BOAC, not sure how u guys cross-check planned burn to actual burn throughout the flight(I'm sure it's pretty standard). Here is the scenario, on a 5hour flight, we were not really losing any fuel as compared to planned burn off at different waypoints when we had fuel in the center, in fact we had a gain, right up to the point where center fuel was depleted...or as the flight progressed, nothing changed dramatically, winds were pretty spot on, sats were give or take....yep scavenge wouldn't be the question here as the center tank was dry on the B738 at this point.
We climbed when the computer flight plan did...(just to figure out why we would lose fuel on these 5hr+ sectors)
Eventually just before landing we were at a dismal -800kgs when compared to the planned. Can't really explain why it would be so....
Also noticed it doesn't just happen on one airplane happens on pretty much all or B738s