MadScientist,
With the AF 447 BEA Report at hand:
Based on the BEA graphs, in the AF 447 case, the start of the Autotrim THS move towards max NU coincides (pretty much) with the start of the constant/continuous 53 seconds Stall Warning. During that period, from the beginning and close to its end - perhaps for about 40 seconds, the graphs also show NON-oscilatting (i.e. stable) AOAs emitted by IRSi(i=1,2,3) (page 111).
In this case, the system didn't really have to move the THS, it only had to stop the Autotrim from moving it from its -3 degrees position.
Back to the suggestion, perhaps there is a misunderstanding.
The suggestion in essence was not intended as to provide more automation during a Stall, but rather slightly change it, by reducing or eliminating the automation of the THS, in association with placing the THS in the "most optimal" position for the Stall Recovery. What the latter means, in terms of "optimal position" can certainly be the subject of more studying, but as a start can be considered THS Neutral.
Lastly, resolving the stability of the Stall Warning may provide the additional stability for the THS mechanism.
Originally Posted by
Mad (Flt) Scientist
I can see no reasonable means of engineering a system which would "reposition" the THS as a result of being in a detected "stall".
The whole point of being in a degraded mode with reduced or no envelope protection is that the system can no longer rely on one of its primary protection inputs - alpha.
if, despite that lack of reliability in alpha the system were, in a degraded mode, to have authority to move the most power single surface on the aircraft through close to full authority ( NU to neutral is a LONG way) it would be utterly impossible to show the system as being safe - in the event of an erroneous AOA input, you'd still be exposed to a large stab runaway, in effect.
With the AOA reliability suspect, the system can no longer act, automatically, on AOA. All it can do is draw the attention of the crew to the apparent high AOA situation - shake, shake - and hope they, with their human reasoning and airmanship, can resolve the situation which is outside the design parameters of the FBW system.
Unfortunately, in this case, they couldn't or didn't.