View Full Version : 737NG minor fuel imbalance

17th May 2015, 08:35
A recent experience has been bugging me, so I thought I'd open it up to the canny observations of the pprune chattering classes.

We were on the ground at out station with approximately 4 tons onboard after a tankering flight. The centre tank was empty. We uploaded fuel for a block of just over 6 tons, spread between the main tanks. I joked to my co-pilot that it was curious that the fueler had also put 60 kgs in the centre tank, though of course I actually presumed that this was left over from a previous sector even though it occurred to me that we hadn't used the centre tank on the previous 3 sectors. The APU was only used for approximately 7 minutes for engine start.

At TOC the centre tank was empty despite the pumps never having been touched. Not long into the short cruise we experienced the start of a minor imbalance. Curiously it was No.2 main tank that showed less fuel. By TOD the imbalance was 170kgs and on the ground the imbalance was 220kgs with nothing in the centre tank. The flight was only 1:25hr.

According to the lower DU fuel used on each engine reflected the fact that No.2 was started before No.1 but nothing more than that.

Crossfeed valve was closed throughout. Aircraft is a 737-800 which is less than 6 months old.

I discussed it was engineers on arrival and a bite test didn't reveal anything, nor did they have an ideas as to the cause of the imbalance.

We've all seen regular imbalances due to APU use or following a climb using centre tank fuel etc etc, and I know 200 odd kgs is not technically a fuel imbalance, but this was a short flight and I don't recall seeing such an imbalance on no.2 main tank before. On a long flight you would certainly need to balance fuel.

I have a couple of theories but don't wish to colour responses by saying any more. But perhaps there is a very simple explanation that is eluding my tired morning brain?

porch monkey
17th May 2015, 09:04
The jet pump empties the centre into the left. Could that be it?

17th May 2015, 09:33
not sure on this but does not the refuel manifold drain into the centre tank on completion of fuelling

cosmo kramer
17th May 2015, 10:29
The center tank scavenge pump doesn't operate until there is less than 2 tons fuel in the left main tank.

On your first leg, you landed with 4 tons - hence remaining fuel in the center tank had not yet been pumped over to the left main.

On the next leg, the scavenge pump started to operate as left main tank fuel level went below 2 tons. So the 1-200 kg remaining fuel was pumped over.

Would that fit with what you experienced?

In some aircraft, the center pumps low pressure lights start flickering quite early. On the previous flights (where you had fuel in the center), did you wait for the master caution before switching the pumps off? Or did you switch them of at the first flickering?

To avoid (albeit small) imbalance, I usually wait with switching the center pumps off, until the low pressure light is fairly continuously lit or the master caution comes on.

17th May 2015, 15:05
Thank you for the thoughtful answers.

Porch Monkey and Cosmo Kramer, you've both picked up on my first thought. The scavenge pump does indeed empty the centre tank into no.1 main tank and only once the main tank is below 2 tons. This fits with my experience insofar as we started with 6.2, normally expect to burn 2 tons in the climb, and the imbalance started short way into the cruise.

Where the no.1 engine is getting the benefit of the tiny bit of fuel from the centre tank it stands to reason that it will not deplete the contents of no.1 main tank as quickly as no.2 engine depletes no.2 tank, hence an imbalance in the order of the amount sucked out of the centre tank. But that was only 60 kgs whereas our imbalance was over 200 kgs.

Also, the centre tank was not used on any of the previous three sectors and was empty when we started the day.

Lightning5, that's an interesting point. I don't know if you're right, particularly as I would expect to see this phenomenon more regularly if it were commonly occurring.

However, I wonder whether the inadvertent deposit of 60 kgs in the centre tank during the filling of the main tanks, and the subsequent imbalance, could be the result of a partially open crossfeed valve?

17th May 2015, 16:30
You also have to take into account that the FSU is not that accurate.

17th May 2015, 19:50
B737 fuel gauges are notoriously inaccurate and often show "fanthom fuel".

Absolutely normal, especially on Classic.

17th May 2015, 20:23
I have no doubt you're right despegue, though my experience of fairly new 738s is that the gauges, whether accurate or not, are consistent. An imbalance is an imbalance, isn't it.

I should also mention that there was a 120 kg imbalance on the previous sector.

17th May 2015, 22:57
Are you really concerned about a 1-200 kg "imbalance"?

18th May 2015, 00:10
Center tank indication below any significant amount is always prone to error, as it can vary already during loading based on forwards strut compression. In flight, this inaccuracy remains with small amounts.
Best guess would be as already revealed by others, scavenge into the left, which would be consistent with a gain in #1 tank.
However, it cannot be ruled out that there may be some "inadvertent transfer of fuel into centre tank" (a small amount of fuel), which used to be a QRH item [now removed], when it is only a small amount, when occurring will be taken care of by the scavenge system into #1 tank.

18th May 2015, 04:38
Well we all know the scavenge pumps can cause a slight imbalance, but this is fuel fed from the center tank and not added.

It could be this aircraft has a check valve at the center tank pump that is not seating completely. This would allow fuel to be back fed into the center tank from the respective main tank, 1. causing 'mystery' fuel to appear in the center tank, and 2. leading to a (non-critical) imbalance.

I like the faulty crossfeed valve theory, MH152. It could cause the imbalance, due to differences in main tank pump pressures, but it wouldn't account for the visits to the center tank from the fuel fairy.

18th May 2015, 07:58
During my time flying Boeings with centre tanks it was not unusual to see small amounts of fuel in there when it was empty before......

777 737 etc

Even the A330 can mysteriously place 50 or 100 kg's into the Stab tank when it was previously empty on short flight not requiring aft transfer.

This is life.

18th May 2015, 10:40
Well we all know the scavenge pumps can cause a slight imbalance, but this is fuel fed from the center tank and not added.

Correct, but it could have been fed into the CTR TANK from #2 TANK, inadvertently, for mechanical reasons, a slow rate. Then, being suction fed into the #1 TANK it materialises as a slow increase in #1 Fuel as the CTR TANK is scavenger empty.
As the rate of scavenge exceeds the rate of fuel flow into the tank (which to cause and imbalance must come from #2 TANK) this shows up in such manner.
Thus I stand by the (no unlikely) position of INADVERTENT TRANSFER OF FUEL INTO CENTER TANK possibility, from TANK #2.

18th May 2015, 20:16
Oceancrosser and ACMS, thanks too for your input. You are absolutely right, it is a minor imbalance and it doesn't 'concern' me, hence the way I phrased the initial post.

However, in 5 years of flying the same aircraft I have not seen a no.2 main tank low by 200+ kgs on a short flight. I have flown many 4-5 hour flights in the 737-800 and rarely seen 100 kgs difference, and normally there's an obvious reason.

When combined with the phantom fuel in the centre tank and an equal fuel burn from the engines I was curious as to whether the two 'unusual' occurrences on this flight were possibly related.

Thanks Skyjob and vapilot2004 for your thoughts. You might be on to something. Next time I fly that aircraft I'll check the tech log and see if it's got a history of similar events, but I won't be losing any sleep over it. :p

de facto
18th May 2015, 20:21
Maybe a simple fact that your low fuel side had stronger than usual fuel pumps output..

its easy
18th May 2015, 20:49
Also as skipper on flight back to pisa, only about 1.30 and we had an imbalance of over 250 kg less in no2. Balanced on dep and nothing showing centre. On ground mentioned it to 2 engineers . They frowned, shrugged shoulders, hummed and said they would review it, but had no answers.

I appreciate different engines on fleets burn different but this much? I know it's in limits but my exception to rule in 7 years.

Just wanted to say I couldn't get engineers to answer!

de facto
18th May 2015, 20:52
N1/EGT/FF? Same same?

18th May 2015, 21:33
152 It's possible the bowser driver opened ctr tank switch before he realised you wanted less than 7800kgs? Who's perfect?

The quoted guage tolerance at full tanks is +/- 2.5%, so each main could start off with virtually +/-100kgs per side error.

If you read my previous postings on mystery fuel readings, you may appreciate the NG, even new 'frames, is not particularly accurate in it's fuel indications. The limit of 453 kgs is just that (1000lbs in American units) and anything less is not worth the distraction.

Concentrate on looking out the window and enjoying the view, whilst current SOPs permit that luxury...............? :)

18th May 2015, 22:12
Having flown with an operator whose procedures including a mathematical analysis of fuel on board/burnt/uploaded, etc, corrected for density, which would have impressed Professor Hawking, I can firmly say that those figures do not deserve your attention. Approaching half a tonne is where I'd get mildly interested. A few tonnes would have my attention, and it would take upwards of half-a-dozen to have me concerned.

19th May 2015, 08:32
TheiC - Ha ha. 200 kgs for us LCC operators is the luxury of once around the hold! Methinks we operate in different 'fuel environments'! :p

de facto - Yes, all was normal: engines performed correctly and we didn't 'lose' the fuel. In fact, the flight record shows we made fuel savings on that sector.

its easy - You had the same reaction from engineers as I did, though they were intrigued by the occurrence. As others have said, and Barkingmad has suggested, there a number of factors that can influence fuel gauge readings which don't demand detailed investigation.

The phantom centre tank fuel merely intrigued me further and I wondered whether there was some connection and possible technical explanation.

Barkingmad - the views across the Alps yesterday were truly picturesque and although I kept one eye on the gauges and spared a thought for SOPs my true focus was out the window. :ok:

de facto
19th May 2015, 08:49
The limit of 453 kgs is just that (1000lbs in American units) and anything less is not worth the distraction.

I thought a difference of 230kgs in 30 mins or less could be an indication of fuel leak..SOP surely..

Capt Quentin McHale
19th May 2015, 12:50

My two cents worth. Firstly, I reckon Skyjob and vapilot20014 are onto it. Fuel is being pumped from the R tank and into the Centre tank through the centre tank r/h boost pump due to a failed or partially failed boost pump outlet check valve. Crossfeed valve position (open/closed) is irrelevant due to the valve being mounted between the centre tank boost pumps in the engine fuel feed manifold, so even if it was open, fuel would still flow through the r/h pump and into the centre tank.

You did not mention anything about your first two sectors, so I can only assume that you arrived with <4tons of fuel both times and zero in the centre tank. You have to have <2tons in the l/h tank in order for centre tank fuel scavenge to operate.

You then tanker fuel and arrive with 4tons and 60kgs in the centre. So with 4tons in the wings no scavenge occurred and hence 60kgs in the centre. As Skyjob pointed out, very low fuel levels in the centre can cause erroneous indications. That 60kgs could have been 100kgs for all we know.

Next sector depart with 6tons and approx 4tons at TOC so scavenge pump kicks in and at next port we have a fuel imbalance but no fuel in the centre.

Mikehotel152 and its easy,

I am very surprised that your respective engineers did not carry out a stick check of the wing tanks to confirm ACTUAL tank quantities compared to indicated.


Close but no banana. Fuel will drain into whatever tank that you are refuelling at the time when you finish refuelling.

Hope this helps.

19th May 2015, 16:04

Your helpful input encouraged me to get out the FCOM2, Pat Boone's MRG, Brady's technical guide and Bulfer's book and I found a note in the latter which stated that an inadvertent transfer of fuel into the centre tank can occur if a check valve doesn't seat properly. Not much else in any of those titles to help solve the riddle.

As for figures, I have the following to hand:

Sector 1: Dep 7.8 Arr 4.5.
Sector 2: Dep 6.0 Arr 2.9
Sector 3: Dep 7.8 Arr 4.5
Sector 4: Dep 6.2 Arr 2.8

On our second sector I shut down engine no.2 for the taxy to stand and I do recall that there was a smaller 100-150kg imbalance at that time. The main imbalance occurred on Sector 4. Interestingly, the imbalances seemed to have occurred on similar length sectors.

By the way, the fuel in the centre tank was not evident on arrival. It only appeared after fuelling the aircraft.

19th May 2015, 21:42

In a desperate attempt to try to find out what was happening to the NGs I was driving and as occupational therapy to keep me "alert" on the Hurghada and similar rotations, l took a piece of graph paper and plotted accurate fuel readings every 10 mins. (I know, sad b'std!)

I plotted at the same event, FMS fuel off the cdu-optimistic, guage totals to nearest 10kgs and finally ramp fuel minus engine totalizers at the same moment.

Assuming the pre takeoff apu burn was a constant and did not significantly affect the result, EVERY 'frame I flew showed an alarming "loss" of up to 200kgs at around the time the ctr tank pumps were selected off iaw SOPs.

Later in the sector, the "missing" fuel mysteriously recovered and despite presenting a number of graphs to the engineers, nobody could put forward a reasonable explanation.

All this was done whilst checking bowser uplift in litres against the kg uplift converted with SG at the time, an old Dan-Air precaution which has been forgotten by the newbies in flt ops management on the basis " not invented here, so we don't do that".

Apart from keeping you awake during the dogwatch, this plotting may provide an answer?

However, unless the observed imbalance/errors cause a sphincter-tightening moment or two, personally I'd use the spare mental capacity to look out of the window, or to practise the black art of weather radar tweaking or planning where I'd go in a hurry when something goes 'twang'. Or asking your (junior?) oppo in the rhs what would he/she planned to do if it all went pear-shaped whilst you were in the loo?

There again, it depends if/how soon one reaches the boredom threshold, if at all............ :)

Capt Quentin McHale
20th May 2015, 05:59

Thankyou for the additional info re..sector figures. This has encouraged me to also reach for my "special" book titled.......Fuel Gremlins 101!!! A truly perplexing defect.

20th May 2015, 11:01
Lots of comments about phantom fuel.

Maybe it's time to think not in terms of phantom fuel but fuel that exists due to A/B/C, e.g. due leaking seals etc, but subsequently is actually used as per design using the scavenge pumps.

Unexplained fuel in a centre tank does not mean phantom fuel which implies that it does not exist.

There may be a perfect reason for those tanks to indicate fuel, as explained for various technical reasons, the amount of fuel of which can be indicating a different amount at varying stages of flight due pitch attitude, acceleration/deceleration, turns and subsequent time required to settle again for accurate measuring.

20th May 2015, 15:33
But Skyjob, phantom fuel, as with the existence of the 'Mary Celeste' criss-crossing the Seven Seas to this day is not in doubt. It's how and why it got there, left and returned is a curious phenomenon which I suspect we've done to death. :p

Barkingmad, you live up to the expectation sown in the mind by your pseudonym. What fun. I don't think 220 kgs is quite enough to encourage me to such graphic antics. Funnily enough, I left the FD to 'check my make-up' (he didn't get the joke either) on this morning's Scandinavian adventure and left my 2-striper to see how he got along with the lovely Valentina in my absence. The disappointment at my untimely return was audible... :E

Thank you all for your helpful insights. Unless someone comes up with a wholly new angle on this, perhaps we can move on. :)

28th May 2015, 15:41
So yesterday We took off with 10.4 in the tanks. Wings were full and rest in the center.

After about an hour of flying at a sat of about -40 at .78 I totaled the Burn and Remaining whihc I found to be only 10.2


28th May 2015, 15:48
Apart from the previously mentioned inaccuracies, one thing I note:

I totaled the Burn and Remaining

Burn is measured near the nozzles in the engine.
Remaining is what's in the tanks.

In between you've got a bit of fuel in the pipeline. After shutdown you can 'see' this fuel pouring back by a slight increase in fuel quantity over a minute or so. I believe also the reason why FMC fuel onboard is more than adding the fuel gauges together.

Capt Quentin McHale
29th May 2015, 00:18
172 driver,

Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.

Also, I would suggest that the "slight increase" in fuel quantity after shutdown is due to the fuel settling after sloshing around in the tanks due to landing/taxiing and not "fuel pouring back" due to the tank boost pump non return check valves.

29th May 2015, 08:15
Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.

I'm bettin' 172 is referring to the HMU output transmitter, located a little over a foot upstream of the nozzles.

Regarding 172's comment on draining/returned fuel causing a higher reading, I am not so sure about that one either Cap. I've always understood the change in levels was due to the FQIS averaging out the probe readings more accurately with the aircraft still and level on the ground - with the main tanks having more variation than the center.

29th May 2015, 08:57
Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.

I meant the fuel flow transmitter is located near the nozzles and from that the fuel burn is derived.

About the pouring back , could be an old wives' tale I have just taken for granted. The technical description does not seem to support it.

Capt Quentin McHale
29th May 2015, 13:04

Totally agree with your interpretation of fuel indication with the aircraft at a standstill. But what is a "HMU output transmitter" and what does it transmit and to where?

30th May 2015, 21:42
Apologies for the inexact wording I used Cap M. I should have said the full and proper phrase, "fuel flow transmitter" which is not far down the pipe from the fuel output of the HMU. It measures the flow rate coming out of the HMU on the way to the nozzles in the cans. It is an electromechanical device, converting fuel flow to an analogue signal.

The output of the transmitter goes to the EEC, located on the fan case, which communicates this flow rate via the ARINC-429 bus to the EICAS computers and fuel totalizer which sends this figure to the FMC for displaying and comparing CALCULATED to TOTALIZER fuel. For the classics, the fuel flow transmitter signal is monitored directly by the EFIS and totalizer units.

Capt Quentin McHale
31st May 2015, 02:41

Many thanks for that, now I see the picture. This has been a great thread (as has others) for all to maybe gain a bit of deeper knowledge about the toys we play with. Again many thanks.

1st Jun 2015, 01:42
Cheers Cap!

I too have learned much about many things here at pprune as well! Often, things I have wondered about myself, or sometimes its something I would have never even thought about until coming upon a thread by a fellow member. :ok: