Originally Posted by
Stick Flying
If we are to believe the FR24 data, it does not logically back up a trim runaway scenario. The speed (I'm still unsure whether we are talking G/S or TAS), drops slightly just before descent. This would not be abnormal if this was TOD as the aircraft sequences to ECON descent speeds (Mach). But then at about the time the descent rate increases exponentially, the speed is shown as a good 70-90Kts less than Cruise/TOD schedule. This anomaly lends itself to some other form of event unless the data table has a high inaccuracy capability. Only the CVR/FDR will allow those that analyze these events the opportunity to piece together the possible causes, hopefully arriving on a conclusive reason.
Looks like your speed qualification assumptions are wrong. The FR24 speed data is Ground Speed. So, when the airplane goes down vertically, the Ground Speed will show zero, while the TAS can and will be enormous, largely above VNE, even the speed of sound (hence the reports about "booms") can be reachable.
Your assumption about a decreasing "speed" in the FR24 data, happening before the upset is wrong. Only once the upset starts, the Ground Speed goes down, understandable, since the airplane is diving with more 20K ft/min. So no anomaly in the reported FR24 speeds.