RadAlts
As someone with no first hand experience of airliner systems, (I'm just a light Heli pilot), I am AMAZED that two pieces of kit, in this case Rad Alt units, can operate independently and feed DIFFERENT parts of the A/P system. Surely at least the two radalts should be constantly compared and if the difference in outputs becomes more than x% they are both flagged as U/S and disregarded by any piece of equipment on the aircraft that relies on them. Otherwise why bother having two?
Clearly abnormal conditions occured which should have been noticed by the crew, but due to the split feeds of the two Radalts, the effects would have been strange and confusing to them. A simple alarm should have been raised "Rad Alt Failure" and A/T A/P etc disconnected.
It's so true that the more automation there is the more off guard the pilot (or any operator of a machine) will gradually become as he sinks into the false sense of security that comes after countless hours of flying without fault. And then when it all goes wrong....
James