B757 Fuel Calculated vs. Totalizer

[sa227]

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

[Cougar]

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.

[Waste Gate]

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.:\