https://tonymadgehjg.proboards.com/t...trident-manual
The link is the most comprehensive data I can find which describes the autopilot operation inuding the azimuth switch.
Imput by some knowledgeable guys.
It also shows the after landing checklist Item of setting the tailplane to zero. It is very unlikely that this was a requirement if the whole lot was driven by an elevator tab as suggested. I asked a former colleague who was on a technical committee with Alex but he can't remember although he flew with a captain who had a triple flight controls warning in the flare as the elevator system failed.
As to comments about re trimming the elevator when in a flat spin.. all it will do is to keep your mind away from thinking this is going to hurt.
One of the guys who was a supervisory first officer described to me a stick push exercise where the aircraft wallowed on the verge of entering a deep stall, fortunately it slowly started to pitch nose down. Shortly after that the air exercise was abandoned.
On the DC9 we had an elevator boost system which activated to aid the tab controlled elevators to move as airflow at high angle of attack wasn't sufficient. The tailplane was controlled through screw jacks.