Originally Posted by
CONF iture
Why should it stall when alpha max is still 2.5 deg away ... ?
Well, for one thing because the engines will be trying to pitch the plane nose-up as they begin to provide power. Without the flight control system counteracting this tendency, even neutral elevator could allow the aircraft to overshoot 17.5 degrees in very short order.
What the computers do is not especially clever, the advantage they have is that they can detect and counteract unwanted deltas (changes) in attitude much faster than a human. Again, nothing in the documentation states that full back-stick will command alpha max instantaneously - indeed experience shows that it can take up to around 20-30 seconds to stabilise enough to reach 17.5 degrees. Thus the BEA did not "fail" to do anything - only if Airbus's documentation claimed instant alpha max would the aircraft's behaviour have needed more thorough examination.
@Chris Scott - thanks for the input. I suspect "Crawley High St." was a bit of journalistic licence on the part of the author. I mentioned it only because Saint-Ex mentioned BA, and to the best of my knowledge that was the only BA A320 to have any difficulty around that time.
@rudderrudderrat - the other thing important to remember about the High Alpha modes is that they are designed to allow for a limited amount of roll authority to be retained even at alpha max. What the flight control systems are doing is assimilating all the external forces acting on the aircraft along with the control inputs and responding to control demands while maintaining a stable flight regime. In this case you have the increase in thrust from the podded engines causing an increase in pitch attitude, as well as a somewhat belated full back-stick demand. The Bilbao case you link seems to highlight what us engineers call an unknown "edge case" (i.e. a scenario that was unaccounted for in the original design, and sometimes results in non-optimal behaviour). However it also demonstrates that if Airbus are presented with evidence of such an occurrence, they can and will turn around a fix and will announce it publicly even before the final report is written.
EDIT : Looking at the report on the Iberia Bilbao incident:
http://www.smartcockpit.com/download...rd_Landing.pdf
It seems that the problematic aggressive phugoid damping was initiated by a tailwind gradient which manifested itself as an excessive reduction in airspeed (a delta of around -10kts over one second). The EFCS logic incorrectly considered this to be a result of phugoid motion and applied nose-down elevator to arrest the pitch-up demand.
As there was no such tailwind gradient at Habsheim, and the airspeed deltas were much less extreme, I reckon it unlikely that phugoid damping applied here.