Originally Posted by Aluminium shuffler
Look at the 777 and you have a single ADIRU supplying all main flight instruments
I wouldn't underestimate the abilities of one 777 ADIRU. It has no less than 6 ring laser gyros, 6 accelerometers, 3 sources of pitot, 6 sources of static, 3 power supplies, multiple processors, etc.... The SAARU has four fibre optic gyros, 4 accelerometers, 3 sources of pitot and 6 sources of static. The ISFD uses the centre pitot/static system only and has an independent source of attitude.
If there are certain failures, the selection of good data is being done by the boxes automatically through fault monitoring and voting (i.e. mid value selection). Mid-value selection of certain data is what an older generation aircraft (such as the 747-400 or 767) would be doing during the critical phase of Autoland.
The boxes are automatically supplying the best available data that the pilots would use under normal and some abnormal situations. And if things get really bad, you still have the option of going back to the basics by selecting SAARU "single channel" data if you, say, want to know the data that a particular pitot/static system is producing.