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B737 NG fuel balance technique?

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B737 NG fuel balance technique?

Old 3rd Aug 2008, 17:53
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B737 NG fuel balance technique?

HI all,

In recent discussions with fellow pilots, we ended up divided on when and how to apply fuel balancing between main tanks 1 and 2. Boeing does not help to clear the ambiguity in it's FCOM, FCTM and QRH statements on the matter.

Excerpts:

FCOM 1, Limitations, Fuel Balance:

Lateral imbalance between main tanks 1 and 2 must be scheduled to be zero. Random fuel imbalance must not exceed 453 kgs for taxi, T/O, flight or landing.

FCTM , Fuel Balancing Considerations:

Routine fuel balancing when not near the imbalance limit increases the possibility of crew errors and does not significantly improve fuel consumption.



So I would like to hear your opinion on the following:

1. Does the "zero imbalance schedule" as laid out in Limitations chapter, apply only to pre-departure ground ops, or extends throughout the flight envelope?

1. After you have MADE SURE you are not experiencing a fuel leak, what would be the imbalance value you would start to balance the fuel at?




PS: I would really love to hear Old Smokey's five cents on the matter...
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Old 4th Aug 2008, 11:23
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My practical interpretation of that is I ORDER fuel left and right to be equal (except for a prolonged ground APU run where I might ask for 100 extra in the left) but the aim would be to 'schedule' startup with equal wings. After that anything goes - up to 453 - including differences in tank shut-off and refueller errors. Any more than that and I would ground transfer - never had to do that yet, but I did have to 'fudge' it once with APU crossfeed.

In flight I have no real fixed figure. I fly with co-pilots who want to balance 100kg. I guess if asked I would say around 300kg. The actual 'out-of-trim' effect is small. Of course, there's always the old 'funny' of keeping extra in the upwind tank for landing in a crosswind..............
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Old 6th Aug 2008, 08:58
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Thanks BOAC for sharing.
However, the main point has not been resolved - in FCOM Vol 1, Limitations chapter, Fuel Balance, Boeing uses strong wording:

Lateral imbalance between main tanks 1 and 2 MUST be scheduled to be zero.

They do not mention during refueling, ground ops, or similar.

Does that apply for refueling and ground ops only, or the statement extends throughout the flight envelope?!?

Also, as we all know, you would have "IMBAL" alert at 453 kg difference, and when you start correcting for it, the alert will only disappear ar 91 kg difference..........

So, does the Limitations "MUST" statement stand for condition throughout the flight envelope?

Last edited by NOTSURE; 6th Aug 2008 at 09:29.
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Old 6th Aug 2008, 11:04
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Yes - it is all down to language. I suggest you ask Boeing direct for the meaning or have a lawyer 'interpret' it if you feel it necessary. To me, it is like a 'scheduled flight', and we all know what happens to those.

Personally I feel "Routine fuel balancing when not near the imbalance limit increases the possibility of crew errors and does not significantly improve fuel consumption." answers your question?

Why not also get a definitive ruling from one of your 'suits'? That will cause some shuffling of responsibility

Edit Smiley adjustment

Last edited by BOAC; 6th Aug 2008 at 11:23.
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Old 6th Aug 2008, 11:16
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The way I read it is that "the lateral imbalance must be scheduled to be zero" is saying that you can't plan to have a difference between your main tanks, eg. adding extra to one tank for ground apu fuel usage. However, once you have loaded your fuel and you are underway, the 453kg difference comes into play - ie. that's the limit and if you have a 250 or 300kg or whatever difference, that's within the limitation and so you don't have to correct it. I go along with the FCTM view that fiddling with the fuel pumps unnecessarily is only likely to increase the risk of making a mistake so probably wouldn't carry out balancing below 300kg imbalance.

PW
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Old 6th Aug 2008, 11:33
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MUST be scheduled to be zero means just what it says; clearly at all times. The word "scheduled" - presumably pronounced here with a "k", suggests that you should always have the intention of achieving a zero imbalance, just a legalese way of letting you have some leeway up to 453 as long as you appreciate that effort should be made to return the imbalance to zero at some stage. That could be next time you land and refuel, for instance.


The imbalance figure of 453Kg applies to any time the aircraft is under way, as stated, so a transient imbalance over that during stationary ground ops, ie refuelling or maintenance is excluded, as long as a zero imbalance is scheduled, as described above.

As to when, the statement "when not near the limit" is pretty obvious, I'd have thought. Again it allows some leeway for practicality. I'd personally think that 300Kg was not really "near" 453Kg, 100Kg most definately is not. Somewhere around 400 might be a starting point, but this is a judgement call.

Recreational fuel balancing (eg at 100Kg) is a big no-no imho. Components fail when they are moved or operated, and people screw up too, so you have a finite chance of human/equipment failure every time they move. Thus operating critical components (crossfeed valve, fuel pumps and associated switching circuitry) when it is not necessary is just exposing yourself to unnecessary potential failures. A stuck open crossfeed you can do without...try explaining the ensuing diversion to your CP because you fooled with the crossfeed at just 100Kg imbalance!

The limit is for structural reasons, asymmetric stressing etc. The aircraft is perfectly flyable and controllable with one wing full and the other empty. (about 1/3 deflection in roll on the wheel in flight.)


Ground transfer is surprisingly rapid. If you ever do it, watch the guages carefully, a ton goes across in no time and an opposite imbalance could easily occur with just a few seconds inattention.

Last edited by Agaricus bisporus; 6th Aug 2008 at 11:44.
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Old 6th Aug 2008, 15:54
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I think a boeing symposium would be the best way to answer and make the question clear.
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Old 6th Aug 2008, 16:44
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Thanks for your thoughts guys,
The email is on-the-way to Boeing, and if they shed some new light on the matter, I'll post it here
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Old 6th Aug 2008, 19:18
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The email is on-the-way to Boeing, and if they shed some new light on the matter,
Fer Chrissake's pal, dream on!!! That's surely a waste of time!

Jesus, if you operate a Boeing and are so hung up on procedural trivia as this then you'd perhaps be better off on a Kingair, or maybe a plastic hairbrush This is one of the least of the many logical anomalies in the Boeing books. Get over it!

Everyone knows that Boeing's manuals ar written by lawyers for lawyers and have no relevance to aviation.

Don't waste your breath. They cannot offer an opinion lest they be found wrong.

Accept the Professional judgement above, that is the current practical state of the industry. The legal bottom-feeders cannot second-guess that.
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Old 14th Mar 2018, 11:22
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Fuel balance Technique

Hi, did you have any answer from Boeing about Fuel Balance Limitation?
Is only for Ground ops or it applies to all the flight envelope?
regards
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Old 14th Mar 2018, 15:51
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I go along with the FCTM view that fiddling with the fuel pumps unnecessarily is only likely to increase the risk of making a mistake so probably wouldn't carry out balancing below 300kg imbalance.


IMHO, 'scheduled' = planned. That is at the loading stage in preparation for flight. Being practical, +100kgs for APU, will mean that taxi is planned to commence with balanced wings. Don't see anything wrong with that.

Reference in the air: I used to see guys get board and look for something to do. 100kgs imbalance was one of them. Agh!. The gauges have a 2% FCD error. Willy nilly they'd be switching x-feeds and pumps. No note taking of which side was low, how long you've been in the cruise, how long was it since CTR tanks had emptied, were the mains showing balanced when CTR's were empty, etc. And of course they'd then forget and the imbalance would go the other way.
Then Boeing included the 453kgs caution. Why not leave it until then and make an analysis of the situation and if it seems OK, then rebalance, making a note of the time etc? Did the rebalancing take as long as you expected? Anyone who thinks fuel balancing does not need careful thought need only watch the AirTransat Atlantic glider video to be sobered up.
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