Hi Mono. The Cat figures relate to weather minima, specifically cloudbase and forward visibility. What minima the approach may be flown to depends on the capability of the aircraft systems, qualification/training of the crew and the facilities at the runway. Basic limits for our operation (Avro RJ 100s in Europe) are:
Cat 1: Decision height 200 feet, Runway Visual Range (visibility down the runway) 550 metres at the Touchdown Zone, 150m at the Midpoint, 50m at the Stop End IF the stop end is part of the runway length required for roll-out. The RVR requirement has to be met when crossing the Outer Marker, typically 1000 feet above runway elevation and if before that point it is not, the approach is discontinued. However if the RVR drops below limits AFTER passing the OM, the app. may continue.
At decision height, the non-flying pilot calls "decide" and the other calls "landing" if he/she can see anything, and "go around" if he can't. If landing, the autopilot will typically be disconnected at this point, IF it was flying the approach, and must be out by 160'. Most of us prefer to hand fly when possible but would auto-couple if theres a high workload for some reason. If the airfield facilities support it, a Cat 1 app. may be allowed to continue to an autoland.
A Cat 2 app. has minima of typically 100' DH and 300m RVR (150m midpoint, 50m stopend) and must be auto-coupled (i.e. autopilot doing the work) down to DH. Again, it may conclude with an autoland if the runway facilities allow, and if not, automatics have to be disconnected by 50 feet. Visual reference required for landing is 3 centreline lights (usually 15 meter spacing) and a crossbar (transverse bar of lights at beginning of runway). Note the RVR required to commence the app. is a lot higher than what you're required to be able to see once you get down to Decision Height.
Cat 3a is down to DH 50' with a required RVR of 200m (MP 150m, SE 50m again), must be autocoupled and by definition, autolanded. Visual cues required are 3 centreline lights. Not the crossbar, because at the lower DH you're past the threshold before you see anything anyway.
Cat 3b is same minima, but you only have to sight one centreline light in order to allow landing after the "decide" call at 50 feet. You're pretty well wired by then! If you don't see the runway its a go-around, and many types will actually contact the runway during the go-around maneouvre, as it doesn't instantaneously stop descending and start climbing.
Cat 3c our aircraft don't do, but I believe its "zero-zero" in terms of DH and RVR. The aircraft would need roll-out capability down to a low speed to stay on the centreline, and then taxiing off becomes a major concern. I believe auto-braking becomes a requirement too??
There is a list of aircraft electronics and systems which have to be serviceable for Cat 2 approaches and above, and airports revert to Low Vis Procedures when the cloudbase gets to 200' and/or vis drops to 1500m, to protect the integrity of the ILS system. E.g. aircraft waiting to take off use holding points further back from the runway (the Cat 2/3 holding points) so as not to get in the way of the localiser or glideslope beams, and spacing of aircraft on the approach is increased.
Examples of the runway requirements are: an ILS certified accurate enough, approach and runway lights comprehensive enough (although bear in mind that for a Cat 3 you don't actually need approach lights because you're well past them by the time you see anything), and relatively flat terrain before the threshold. Critical info. comes from the radar altimeters in the final few seconds, like Align Mode, which puts in rudder and drops a wing at 150' if there's a crosswind, and of course Flare Mode, which kicks in at 50', at which point the auto-thrust retards the power aswell. If the surface before the threshold is lower than the runway elevation, i.e. rising ground as you get closer, the radar alt. gets its height cues too late and an autoland may not be allowed. Interestingly a runway may support Cat 3 autoland but not Cat 2 autoland, because for example the decision height is lower on the Cat 3, and the uneven ground has been passed by then.
I never tire of watching the autopilot land the aircaft in lousy conditions, especially if there's a crosswind. Its a thrill every time to see the runway lights come into sight and you're only 50 feet above the ground.
Why can't a Cat 3 approach be autolanded? Because at 50' there's only 5 seconds or so left till touchdown, and this isn't enough time for a pilot to absorb all the visual cues sufficiently to be able to take over from the autopilot. Our procedures for Cat 2/3 are for the co-pilot to fly the approach, one of his calls being "100 above" when 100' above decision height. I then reply "Looking", stop watching the instruments, and look out the windshield. At "Decide", if I can see the runway and we're in position, I say "Landing" and let it get on with it, my hands now on the thrust levers and control column just in case.Taking control from the autopilot at 50' would involve a high risk of de-stabilising things, and if the autopilot fails below 50' a go-around is mandatory (never personally known this to happen. Except in the simulator, where its quite frequent.....)
So to summarise, if the ILS signal is accurate/reliable enough, and the aircraft can accurately/reliably follow that ILS information, an autoland can be done. The Cat designations refer to minimum weather conditions and dictate the finer points of how the approach is conducted, and what we're required to do in cases of abnormalities (interrupted ILS signals etc). Obviously the worse the weather, the more conservative the procedures.
That turned out a bit more long-winded than I intended but I hope it clears up some of your question. regards - Musang.