Originally Posted by birdspeed
Concours77, as the stab seems to trim to full ND at a normal rate, I don’t think there is a mechanical failure of the stab jackscrew.
What perhaps happens is a breakout of the control yokes just before the dive. FDR shows a difference in the applied control force on the yokes. I believe when elevator breakout occurs there is sudden loss of elevator effectiveness. This would cause a sudden bunt over, with a wtf moment and the pilots stopped any sustained opposite trim.
Birdspeed, you may well have the final piece to the puzzle.
Here is the cockpit NG flight control portion diagram courtesy of Infrequentflyer:
NG cockpit control system
See the little goodie named 'Breakout Mechanism'
That is there to restore controlability when something jams part of the flight control system. It's function is to separate the flight control system into two halves.With enough force differential between the two control columns, a cam system will disengage and allow the two sides of the control system to operate independently. From the FDR elevator force charts, the crew were applying force at the limits of their ability against the elevator feel system, and there is no sense in making a disconnect that requires more force than can be exerted. Just how far back in the aircraft would the system remain separated is the next question. It appears that the system would be separated back to the hydraulic power control unit
input torque tube. The NG flight control system appears to have benefited from jammed flight control incidents involving earlier models. See this reference:
NTSB Recommendation
My key takeaway is the following statement: "Further,the elevator system on 737-600 through -900 series
airplanes was improved by the addition of several mechanical override mechanisms. While these
override mechanisms do not mitigate all possible jam conditions, in general, in the event of a
system jam, the mechanisms allow both elevators to be controlled by the movement of the
unaffected control column."
However my reading on flight with jammed controls indicates that the crew should expect reduced control effectiveness. Whether or not flight with disconnected but not jammed pitch controls would experience decreased control effectiveness is dependent on the location and design of the additional override mechanisms required. What I suspect is that there are spring cartridges located both before and after the power control unit input torque tube.