FCOM 4 APPROACH
FAIL-OPERATIONAL AUTOMATIC LANDING SYSTEM
An automatic landing system is fail-operational if, in the event of a failure below alert height, the remaining part of the automatic system allows the aircraft to complete the approach, flare, and landing.
A CAT 3 DUAL system is a fail-operational automatic landing system.
Note : In the event of a failure, the automatic landing system operates as a fail-passive system.
FAIL-PASSIVE AUTOMATIC LANDING SYSTEM
An automatic landing system is fail-passive if, in the event of a failure, there is no significant out-of-trim condition or deviation of flight path or attitude, but the landing is not completed automatically. A CAT3 single system is a fail-passive automatic landing system.
****Note : With a fail-passive automatic landing system the pilot assumes control of the aircraft after a failure.****
Below 200 feet (radio altimeter), the FMGS freezes the landing capability until LAND mode is disengaged or both autopilots are off.
Therefore a failure occurring below 200 feet does not change the category of the system.
ALERT HEIGHT
The alert height is the height above touch down, above which a CAT3 autoland would be discontinued and a missed approach executed, if a failure occurred in either the aircraft systems or the relevant ground equipment.
Below the alert height, if such a failure occurs, the flare, touchdown and roll out may be accomplished using the remaining automatic system.