CAT requirements have tightened up over the years, Boeing built allot land systems triple redundant (probably because they have allot of boxes in their aircraft that are made by them $$$). Douglas went with minimums for instance DC-10 and MD-11 2 ILS systems, the MD-10 being modded after the 3 ILS standard was in place had to rewire 3 ILS receivers to 2 loc and 2 gs antennas to be in compliance.
Thx torque 2
Yeah each aircraft has it's own list of minimum equipment to comply with the aircraft's CAT requirements.
Alot of aircraft will autoland and rollout on a single A/P and ILS. Just about all aircraft I know of only use 1 A/P active, the other 1 or 2 are just there to compare data.