738 FMC fuel predictions

[nick14]

Hello all,

We have a common question at our base regarding fuel predictions on the FMC PROGS page.

When the centre tank contains fuel, the fuel prediction over-reads and as the fuel is burnt during the cruise, the prediction reduces to a sensible reading. I have personally seen differences of around 1000kgs from start to approach.

Does anyone know why? I have heard talk about temperature sensors in different tanks etc?

Thanks

[misd-agin]

1500+ hrs on the airplane and didn't notice it as you describe it(center tank related)

Did notice, as on all Boeing fuel/FMC readouts/predictions, that total fuel reads low after climb/level off and FMC prediction is optimistic early in cruise but increases accuracy of predicted arrival fuel later in cruise, which is similar to what you appear to be describing.

IMO the typical discrepancy between early cruise predictions of FMC predicted arrival fuel would be 1000-1500 lbs (450-700kg) optimistic vs. the expected/actual arrival fuel.

Did not appear to be center tank related as it was often, if not typically, empty.

[BOAC]

http://www.pprune.org/tech-log/238854-737ng-ctr-tank-density-error.html has more discussion

[framer]

I didn't think it was center tank related either.
The progress fuel over destination seems to be optimistic by about 400kg at top of climb compared to top of descent in our machines, although I have seen 1000kg once as well. I put this down to ambient conditions being different from FMC conditions, ie different wind, temperature, and less than perfect aircraft trimming etc. If you get more favourable winds than loaded in the FMC for example , it does go the other way as well.