As I understand, you trim to reduce the effort needed to maintain the stick in the right position. Thus, you may reach steady flight before moving the trimming wheel.
In fly-by-wire aircraft (to fix things, let's consider an A320), there is no such effort to maintain as you use sidesticks. Moreover the elevator may move almost constantly (and thus always apply some effort) to compensate turbulence (enhance stability feature).
My question comes in several parts:
- In flight in normal law, how and when does the computer decide it must trim the plane?
- What does it take into account to trim the plane (speed? flaps configuration? average position of the elevators for the last few seconds? a mix of all? I guess average position of the elevator in a sliding time window is enough)
- Does it anticipate the trim (e.g. trim before or while changing the flaps configuration)?
- What happen on the ground? For example during the take off roll, I don't think there is enough data to feed an auto-trim system, but I may be mistaken.
- What happen if not in normal law (I guess auto-trim would be one of the feature unavailable in alternate law)?
EDIT: althought the question may seem similar to this one, it differs as my question address the decision process used by the auto-trim system to command the flight control surface, and not the flight control surface itself (the use of stabilisator as trim in the A320)