BOAC asked :
With 1 in 'normal' and the other two in 'Alt2' what would be the governing control law?
A
simplified approach to this problem is to examine which computer governs which flight control surface, and examine carefully how the different flight laws work, and how they interfere with pilot actions. Reality would tend to be even more complex.
Occam's try on these hypotheses is the triggering of an undesired normal law (full authority) overspeed protection by PRIM2
only which uses erroneous airspeed data after having unexpectedly returned to Normal law. This protection reacts to speed only, and simply adds a nose-up order, with authority superior to that of pilot sidestick. Alarms and warning triggers to the crew are a big unknown in this situation...
This situation I described is absolutely not what should happen on a FBW aircraft. Flight Controls Computers are not supposed to disagree at every turn. They are almost always in full agreement to the 0.0000001% precision.
I have determined, however, that in this particular instance, they were likely not in agreement. After such an unprecedented problem, all bets are off.
This is not fundamentally different from the unexpected rudder hardover in a 737, or the unexpected loss of all hydraulics on a DC-10 or 747, or whatever... These happen extemely rarely and are always specific to a certain design. No design is perfect, even this one.
Designs always contain certain trade-offs, carry certain basic assumptions, and try to cater for the unexpected in the best way possible. The more complex they are, however, the more prone to human-like error they become.
James Reason is always quoted for his Swiss-cheese model, but he could also be quoted for this : "
intelligence and error are both sides of the same coin".