One of my ground school instructors had been accident investigator in the USAF. He said victims of a flat spin were intact in their seats, just dead.
A gross appproximation maybe! The tell-tale would be as in the LHR Staines accident, yes 'likely' to be still strapped in, but with compressed verterbae almost uniformly.
Look, there's a whole host of people here, maybe pilots maybe not, but this is engineering and kinematics stuff, or a lot of it is... accident investigation - certainly impact dynamics is NOT necessarily just about flying and piloting knowledge, unless so many have had a lot of accidents & walked away... there's opinions flying about like wasps in a jam-jar here, and that's all most of them are, 6 possibilities back ther in ONE post!!
We need to take deep breath... and let some experts think this through a bit.
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For what it is worth, 'high acceleration' an
engineer usually means in this context... a/c arrived with large deceleration upon impact e.g. it was travelling fast & stopped very quickly.
High acceleration is just they way it is said... inplying large forces.
I also interpret the previous line to mean the
trajectory was predominantly vertical but the
attitude was predominantly horizonatal (line of normal flight)
In fact much as takata said...