The departure from controlled flight appears to have been sudden based on the data I keep posting, followed by a very steep descent profile. That profile is NOT an emergency descent profile. The sinkrates involved are not achievable in normal, controlled flight (~167 kts vertical speed component alone, as posted above). There appears to be porpoising prior to the onset of the sudden departure, along with a significant speed decay. That could very well be indicative of a stall induced spin.
I'm adding another plot here, ground speed (GPS derived speed in the lat/lon reference frame), vs. the velocity vector (VV), or total velocity in X,Y,Z reference frame. It's interesting to see that the VV has a couple of very high excursions but otherwise remains at similar magnitude to cruise flight (conservation of energy) - it would seem plausible from this that the airframe remained largely intact but spent most of its energy on vertical speed (as in a spin), the two outlier data points probably being just that - momentary glitches from poor GPS geometry due to the unusual attitude. Note that the geometric vertical rate is also GPS derived. I didn't use the barometric altitude due to hysteresis. This particular aircraft did not report barometric vertical rate.
What's odd is that the speed already decayed to a similarly low level at time 03:44:38, ~4 minutes prior to the rapid descent.
I'm fairly certain the potential stall/spin is the result of events prior to that (icing and/or incapacitation) - it's a more complex sequence of events than many here speculate.
