Originally Posted by
Tonic Please
So, next question would be: why three? If one provides roll and pitch during regular flight phases, and another provides yaw (which one?), then why have three and not 2? I just don't see how the numbers add up and it's somewhat frustrating me that I can't find a clear answer anywhere.
Three Autopilots equals Fail-Operational CAT IIIB ILS approaches with no decision height and 50 meters visibility(assuming all other required approvals from CAA etc and ground equipment allow this type of maneuver). If one is inop, then two Autopilots equals Fail-Passive with a 50 foot DH and higher vis requirements. Both situation allow autoland and rollout. Only one Autopilot(could be any one of the three) allows a coupled approach with no autoland allowed and Cat 1 minimums. A matter of redundancy.
So what happens if one of the autopilots fails on the CAT III approach? It depends where it happens. If below what is called an Alert Height which is 200 feet radio altitude, we continue and land. In fact, I don't believe that there will be any indication of the failure while airborne below the Alert height.
Whichever of one of the three autopilots is individually selected in normal flight will only provide pitch and roll inputs. The three of them together provide rudder input as well once all three are engaged which will happen automatically without pilot input on an ILS approach around 1500 feet. Of course that is necessary for crosswind correction for autoland and helps in an engine inoperative situation for approach and go-around(although one has to be ready to apply rudder when the aircraft automatically goes back to single autopilot operation). In other words, the pilot always selects one autopilot on. The rest engage automatically during the approach when appropriate conditions are met.