Originally Posted by
HalinTexas
I'm not buying that they could trim manually. They were doing less than 350KIAS. I suspect they didn't pull the handle out to crank it.
The problem was not just the airspeed but also the extreme out-of-trim state combining to put an excessive load on the jackscrew. Once the aircraft is at or near neutral trim, manually trimming is a pain but doable. However, you absolutely need to keep ahead of it.
Even if it was possible to manually trim in their state, the crew may not have been aware of just how much force it would take. The ops manual does not provide much guidance on this. Since the Captain presumably had all his hands full with the yoke, the FO probably tried the crank handle but either could not turn it or simply did not apply enough force.