She had five total posts on Beechtalk, one of which was inquiring about her Century 2000. Both responses told her that it was broken and one even referred her to a good shop (apparently not done).
Several posts on this topic are speculating if they autopilot trim was or was not the problem. It wasn't because this airplane did not have electric trim. But this does not rule out the autopilot as being a contributing factor. I wonder if the autopilot enganged and for whatever reason would not disengage?
Let's say hypothetically the trim was way too high. The autopilot would try to push the nose over. A confused pilot might have trimmed in the wrong direction, making the situation worse. Then the autopilot while still turned on pushes as hard as it can until something gets stuck. I'm not sure what would happen if the airplane was in full opposite trim from what the autopilot was trying to do. I'm just saying it would be possible that something could get stuck in this situation because it's an old autopilot.
A trained pilot should know where the breaker for the autopilot is. I'm going to assume she didn't know how to hard-kill it from the breakers. So then what would a new-ish pilot instinctively do? Probably call mayday and wait for a repair guy?
I didn't notice in any of the videos if the breakers were the pull-out style. In my old Beechcraft, it had the breakers that were flat when not tripped. These are not pullable by the pilot. So the only other option would have been to kill the master switch. Doubtful that anyone would have taught her that.
If the autopilot is still on, did she or her passenger have the strength necessary to overcome the autopilot? Did they know this was a feature? Was the feature adjusted correctly? (it works from measuring how many amps/volts it needs to maintain force, and when a threshold is hit it shuts down. The threshold could have been out of alignment).
Without any additional info, this is the only scenario that kinda makes sense. Why else would they keep the airplane at cruise power straight into the ground? Reducing power would have caused it to descend faster. Let's assume she at least knew that much.
We know from people who do math (thanks for that) that the airplane was approximately 30 degrees nose down. If the seat belts were not adjusted tightly, the body weight would have been quite forward after the initial G-unloading. So she would have had to overcome about 1/3 of her own weight (here comes the math fact checkers... go ahead...) PLUS the force required to overcome the autopilot. So let's say she was about ~130 lbs. The autopilot would require about another 30-40 lbs to overcome? So that means she would be doing about half a pull-up excercise of her own body weight, while nose down 30 degrees, and probably the only leverage would be her feet on the rudder pedals.
I'm assuming the autopilot was engaged to the end because it was basically wings level for the entire decent. For a pilot who couldn't keep it wings level flying by hand when she turned her head, let's rule out that she was hand flying it.
Then at some point the airplane want past Vne. Game over.