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.
Dear Colleagues,
I have committed here a HUGE error! And I would like to apologize for that.
In the above said statement I said
ALL ADIRU's are switched OFF and I meant to say
ALL ADR's! (post #1907, now corrected to prevent others to be misinformed).
I studied the system again and noticed my mistake.
This led me to think that it is a common mistake in recurrent flight simulator sessions,
to see flight crews, switching off one system, when they're actually thinking on another. (That is why we have the "confirmation" philosophy of asking for PF to visually confirm PNF actions, before switching of guarded or important systems when handling an emergency procedure)
I wouldn't like to speculate on that, (for the sake and honor to our fellow death colleagues) but, it now poses a reasonable doubt to me and a reasonable explanation for their lost of control of the aircraft.
Till now, I was strongly convinced that the failure of all IR's were responsible for the crash. Now I have a stronger and stronger (seasick) sensation that Human Factor was.
I'm sorry for being (as far as I remember) the first one to bring this speculative "explanation" for this accident. I do know that accidents have multi-causes. But, what if, instead of switching off the ADR part of the ADIRUS, they have inadvertently switched off the ADIRUS (or the IR part of the ADIRU's), in response to a Unreliable Speed Indication Check-List?
We all know how disturbing is to fly in turbulence.
Was the junior FO seated on the left seat?
Would the ISIS still give them attitude indication?