Hi, this is from the FCTM regarding the C.G. entry. Basically your FM predictions and characteristic speeds will be in error if you enter a wrong zero fuel weight and to a lesser extent a wrong zero fuel weight C.G.. However in flight the FAC will calculate its own gross weight, C.G. and characteristic speeds based on aerodynamic data. Hope it helps.
The GW and CG computation is as follows:
1.The pilot enters the ZFW and ZFWCG in the MCDU INIT B page
2.The FMGC computes the GW and CG from:
The ZFW, ZFWCG inserted in the MCDU INIT B page
The fuel quantities from the Fuel Quantity Indicator (FQI)
The Fuel Flow from the FADEC.
3.This current GW and/or CG is used for:
FM predictions and speeds
ECAM (GW only)
MCDU (GW and CG)
4.The FAC computes its own GW and CG from aerodynamic data.
5.GW and CG FAC computed are used for:
Minor adjustments on the flight control laws
Characteristic speeds (VLS, F, S, Green dot) display on PFD.
Note:
1.On ground, FAC uses the GW FM computed.
2. In flight, at low altitude (below 15 000 ft), low speed (below 250 kt) and flight parameters stabilized, GW FAC computed comes from aerodynamic data. If these conditions are not met, GW FAC computed equates to the last memorized GW - fuel used.
3. If the GW FM computed and FAC computed differs from a given threshold, a CHECK GW message appears on the MCDU scratchpad.