How does this sound: When you refuel from below, (pressure), you allow the displaced air to escape via the vents that are a part of the surge tanks, it is a natural process, the fuel level rises the air moves out.
Alternatively, if you fill from above, (gravity), somehow air gets trapped as there is no significant pressure differential?
That said, I remember on the BAC1-11, when operating Tenrife to Manchester, we would refuel at the panel until cutoff, then ask the refueller, (now armed with several packets of fags), to complete overwing.
An interesting question.
Last edited by BlueEagle; 9th July 2002 at 10:41.