Autotrim
Quote from DozyWannabe (Jun13/2255z):
"The only way autotrim is activated is if the stick is fully deflected and held there past the elevator travel limit. It's there to give the pilot more authority when the chips are down."
Like CONF_iture, I think you are misunderstanding the role of autotrim and, in your latest post (Jun14/1010z) you also seem to be misinterpreting the following piece you quote from PJ2:
"...to put it simply, the THS will follow the sidestick orders in Alt 2 Law.
The elevators are short term pitch control, the THS long-term, providing the elevators with full authority throughout their range."
The role of the autotrim is to enable the elevators to return to neutral as soon as possible after a change of pitch or of pitch-trim has been executed. The latter may have been ordered by PRIM or SEC (for example, to maintain 1G with a decreasing airspeed). In the case that you are discussing, it is by sidestick movement.
Even if the PF moves the stick back a small amount, he is ordering a pitch-up. The elevators deflect slightly up. This reduces the available pitch-up capability slightly, so the autotrim adjusts the THS slightly in the nose-up-trim sense, allowing the elevators to neutralise again. As PJ2 says, "The elevators are short term pitch control, the THS long-term, providing the elevators with full authority throughout their range."
But even with no sidestick displacement, this FBW system would provide up-elevator as the aeroplane slowed down: initially to maintain the 1G-trajectory that the pilot's inaction would have – in effect – commanded. Again, the "short-term" up-elevator would be backed up by "long-term" THS.
Hope this makes sense.
Last edited by Jetdriver; 14th June 2011 at 14:09.