Alert Height (AH) is not like a Decision Height (DH) -
At "DH" (obtained from radio altimeter for Cat.II) you have to make a DECISION to land or go-around...
In Cat.III operations, there is no DH
... but you have to make a decision to land based on "what you see"... pilots find the DH
"decision" very convenient for Cat.II, but did not exist for Cat.III...
So in "pratical operations", the AH
is used somewhat like a DH
, but is not regulatory. In other terms, we expect to "see the runway" at that point... which is about 50 feet radio altimeter, just about where the runway threshold is located, in Cat.IIIa minimums. In Cat.IIIb, happens at about 35 feet...
Many 747 are equipped for Cat.IIIa operations (not Cat.IIIb), although most of the "Classic" 747s (with 3 autopilot channels) have the LRCU that is required for Cat.IIIb... LRCU = landing roll control unit... keeps the nose wheel on the center line, using the localizer...
A reminder - Cat.III is (in the 747) flown by the autopilots. Pilots simply monitor the progress of the approach, with autoflare, autoland, autothrottle and autobrakes... I only did one real Cat.IIIa ILS down to minimums in my life...
Hope this answers your questions
Happy contrails -