OK
Homo Ludens, it's a bit long winded, but here goes. Bear in mind that what I say here came about after experimentation, and VERBAL advice from Boeing, I have no reference to quote.
It is an FMC problem, specifically, VNAV, it has nothing to do with Flowmeter performance or fuel density, and applies to RR Trent versions as well as GE 115 versions.
Like yourself, I had often noticed the difference en-route between Indicated and Calculated fuel, but unlike you, I often landed with about 1 Tonne less than the FMC prediction, even after flying the approach and arrival exactly as programmed in the FMC (no additional radar vectors etc.). The Calculated prediction was in error on the "wrong" side, leading one to believe that there would be more fuel on arrival than actually was.
The first "fix" was accidental. Most pilots calculate Vref by adding the predicted fuel on arrival to the ZFW, and entering this figure on the APPROACH REF page to get the Vref. After leaving the APPROACH REF page, the manual weight entry reverted to Actual Weight (Important to remember).
In earlier times, the Boeing technique for calculating Vref was messy and seldom used (most operators used the technique described above). Hints of it are still there in Vol 2 if you want to go searching. The technique was to note the FMC Fuel prediction for arrival, go to the
PERF INIT page, and Manually enter the FMC Fuel prediction into the Fuel line. (This immediately resulted in an "INSUFFICIENT FUEL" message, but we knew why it was there). Then proceed to the APPROACH REF page to get the Vref.
VERY IMPORTANT : Now return to the
PERF INIT page and DELETE the Manual fuel entry, it DOES NOT automatically erase when you leave the page. This is done by pressing DELETE and Line Selecting the Fuel Quantity line.
After doing this return to the PROGRESS page, and Huh!, where did 800 to 1000 Kg of fuel go to?, the Arrival Fuel Prediction often dropped by this amount. (Yesterday the Arrival fuel dropped from 9.1T to 8.2T after this procedure, necessitating a second calculation of Vref). Go now to PROGRESS Page 2, and et voila, the Calculated and Actual Fuel matched, or nearly so.
And what fuel did we land with after STAR/Approach as planned ..... 8.2T
My Boeing informant tells me that it's a VNAV glitch that will be addressed ..... someday. By going to the PERF INIT page, and inserting a Manual figure, and then deleting it, it forces the FMC to do a "Soft Reboot", removing the cumulative VNAV errors. (Being an INIT page, you're forcing a "Soft" Re-Initialisation and internal re-work of the figures).
So there you have it. The
DANGER in the procedure is that most crews are accustomed to temporary Manual entries erasing after moving to a new page. It DOES NOT for the PERF INIT page, and some crews have been perplexed by subsequent INSUFFICIENT FUEL warnings. For this reason our company (and most others) avoid the original Boeing technique, but a side effect of this Vref calculation, although messy, is correction of a known Calculated Fuel error caused by a VNAV glitch.
BE CAREFUL!!!!!!
Regards,
Old Smokey