The "Human factor"(ie.correct diagnosis followed by correct transfer switching,inadvertent shutdown of a good unit etc) theory following ADIRU event(Aguadalte) needs much closer scrutiny IMHO.What those Qantas pilots had to go through is above and beyond the normal scope of "airline pilot".
As can be seen, the Captain’s
Primary Flight Display usually presented data from ADIRU 1, following the presentation of the NAV IR1 FAULT,
the source of data was switched to Inertial Reference unit 3 on ADIRU 3. However, this did not automatically
switch the source for Air Data References, which continued to be ADIRU 1. This illustrates the complexity of
interaction with redundant systems as crews struggle to ensure that they receive data from a reliable source without
knowing for sure which of the alternate ADIRU’s is providing reliable information. The problems with the Captain’s
Primary Flight Display could not be resolved before a second uncommanded pitch down. The uncertainty created by
crew interaction with their redundant systems was exacerbated by the way in which the master flight control primary
computer was switched from PRIM1 to PRIM 2 following the first pitch down event. The subsequent indication of
a fault on PRIM 3 then triggered a further change in the master from PRIM 2 back to PRIM 1 and it was only in
subsequent discussions with the operators maintenance watch unit in Sydney, while the flight was still in the air that
the crew decided to switch off PRIM 3.
KISS