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Old 14th Jan 2016, 15:19
  #90 (permalink)  
Join Date: Jul 2009
Location: Not far from a big Lake
Age: 77
Posts: 1,458
The autopilot will command pitch trim to zero out any required elevator servo commands, after a short time delay (to prevent it from commanding trim in response to short duration elevator commands). The autopilot receives elevator position information, and I believe it would disconnect if the elevator position did not match the result expected from the amount of servo current, but I'm not completely certain of this.
Thank you again for your helpful descriptions.
This implies that there must be both right and left elevator servo valves and that the linkages between the control columns and the elevator servo valves are duplicated, probably all the way back from the cockpit. Since each elevator side is actuated by 3 cylinders on 3 different hydraulic systems, the servo valves are either a single valve body with 3 sections or actually 3 separate valves on each elevator side. Followup signals would probably be by means of a linkage attached to the individual elevators which is then summed with the associated input linkage. Autopilot trim inputs would then be taken directly off the summing linkage per your description. Then the question arises, are autopilot trim inputs taken from both sides and compared or from just one side? Or have I run well off the track?

Those design engineers are clever fellows and often do the unexpected to achieve their goals. I tried to download system information from Scribd, however didn't find much of great use.
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