surplus1, thanks for your views and the comprehensive explanation. (#3133)
First the stall: my hypothesis is that with the loss of airspeed procedure the aircraft was likely to slow down (power, attitude), which in the circumstances of weight and altitude would progress towards the stall.
Modern swept wing aircraft, in general, tend to pitch up into the stall (usually gently) – I don’t know if the A330 does this. If the nose-up pitching moment remains at post stall AOAs then without any nose down elevator the stall may be described as ‘stable’ (not the best of terms, as the condition may involve pitch and/or roll oscillations). Like a stall with full back stick, the wings may rock, but some lateral stability usually remains – why should the wings ‘roll off’.
My understanding is that in ALTN law (no speed protection) a stall is possible – AOA protection may be available, but I await an answer to my previous questions on this.
It is also my understanding that the normal control law in manual flight provides a ‘follow up’ trim function – at least with the stick free the system holds attitude. If ALTN law is the same/similar, then the aircraft may be trimmed into the stall, or at least at a very slow speed before the stall. The alternative is to use manual trim.
Without trim stick-force feedback the crew might be unaware of the trimmed state, they would have to look at the trim indicator – I have asked validating questions on this elsewhere; note similarities with conventional auto trim (also covered by TheShaddow), but the A330 systems appears not have the same mechanism as conventional auto trim, which disengages with the autopilot disconnect.
The slow speed trimmed state, together with any nose-up pitching moment could maintain the stalled condition (stable stall), which the crew may not deduce, and even with full nose down control input (not an instability), the situation is not recoverable without applying nose down trim: c.f. 737 AMS acct & UK incident. A conventional aircraft has stick force proportional to displacement from trim, whereas ALTN appears only to have to have force proportional to stick position – validation required please.
The important issues are; as stated by TheShaddow (#3106)… “But what was happening to the THS (trimmable horizontal stabilizer)” and “…. mask the back-trim and leave the pilot totally flummoxed?”; our difference is that as explained above, the nose remains high in a stalled condition.
Re turbulence: in general terms, the certification requirements require that an aircraft can withstand the ‘worst case’ gust (turbulence) from both aerodynamic and structural perspectives when flying at Vra; at slower speeds, a stall (or roll upset) is possible. Engines have similar requirements, but also like aeros/structues not necessarily considering all contributing issues. Furthermore, “what is worst case” and how is it affected by crew action; the industry still has to learn, unfortunate from accidents like those you cite.
Our views don’t differ on Cb encounters, my “inadvertent” is in the context of not premeditated or not deliberate.
Re pitot balance; I have corrected my post, and in other replies explained that the point is still relevant. Turbulence is most unlikely to cause the abnormalities reported – consider that a gust is more likely ‘alpha’ opposed to IAS change.
MFS #3136, thanks for the parallel view. In addition to your post-stall description I would appreciate comment on the trim issues.