Hi butcher,

I don't know how old your A320s are, but have you read the previous posts, in particular from #6 onwards? Until I saw that, my understanding was similar to yours and I was not aware that the system architecture may be different in newer aircraft.

http://www.pprune.org/tech-log/59331...ml#post9735039

Initial calculation
 

Hello Chris,
My company's MSNs are in between 3000 and 5000ish.
At the beginning of the flight FAC uses crew entered data on MCDU but it changes to FAC's own calculation after switching to aerodynamic calculation model. (#6 says a bit different) At least that's what I know. I found an article as an answer from Airbus on old WISE which states that.
I think this is an important subject and FCOM should be more detailed on matters like this one.

For older aircrafts FCOM says:
The characteristic speeds displayed on the PFD are computed by the Flight Augmentation Computer (FAC), according to aerodynamic data.

For newer:
The characteristic speeds displayed on the PFD are computed by the Flight Augmentation Computer (FAC), according to the FMS weight data (for PFD/MCDU display consistency and accuracy purposes), and aerodynamic data as a backup.