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This question already has an answer here:

My understanding of trim (on non fly by wire aircrafts) is that when the pilot feels force on his column or side stick, he trims the aircraft (either up/down) until the forces on the stick or yoke are neutralized. But in fly by wire systems, the pilot does not feel any force on the stick or yoke. In such case how does pilot know when to trim the aircraft (be it pitch/roll/rudder trim). Or does flight control computer computes the trim required automatically? In such case, what happens during the direct mode (or alternate law) when flight control computers are no longer in control and pilot commands directly go to actuator controller electronics.

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marked as duplicate by fooot, SMS von der Tann, Federico, CGCampbell, Simon Mar 18 '16 at 19:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ The question you are referring talks about auto trim on FBW aircraft but my question is about manual trim $\endgroup$ – user2927392 Mar 18 '16 at 17:23
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    $\begingroup$ More generally, see How does trim work on an A320? $\endgroup$ – fooot Mar 18 '16 at 17:38
  • $\begingroup$ When the controls degrade to direct law the autotrim doesn't function. It puts a warning on the display to tell you autotrim is deactivated. That is what happened on XL Airways flight 888. The AoA sensors failed and the control law degraded. The pilot apparently did not realize the need for manual trim, lost elevator authority and stalled. $\endgroup$ – TomMcW Mar 18 '16 at 17:56
  • $\begingroup$ @mins the last two paragraphs in this answer to the second linked question seem to address how trim works without force feedback. $\endgroup$ – fooot Mar 18 '16 at 19:19
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Fly-by-wire just means that the control column is not connected to the control surfaces (hydro-)mechanically, but using digital (electrical or optical) signal wires. That says nothing about how the controls behave. There are two basic ways they can:

  1. The Boeing way:

    Force and position is transmitted in both directions, emulating control with mechanical link. The pilot feels force on the control surfaces and trims as usual.

  2. The Airbus way:

    The position of the control column corresponds to wing loading in pitch and roll rate in roll axis. The computer manipulates the controls to achieve the target values. The control column is spring-loaded, so force always corresponds to position.

    The trim is operated by the computer to balance the control surface for wing loading of 1 G (or is not a separate control at all as in case of FBW fighters with all moving stabilator).

    But when the system is in direct law (during take-off or if degraded due to fault), the trim still works more or less normally. The centred position of control stick corresponds to the trimmed position of the control surface, so if the pilot feels force on the control stick, because it is displaced, they dial trim and reduce pressure on the stick until the plane flies straight with the stick centred.

The Airbus way is used on Airbus A320 and all newer models, Sukhoi Su100 and some fighters (since F16).

The Boeing way is used on Boeing 777, 787 and, IIRC, some regional jets.

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  • $\begingroup$ I suppose Dassault Falcon's way is similar to the airbus way. Do you have any further readings for this "airbus way"? $\endgroup$ – Manu H Mar 19 '16 at 12:27
  • $\begingroup$ @ManuH, wikipedia has quite a lot of details already. There is also some pretty detailed document at smartcockpit (look for document named "Fight Controls"). $\endgroup$ – Jan Hudec Mar 19 '16 at 19:53
  • $\begingroup$ I was thinking of decision process behind autotrim (subquestions of this question) $\endgroup$ – Manu H Mar 20 '16 at 17:58
  • $\begingroup$ @ManuH, From pilot's point of view, the aircraft is always perfectly trimmed for straight flight path under autotrim. But I've never found a single word about when the ELAC actually decides to move the stabilizer (though in the plane it can be observed, because the trim wheels have hydromechanical link and turn when the stabilizer moves). Probably it's considered an implementation detail nobody needs to know. $\endgroup$ – Jan Hudec Mar 20 '16 at 20:28
  • $\begingroup$ Fly by wire isn't just that the signal is sent by wire, but is also interpreted and modified (mostly restricted) by a computer (either analog or digital) $\endgroup$ – Chris V Mar 20 '16 at 21:26

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