND page are shown in Farenheit, while the pack temps on the BLEED page are in Celsius...

Metric is easier to work with, by design, but I'm not looking forward to being told to maintain 1000m, and fly heading 2 pi :D