Originally Posted by
F4E Mx
"A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable."
So basically we do not know if the 737 was designed to be controllable in the instance of full runaway stabilizer travel or not. The instance of the crash might suggest not.
Extremely improbable means that it is never expected to happen to the whole fleet, ever.
If a simple AOA sensor failure makes it happen and there's no compensation method for it, this will be nasty, NASTY, for Boeing.
I would say this failure mechanism is "probable" according to the guidance in 25.1309.