Originally Posted by
SteinarN
I did some calculation.
Starting at 6000 feet/2000 meter, the time in free fall to ground is 20,2 sec. This aircraft took about 18 sec from 6000 feet to ground. This means the G throughout the dive, from start to end, would have to be close to zero or slightly negative.
This seems to imply that the elevator/h.stabilizer must have been at average at approximately zero degrees angle of attack thus producing zero force.
I do wonder, if the jack screw for the stabilizer breaks loose, in which position will the aerodynamic forces put the stabilizer into? Would it "free float" into zero degrees AoA?
I don’t think it would free float. It’s not a hinged device. Having said that if the aircraft was loaded with a forward CG and the stabilizer system suffered a catastrophic failure it would have thousands of lbs of force trying to move it to a neutral airload position like the Alaska MD80. That kind of force could drive it to that position and I doubt there is enough elevator force available to override it. To prevent that Boeing has both a main and secondary brake system on the stabilizer if I recall correctly. It would be interesting to know the CG of the aircraft as loaded that day.
Another possibility is that as the aircraft slowed the stabilizer was frozen and not moving. The autopilot would compensate with up elevator until at some point the autopilot desengaged which would produce a instant negative G nose down pitch mode.
It might be very difficult under negative G to apply much if any nose up elevator via the yoke depending on seat position and how tight the lap belts were plus shoulder harness usage. In fact I think it would be nearly impossible. The catch to this theory is the system has a stab out of trim caution to alert to this type of failure and the yoke itself would have been visually aft of its normally position.
If the aircraft had recent work in stabilizer area it would be interesting to know what was done.