What a strange crash.
Aviation
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It's a stall. When the plane is like that it's nearly impossible to maneuver because the control surfaces aren't exposed to airflow so they don't do anything. Your only hope is to somehow make it face the ground again and pull up in time.
As to how it got that way it usually happens when someone loses track of the horizon and tries to fly up.
It's a flat spin. It's unrecoverable for many 2 engine aircraft. Sounds like heaving icing was reported between 12-21000' this morning. They had been in flight for over an hour. Scary stuff.
How would icing cause the plane to stall like that? Shifting the center of gravity towards the tail?
Wings have a very delicate engineering behind them. If the airflow over the wing is disturbed too much you can lose lift and stall the wing. Plus ice build up on engines and instruments could mean bad info to the pilots and not enough power to get out of it. This was a prop driven plane so those are airfoils too. Ice on a prop and you might have no thrust to keep going.
From a video about another incident with ATR 72: https://youtu.be/DOrK_5cZTlE?t=1692
This is probably not exactly what happened to this particular plane, but the section by this time code explores your question.
Tragic. An awful way to go. They must have been terrified. Rest in peace.
It's very disheartening to watch. So much time and so little that can be done.