Hi, STBYRUD,I figured it out!
cause the aerodynamic shape of the stabilizer is different to the wings,it produce the tail down force for balancing the pitch down force which cause by the center of lift after the CG.
1/if the stab runaway all the way to pitch up position,means the trim wheel rotate to backward,then push the yoke forward,try to stop the runaway,but it won`t, push the yoke will cause the angle of stab chord line between the relative wind become smaller, that produce less tail down force.that is kind acceptable~
2/if the stab runaway all the way to pitch down position,means the trim wheel rotate to forward,then try to pull the yoke backward,to stop the runaway,but it still doesn`t work, pull the yoke will cause the angle of stab chord line between the relative wind become larger, that produce much more tail down force than needed,it will accelarate the aircraft to the ground,that is totally not acceptable.
That`s only my guess,sounds make sense
please correct me if I am wrong.
Thanks a lot~ STBYRUD