Professionally speaking …
Even those of us who are conscientious professionals make mistakes. And more often by omission than commission; that’s a feature of imperfect human cognition. And even with two of us, sometimes those mistakes are uncaught by both. Happens every day on many flights.
The vast majority of such fully uncaught mistakes are harmless. But when something bad has happened, it’s a near certainty one of those rare mistakes was part of the accident sequence.
It’s a form of Bayesian probability looked at from the other end of the process. Mistakes are kinda rare. Accidents are extremely massively rarer. But given an accident, what’s the probability of a significantly contributory mistake? Nearly 100%.
Switching from pro mode to amateur accident observer / explainer mode:
Something else to consider about a sliding seat is that the pilot’s weight sliding aft as his seat became unlatched would itself be unhelpful to an already precarious W&B situation. And that’s in addition to the monster control input as he inadvertently hauls back on the yoke.
I admit that I don’t place a great deal of faith in a lot of NTSB GA accident investigations. They often take a few pictures, talk to a few witnesses, publish a guess, and move on. I wish there was some more objective rating scale they applied to now rigorous the investigation and how confident they are about their conclusions. Them doing a lick-and-a-promise job on yet another VFR flight into IMC or light plane fuel exhaustion mishap is a completely reasonable prioritization of resources versus future accidents prevented. I just wish they were able to say up front in big print that that’s what they are doing. And therefore to apply appropriate caveats to what turns into the pilot’s aviation epitaph.
As to the seat stop witness marks, there may well be one set of marks where it slammed to the aft stop, and a different set of marks when the airplane hit the ground.
Digging into the photos in the NTSB docket, it’s pretty clear the seat was latched in the full aft position at impact. And failed forward from there. Both the pin(s) on the seat and the hole(s) on the track confirm that. So the question becomes: “How did the seat get full aft and locked when the pilot is too short to reach the controls with the seat there?” And the answer, which is surmise, but reasonable surmise, is “The seat wasn’t well-latched at takeoff and slammed aft when he first rotated, then over-rotated as the unlatched seat slid abruptly aft.”
From the vid as I interpret it, the airplane rotated so abruptly it probably drug the tail, or nearly so. Which is totally not the way to take off in an ICE or turboprop straight-winged airplane. “Rotation” is a swept-wing jet idea; that airplane should have more or less flown off in a level attitude with a smidgen of pitch up upon breaking ground.
The presence of this improper rotation suggests to me that the seat situation, or the trim setting, or the W&B situation was already going haywire while they were still on the runway. Maybe it was already unrecoverable by then, maybe it was not. But very quickly things got utterly out of hand.
I tend to discount a trim setting mistake as a full-up cause. Pitch trim in a low speed non-transport just doesn’t have enough authority to render the airplane unflyable all by itself. Unlike the all-moving stabs of big airplanes. Part of the reason big airplanes have elaborate systems to prevent stab trim runaways and elaborate backup systems to ensure failures don’t leave a stuck stab is precisely that they have enough leverage within their normal operating range to leave the airplane flat uncontrollable.
Lightplanes solve the problem the other way, by limiting the trim authority (by range of motion and size of tab) such that a pilot of normal strength can outmuscle even worst case trim stuck at one or the other stop. To be sure badly mis-set trim can be a real startle as you’re accelerating down the runway and the pitch situation starts developing a mind of its own. But if nothing else goes wrong that “should” be handle-able by the reasonably prepared & skilled private pilot.
We know, or strongly suspect that “nothing else” wasn’t the case that day.