Originally Posted by captainpaddy
Surely whether the FAC weight and speeds are inaccurate is sort of irrelevant. You have to honour and follow them regardless??
Not true. If you get a Check Gross Weight message on the MCDU ( difference of 5 or 7 tonnes depending on standard between GWT FAC & FMGC ) you need to establish the error source. If loadsheet error, update Fuel Pred page and if FAC error you must ignore FAC character speeds and get them from QRH.
I have had the FAC error myself. First thing to remember is the system uses an ELAC failsafe where the aircraft can have inaccurate speeds however the ELAC only considers alpha information and hence, offers protections- stall warning or alpha prot depending on laws. A small alpha error of all probes results in a significant speed error, however, this small error is negligable in critical angle of attack information for protections considered by ELAC. So you use QRH and trim sheet weights for speeds accordingly.