Aquadalte: "If this aircraft was equipped with ISIS, it was surely also equipped with Back-Up Speed Scale and Back-Up Altitude, that displays on PFD's and gets information from AoA probes and GPS, respectively.
This is very important, once Back-Up Speed Scale and Altitude are only displayed when ALL ADIRU's are switched OFF. The old procedure (for the stand-by horizon equipped aircraft), would call for the trouble-shooting of the IR fault, and in case of all confirmed to be faulted, one would be advised to switch off those connected to Captn PFD and FO PFD, i.e., IR1 and 2. This would allow the aircraft to revert to Alternate Law.
This system (although brilliantly designed) seams to work fine in rather smooth air and on flight simulators, but I find it very difficult to see it properly working in a heavy turbulent environment, as seemed to be the case of AF447."
Believe that the Back-Up Speed option was available in 07, the ISIS does indicate Speed/Altitude and Attitude...the instument is only about 1.5" square.