Even at the start of a new flight with fresh everything and loaded tanks, the GToW is always listed on the PERF page of the FMC as about 1-2K lbs lower than the "Dual" weight. This is because the fuel calc is always less than total fuel listed on upper EICAS (fuel totalizer.)What causes this difference to occur and does it matter? Should one adjust for it by simply raising the ZFW on the PERF page by the amount of fuel diff?Jonathan

FMC's which show DUAL are found on aircraft which have a Weight and Balance System. This automatically computes the weight and CG of the aircraft. Unfortunately, this system is known to be unreliable on the real aircraft and the values are always cross-checked against a manually computed flightplan.This system is not the norm for pax aircraft (usually only freighters and combi's have it), but is sometimes added in simulators to make the simmers life a bit easier).As far as I know, the two values should be precisely the same at the start of the flight (i.e. when the fuel levers are put to RUN). The FMC uses the DUAL (or manually inserted) GW value as a starting point for it's calculations. From then on, the FMC has to rely on the accuracy of the fuel flow sensors on the engines. Fuel flow meters are very crude in the way they compute the fuel flow. Basically, they consist of a fuel-turned propeller and a propellor speed sensor, so their accuracy is not 100.00%The totaliser system accuracy varies also. There are dozens and dozens of sensors in the tanks, but some of the tanks are very odd shapes, and it's difficult to compute a precise mass value for all fuel levels. Also, even though there are densitometers in the tanks, fuel readings can be affected if, say, the fuel uplifted is a different density to that fuel already on board.Of course, pilots/flight planners always allow for these minor inaccuracies in their calcs. Minor variations between calc and totalizer values are the norm as the flight progresses. The FMC will provide a warning if the variations become significant.Hope this helps.Cheers.Q>

Q:Many thanks for this helpful reply. Good to know it is not me doing something wrong.Cheers,Jonathan