Hi,
The FACs use MCDU weight for its flight envelope calculations, as long as an MCDU is operative. FACs also calculate the GW and CG independently for ELAC, for backup in case the MCDUs go out and also for the FMS' gross weight cross check. FACs calculate the GW and CG based on live aerodynamic data from the ADIRS. If there is GW discrepancy in flight between what FACs calculated and what FMS is programmed, then "CHECK GW" MCDU message will appear to let you know.
This is from DSC-22_40-30 Airbus FCOM.
My question is then, will the envelope speeds be based on FAC calculated GW and CG if there is a discrepancy between the two?
It seems there are two possibilities here.
1: FAC will display envelope speeds on PFD based on its own calculated GW and CG and ELAC will follow it. Which means VLS indicated on PFD and MCDU PERF pg will be different.
2: FAC will display envelope speeds on PFD based on MCDU GW and CG but ELAC will follow different speeds than what is shown on PFD.