737NG minor fuel imbalance
 

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?

The jet pump empties the centre into the left. Could that be it?

not sure on this but does not the refuel manifold drain into the centre tank on completion of fuelling

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.

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?

You also have to take into account that the FSU is not that accurate.

B737 fuel gauges are notoriously inaccurate and often show "fanthom fuel".

Absolutely normal, especially on Classic.

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.

Are you really concerned about a 1-200 kg "imbalance"?

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.

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.

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.

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.

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

Maybe a simple fact that your low fuel side had stronger than usual fuel pumps output..

Had the same
 

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!

N1/EGT/FF? Same same?

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...............? :)

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.

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: