Originally Posted by
Educated Airman
I am still thinking about the missing bolt on the horizontal stab actuator theory. If this happened the stab would most likely have moved beyond the design limits and could possibly have caused contact with the elevator control cables resulting in an overload of one of the many pulley quadrants. This would cause the cables to go slack and the elevators to assume and retain any arbitrary position. If there is a control column position channel on the FDR and it does not agree with the elevator position, a probable cause could be stab movement beyond design limits. Once the horizontal stab stalled out due to pitch momentum, the excessive nose down AOA could have a wing area effect on the stab (Due to center of lift and hinge location) to blow it into a nose up position resulting in the final pull up.
The 767 is a conventional geometry aircraft: The stab provides a negative lift force to balance the nose down pitching moment of the wing. If the actuator becomes fully free of constraint, the aerodynamic load on the stab will drive it to stabilizer leading edge down, which increases the negative lift, and results in a pitch up. The normal force from the section is forward of the remaining hinge line, and will result in the stabiliser increasing LE down (pitch up).This was not the case for an aircraft that pitched nose down in wings level flight. (Sorry...)
[if you get up close and personal with the stab, you will see that it is a cambered section, with the camber towards the bottom, not the top]