Originally Posted by Bearfoil
But which pilot?
Dazzle me with a theory of a Near STALL widebody at handoff. Because if you cannot, I will continue and suggest that the unreliable airspeeds were caused by the Rolling moment of a fast widebody in chop, and that the discrepant reads were made so by the airstream losing its integrity at the lower nose, while she mushed on full of gusto and KE. Not to mention plenty of NU. Hopefully BEA will fill the void they created between 2:08:07 and 2:10:05. Then I'll buy you an adult beverage of your choosing. My soul is ready, how's yours?
Well, it seems I'm the one making wishful thinking while an endless Bearboiling of circular points seems to make your own days. Obviously, what makes you think that she was near STALL at handoff, when AP kicked off, is that the STALL WARNINGS sounded twice, then stopped. On the other hand, in many UAS case, without changing anything about the flight parameters, this happened exactly the same.
From here come your insistence that she could not have been in "controled flight", no matter if everything else is pointing that she was, except a starting roll to the right.
On the other hand, I have already underlined many other cases of UAS events recorded that are obviously pointing at exactly the same "suspect" stall warnings when the FLIGHT LAW is switched from NORMAL to ALTERNATE. For me, this is certainly a major issue (if confirmed) of the Warning System because I really think that it could have played a major role during the following sequence leading to the total loss of control by the crew.
In fact, there is a VERY simple BASIC rule in the system :
* NORMAL LAW -> COMPLETE SAFE FLIGHT ENVELOPE USED BY ELECTRONIC FLIGHT CONTROL SYSTEM
* ALTERNATE LAW -> REDUCED SAFE FLIGHT ENVELOPE USED BY ELECTRONIC FLIGHT CONTROL SYSTEM
Hence, any switch from NORMAL to ALTERNATE LAW should be immediately followed by a reduction of the safe flight envelope at both ends : in clear, the safe margin should be expanded concerning both the overspeed warning and the stall warning (nonetheless, the aircraft real "flight envelope" is still the same, but its defined "safe limits" are changed due to some certification concerns about its systems state and possible manual flight).
1) In our case, the high speed limit warning would be reduced from Mach 0.86 to Mach 0.82. Also, the low speed warning (alpha-prot) could be rised (I did not find the relevant data about it, then, if wrong, see point 2).
Nonetheless, neither overspeed nor stall protections could be applied because of the current declared airspeed monitoring by EFCS : ALT2 is immediately applied, no protection could kick without a valid airspeed ; After this point, the PFD function displayed (overspeed and stall warning speed limits) will disapear from both PFD.
Next, following an UAS confirmed in ALTERNATE LAW, the calculation mode of this stall warning limit will change again from one function using an AOA corrected by a Mach value supplied by those ADRs to another one using a default Mach parameter. This would change the granularity of the AOA measured and subsequently will increase the margin for the stall warnings.
My explanation (so far) is that, when UAS is declared, there is a small time window where the limitation of the safe flight envelope is applied to the stall warning limit which last during the resolution of an UAS monitoring : it takes 10 seconds for the system in order to confirm ALT2 (after that, the system could not subsequently revert to NORMAL LAW without a full ground reset).
So in fact, the aircraft, during those 10 seconds window, without changing any flight parameter, will fly at speed closer -or below- the threshold of the STALL WARNING speed. But, following the UAS confirmation, the new function will apply another AOA warning limit. Then, this would stop the WARNINGS... if the reduced threshold has been overshot during those 10 seconds. So my bet is that the STALL WARNINGS stopped at 0210:14 and would have sounded during the 0210:05-0210:14 time window.
2) If alpha-prot calculated speed do not rise when switching to ALTERNATE, another simple explanation is that a drop of the polled speed, due to unreliable airspeed situation, is not filtered by the stall warnings during the same defined 10 seconds time window. Hence, if the polled (unreliable) speed goes below alpha-prot speed, this alarm is sounding. Once UAS is confirmed, as precedently, the new UAS STALL WARNING function is declared and the alarm is stopped.
Those 2 points are based on the history of published UAS events :
Here are the tables of the 36 UAS events declared before the publication of 2nd BEA report; one may see that 12 cases of "suspect" STALL WARNINGS were reported (33%). Moreover, those stall warnings do not let any trace outside the CVR or crew reports (they are not part of the maintenance post flight reports), and this survey was mostly incomplete about many cases listed. So it may be assumed that more cases of "suspect stall warnings" were not recorded.
