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?