B757 Fuel Calculated vs. Totalizer
 

Can any one tell me which fuel quantity is more accurate? If there is a difference between the overhead total and the from the FMC which one should you use? Are there different probes to get these values?

Cheers

We use the overhead totalizer rather than the FMS. We have an update capability in the LEGS page which you can use when a discrepancy occurs at a waypoint to update the FMS.

Can't help with the 757, but can offer this from the 767. . .
 

The totaliser quantity is derived from probes in the fuel tanks themselves. The calculated (FMS) value is initially set equal to the totaliser value.

Once the fuel-on signal is received from the fuel control switches during engine start, the FMS then uses fuel flow data to continually reduce the calculated value.

I guess your question concerns which value to place more faith in. All I can suggest is that totaliser value is subject to probe measuring errors, admittedly small in these A/C, whilst the FMS value is only accurate assuming there is no fuel leak between the tanks and the point at which fuel flow is measured.

WG.:\