Thinks!! whatever happened to the blind landing capability we had on the Tridents and Tristars of so long ago?
Still there, still used, still works very well. However the problem at places like LHR is that you need more spacing between "blind landing" aircraft in foggy weather than you need on a relatively clear day, which reduces the rate at which aircraft can land. The system used by most current aircraft (ILS) in foggy weather relies on there only being one aircraft flying down the approach at any one time - and if you have ever watched aircraft landing at the likes of LHR on a good day you'll see that isn't normally the case. So on a foggy day rather then a string of aircraft flying down the approach with 3-4 mile spacing the aircraft are backed up into the holds, waiting for a slot for their individual approach. If you haven't got the fuel for very extended holding for whatever reason then you're off somewhere else. The newer technology (Microwave Landing System or "MLS") on some types allows decreased spacing between aircraft, but at LHR at least that doesn't yet seem to have radically improved the landing rate in foggy weather.
What are the rules for category 3 landings ?
Blimey, how long have you got? That's a book in itself. very basically each airline has it's own set of in house rules, each aircraft type will have it's own capability. Typically the rules will allow approaches in visibilities typically down to 100 or 75 metres, but it's subject to
lots of caveats.
R all aircraft equipped for such landings ?
Most airliners are these days - which is why part of the reason for the extended holding in foggy weather. Once upon a time when "blind landing" was restricted to a few types and/or a few airlines you didn't get extended holding, because most flights simply couldn't even attempt an approach. They had to divert immediately to somewhere not foggy, leaving the approach clear for the few "blind landing" types. Now that almost everyone's got "blind landing" everybody just joins the queue......