Chris,
That is a good summary of the trim function. It was surprising how stable AF447 was in the stall. I initially expected that such an aircraft would drop a wing and progress into a spin, but that did not happen. I have to attribute that to the automatic action of the yaw damper.
In Normal Law THS trim is limited by AOA protection activation.
From FCOM
When angle of attack protection is active, THS is limited between setting at entry in
protection and 2° nose down (i.e. further nose up trim cannot be applied).
For some reason, in Alternate Law, there is no inhibition on nose up trim related to angle of attack. It would seem logical to inhibit THS trim at stall warning activation when in Alternate Law. From Owain's comments recovery from stall would have been greatly enhanced.
Originally Posted by Owain
If it had been so inhibited the THS would have been frozen at 3 deg NU. That would not have prevented the accident because the pilot's continued pitch up command would have resulted in the application of full up elevator which is quite enough to drive the aircraft into stall.
It would of course have made recovery easier, and incidentally would have prevented the aircraft from developing a 50 deg angle of attack so the position error would not have gone haywire, the indicated speed would not have dropped below 60 kts and the stall warning would not have been inhibited.
The BEA did not address the lack of THS trim limits in Alternate law in the accident recommendations. I have to assume that there were good engineering reasons for not adopting limits on the THS trim system in Alternate law, but I have no clue what they might be.