Hi Bochos,
Alert height only applies when fail operational mode is announced on fma despite you are on cat 3 a approach?
Answer: Yes, regardless what minima you are flying to (even CAT 3A) if your system is capable to be Fail Operational (for Airbus when CAT 3 DUAL displays on FMA), in fact Alert Height is applicable. For CAT 3A minimum required system is Fail Passive system. However if your airplane is capable to fly Fail Operational, of course it is preferable to fly the same approach with Fail Operational system (that is why we are always engaging 2 A/P for even CAT 3A Approach in Airbus). As doing this, we are using the extra benefit coming with Fail Operational system (like Alert height)
Alert height for A320 is 100 RA is this the lowest possible AH or it could be lowered down to cat 3a minima of not less than 50ft?
For A 320, 100 RA is the maximum available AH, not the minimum. 100 RA is the approved altitude where above this altitude the required probability can not be achieved.
The following is an extract from Getting The Grips with CAT 2-3 (airbus)
The AH is only linked to the probability of failure(s) of the automatic landing system. Operators are free to select an AH lower than the AH indicated in the AFM but not a higher value. Airbus procedures include both AH and DH concepts for all Fail-operational (see later) Category III operations.
Consider like this, as a pilot I would like to sit in the flight deck of an airplane type so that the approved AH is as high as maximum. Because this means, I am sitting in the flight deck of a more reliable airplane. Further explanation: As the AH is higher, the duration passing from AH to completing App and landing is longer, consequently the airplane systems are so reliable, even at this longer period, the failure probability is still below threshold value.
Practically flying CAT 3A (lets say to DH 50 RA) with Fail Ops system and your AH is 100 RA, below 100 RA continue to land unless the designed warning system is triggered (e.g AUTOLAND light for A 320) and provided visual requirements is achieved by the time of reaching 50RA.