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?
Last edited by Mikehotel152; 17th May 2015 at 14:32.