777 Fuel Index Question

[trigger21]

To any 777 trainers / people that understand it...

Why do you enter the MACTOW into the CRZ CG above 98 tonnes of fuel, rather than MACZFW. I understand its something to do with the fuel index but its unclear.

Also does it effect the optimum altitude in the FMC?

Thanks in advance! :ok:

[5LY]

This is done because the default is very conservative. You'll recall that an a/c with an aft c of g can climb a bit higher than one with a forward c of g. The entry you're making is intended to refine the computation in the FMC. Otherwise your FMC computes too low a cruise regime and your trip fuel suffers.

Why this couldn't have been built in is another question.