some copy+paste from
ILS Categories -
Cat.I - 200 feet DH
- 2,400 feet (or 1,800 feet) RVR
Metric: 800 metres of 550 meters RVR...
Restricted - 150 feet DH
- 1,600 feet RVR
metric: 500 metres RVR
- 100 feet DH
- 1,200 feet RVR
Metric: 350 metres RVR
Cat.IIIa - 700 feet RVR - no DH
(alert height generally 50 feet)
Metric: 250 meters RVR
Cat.IIIb - 600 feet RVR - no DH
(alert height generally 35 feet)
Metric: 175 metres RVR
Cat.IIIc - zero ceiling, zero visibility - "blind" landing...
RVR is Runway Visual Range, basically a distance in feet that the pilot can expect to see forward in his airplane.
The ILS equipment at the airport must be certified for it, as well as aircraft type (actually individual aircraft) and crew have to be certified.
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...
i think that sums it all up perfectly. good thing we have such knowledgeable people on the boards here like those in the thread i copied it from.