Hey guys
We have the exact same thing on our fleet. I've been asking around about this, and it seems to be fairly common, but if you get an answer please let me know.
We operate 738's and employ fuel bias calculations.
Often you loose about 500 kgs when center tank is depleted, but approaching the end of the sector (for us everything from 3 to 6 hrs) it rises again to a difference of appr. 2-300 kgs.
I see from CaptainSandL's webpage that the 738 has a accuracy of about 2% of full capacity without densiometer which puts us right there (13066 x 0.02 = 261 kgs).
I've heard a theory that without the densiometers it measures all the fuel from an average value of capacitance between all the three tanks. When you then deplete the center tank, the average specific gravity is higher than the SG for the cold main tanks, and it underreads.
I don't have access to any maintenance manual, so I have no idea, and no opportunity to check this I'm afraid. There is little to deduct from our companys OM's.
I hope some clever heads here figure it out.
Cheers
