Originally Posted by
Graybeard
I don't understand why the AP/AT had to drop out of Normal for flaky airspeed.
Because without that information, some of the protections can't work. Any protection out is an immediate drop to Alternate Law 2, which puts the pilots in near-absolute command of the control surfaces (any remaining protections can be overridden with sufficient control deflection).
It may surprise many, but it was part of the original specification for the systems that drive the A320 and her descendants that if, for whatever reason, the computers think they can't behave as they should, they defer to the pilots - on the understanding that they have more information available to them, especially in daylight when there's an external reference as to which way is up. The human brain can at least try to work with partial information to rationally attempt a solution to the problem at hand, whereas a computer can't.
Also, I think you may be getting the FCU (Flight control unit - i.e. the FBW part of the design) confused with the FMC (aka A/P and A/THR), which is much like that of a conventional airliner).
At least not in cruise. It could hold pitch and power for awhile without dropping out, giving the pilots time to analyze the situation without having to hand fly, too.
This would add unnecessary complexity to the system. In fact the A/P can be engaged again in Alternate Law, but with airspeed data out, it's not considered a good idea. I know I keep harping on about this, but if you look at the Birgenair 757 incident, you can see what happens when an autopilot tries to fly the aircraft with a blocked pitot tube (and that was on a calm night, with little or no convective activity for miles).
I've lost count of the number of times I've heard "Switch the damn computers out and give control back to the pilots", or words to that effect. That is *exactly* what happened in this case. Unfortunately, with no airspeed data over water in the middle of the night and threading their way through patches of very inclement weather, even the best and most experienced pilots can be overwhelmed.