Here's a couple of things for people to consider regarding rad alts and autopliots.
Rad alts are primarly autopilot sensors not primary flight instruments. There is one rad alt per autopilot which is used in the landing phase.
To meet the design requirements for autoland with multiple autopilots, you must have the autopilots operating independantly. Autopilots will be powered from separate electrical busses, utilize separate hydraulic systems and receive information from separate sensors. Because of this requirement, it is not possible to use rad alt info from another side to replace a bad sensor.
Rad alts monitor the aircraft height above ground. Its operates between 0 and 2500feet. Height is determined by bouncing raido signals off the ground. The frequency is in the 4.3Ghz range. Above 2500 feet, it is not used. Each rad alt has 2 antennae. One transmit and one receive. On most of the modern Boeings, the rad alt R/T and both antennae are monitored for faults and will supply fail flags for a hard fault. Intermittent faults are much harder to detect and may not show as a failure.
I spent most of my 30 years in aircraft maintenance working on B747s. I never saw a rad alt stop one from flying. I don't know of any aircraft that doesn't have an MEL to cover the dispatch of an aircraft with an inop rad alt.
I thought this was a miscommunication between the RA and the A/T?
And why are people still harping on about the system architecture of the RA, A/T and A/P? They simply ran out of airspeed because no-one was looking.