Originally Posted by CONF iture
You are misinformed here : AP but also A/THR could be reengaged under alternate law. But a RECENT OEB warns pilots not to do so, at least not instinctively ...
This recent AD is for wording it more clearly: it is possible to (re)engage AP/FD if 2 ADRs are (re)displaying consistent, but erroneous, data (hence danger). I wrote it many times that it would be possible in such circumstances. But in AF 447 case, it seems very unlikely that they ever had such an opportunity due to the RTL settings still frozen at impact; a PRIM reset is also mandatory before doing it, but it was not made until very late in the sequence. In any case, it would be due to pilot's wrong choice of erroneous data as the computer will not activate AP/FD by itself.