|Quoting Tozairport (Reply 5):|
For CAT 3 approaches, visibility is the sole controlling minimum, and there are only alert height, no decision heights.
Tozairport, not entirely correct. A CAT 3a approach has a Decision Height, usually 50ft radar altitude. CAT 3b/3c approaches have no DH
Alert Height is NOT the same as DH
! Alert height is a different altitude (usually 200ft) at which you have another decision point during the approach if you have a fail operational autopilot.
Most 737s only have a fail passive autopilot, which means that if a significant degradation in the system occurs (eg. one autopilot fails), you have to make a go-around at all times, as long as you do not have the runway in sight.
Aircraft that have a fail operational autopilot can still do an autoland when a significant degradation occurs, but there's an extra decision point before which you should make a go around and investigate the problem. After that point, you go ahead and do the autoland anyway. That's the Alert Height.