Check Airman,
We have the following warning in our manual:
In some cases, one FMGC may fail but continue to send invalid guidance targets to the autoflight system. Both the autopilot and flight director may remain engaged, but are invalid. 1FD2 remains on the FMA display but the guidance is erroneous on the side of the failed FMGC. The following messages may display:
• MAP NOT AVAIL
• GPS PRIMARY LOST
• OFFSIDE FM CONTROL
• SET OFFSIDE RNG/MODE
Engage the autopilot on the operative side and disregard the information coming from the failed side. If FMGC 1 failed, do not use AP1 for approach.
On two of the failures, the first indication of a problem was the cabin pressure landing elevation fault ECAM. The second indication was the MCDU message "SET OFF SIDE RANGE". The third failure was indicated initially by a "SET OFFSIDE RANGE" message. On one event I noted my ND on the green line and the FO's diverging. All three displayed 1FD2. FiFi is a wonderful airplane, very smart and "most" of the time she will tell you when she is sick but she still needs constant attention.