This appears to answer the question of why, given that there is a "memory item" for dealing with runaway trim that would have helped, it wasn't followed by the crew:
> “They didn’t seem to know the trim was moving down,” the third source said. “They thought only about airspeed and altitude. That was the only thing they talked about.”
If this is true, not only did the pilots not realize the trim was the cause of the dive and try to fix it, it seems that they didn't even realize the trim wheel was moving.
"Runaway trim" would be something they trained for, but has a presentation so vastly different (of a visibly spinning trim wheel causing quick descent) as to not be recognizably the same emergency to these pilots.
Of course, this crew had no way of knowing that there was a system on the plane that could slowly make erroneous trim changes over a long period with autopilot off, and it makes sense that they might miss it happening while focusing on their stall alerts and speeds and trying to come up with an explanation and also trying to retain control.
> “They didn’t seem to know the trim was moving down,” the third source said. “They thought only about airspeed and altitude. That was the only thing they talked about.”
If this is true, not only did the pilots not realize the trim was the cause of the dive and try to fix it, it seems that they didn't even realize the trim wheel was moving.
"Runaway trim" would be something they trained for, but has a presentation so vastly different (of a visibly spinning trim wheel causing quick descent) as to not be recognizably the same emergency to these pilots.
Of course, this crew had no way of knowing that there was a system on the plane that could slowly make erroneous trim changes over a long period with autopilot off, and it makes sense that they might miss it happening while focusing on their stall alerts and speeds and trying to come up with an explanation and also trying to retain control.