B777 vref check -0 to +1 knots

[Wordsworth]

Maybe someone could help me and a buddy with this question?On our B777 fleet we have a gross error check for our vref. We compare our laptop performance calculations with the FMC after all info has been entered. In our SOP s we have a tolerance of 0 to +1 knot. anyone know why?

Is it to do with the FMC working out the speed based on calculated weight at that precise moment compared to a planned TOW in the perf laptop? Or is there a more complicated scenario going on?

It's a possible upgrade question so sorry to be such a "spotter"

Thanks in advance

[Stel]

Exactly that. The OPT calculated VREF30 is based on your planned TOW as entered, the FMS displayed VREF30 is based on the current a/c weight which at this stage will be greater than planned TOW as it includes the taxi fuel allowance. Under worse case scenario the VREF will increase by 1kt for a weight change of 3.3T, therefore, the most it could be with a sensible taxi fuel allowance would be 1 kt higher and 0 kts less. Hope that makes sense?