For non-Airbus readers, the following may be useful regarding just what functions the FAC computers perform. Bolded sentences are by my action.
AUTO FLIGHT
FLIGHT AUGMENTATION
1.22 40 P 1
SEQ 001 REV 25
GENERAL
The aircraft has two flight augmentation computers (FACs) that perform four main
functions:
• Yaw function
— Yaw damping and turn coordination
— Rudder trim
— Rudder travel limitation
• Flight envelope function
— PFD speed scale management
* Minimum/maximum speed computation
Maneuvering speed computation
— Alpha-floor protection
• Low-Energy Warning function <I
• Windshear detection function <
In performing these functions the FAC uses independent channels :
Yaw damper
Rudder trim
Rudder travel limit
Flight envelope
Each FAC interfaces with the elevator aileron computers (ELACs) when the APs are
disengaged, or with the FMGS when at least one AP is engaged.
Both FACs engage automatically at power-up.
The pilot can disengage or reset each FAC (in case of failure) by means of a
pushbutton on the flight control overhead panel.
When a FAC is disengaged (FAC pushbutton set off) but still valid, the flight envelope
function of the FAC remains active.
If both FACs are valid, FAC1 controls the yaw damper, turn coordination, rudder trim,
and rudder travel limit, and FAC2 is in standby.
FAC1 keeps the aircraft within the flight envelope through F171 ; FAC2 performs this
function through FD2.
If a failure is detected on any channel of FAC1 ,
FAC2 takes over the corresponding
channel.
Note that the FACs also control the rudder operation in certain important ways. For that reason, it may be more than coincidental that the following comment by 747SP5 reported repeated problems with that aircraft's rudder system, the same computer system that the crew was attempting to shut down during their departure from controlled flight.
http://www.pprune.org/rumours-news/5...ml#post8841170
Did they use the switches on the overhead panels? That would not work too well if the flight envelope protection part of the FAC system was malfunctioning without failing its self test.