Machinbird,
I agree that a crash site not far from LKP may indicate a sustained stall, though not necessarily from cruise altitude. However, as you know, the plane with all system components operating as designed is well protected against stalling. So how could the airplane be stalled at altitude?
One obvious possibility is that the pitots became obstructed with ice crystals, airspeed indications were lost, FCS reverted to alternate law without stall protection, pilot pulled up into a stall and maintained it until impact, as in AF447. However, in view of the extensive publicity that accident has received, it is almost unbelievable that it was repeated.
The sequence that I have in mind starts here:
The aircraft was observed on ATC radars to both climb and to decelerate (GS) significantly before descending rapidly.
That observation is consistent with an encounter with an upward gust, that momentarily increased the AoA to beyond alphaprot, and put the Flight Control System (FCS) in high-angle-of-attack protection mode. When that happens, the FCS maintains an AoA equal to alphaprot, until the pilot moves the sidestick forward (or some other criterion is met). The result is that the airplane enters a steep climb at the expense of airspeed, levels off, then starts to descend. The cited A340 incident started at FL350, M.84, reached FL380, M.66, 205 kCAS, and max RoC was about 6000 fpm. About 100 seconds after the upset the pilots moved the sidestick forward and resumed normal flight. During that time crew changed parameters of the disconnected autopilot and evidently did not understand what was happening. Although the lowest speed came close to the stall speed, there never was a risk of stalling, as long as the FCS remains in Normal Law.
The high-AoA protection is lost when the FCS reverts to Alternate Law when certain system components fail or are switched off.
The emergency procedure that I refer to is quite recent, the EASA EAD is dated 9 December 2014 and demands compliance within 2 days after receipt. The background states:
When Alpha Prot is activated due to blocked AoA probes, the flight control laws order a continuous nose down pitch rate that, in a worst case scenario, cannot be stopped with backward sidestick inputs, even in the full backward position. If the Mach number increases during a nose down order, the AoA value of the Alpha Prot will continue to decrease. As a result, the flight control laws will continue to order a nose down pitch rate, even if the speed is above minimum selectable speed, known as VLS.
This condition, if not corrected, could result in loss of control of the aeroplane.
Possibly the pilots have taken notice of the change just a few days before the accident flight and, being kept very busy, not given it more than cursory attention. IOW, they may not have fully appreciated and remembered the precise system failure scenario behind that procedure. If the high-AoA protection mode was was triggered by a gust, they may well have been as surprised and confused as the crews in the A330/A340 ‘level bust’ incidents. They may have remembered the recently introduced procedure and switched off 2 ADR’s, losing the stall protection, and stalled during the recovery from the upset.
That may all seem rather far-fetched, but is it possible?