Is there a limit to autopilot trim?

Is the Autopilot limited to how much trim it can use? In one of our planes the autopilot cannot go past 63% of elevator trim, but I can’t find any setting that could be limiting it.

Hello @SWSAlexVletsas

Have a look at elevator_trim_up_limit and elevator_trim_down_limit in AIRPLANE_GEOMETRY section of flight_model.cfg maybe?
flight_model.cfg - [AIRPLANE_GEOMETRY] (

That limit will also apply to autopilot.


From reading that, I understand that if max trim is set to 10 degrees but the effectiveness set to 60%, then the maximum trim is 6 degrees. Is that right?

Still, the autopilot should be able to use the full trim range available to the aircraft. As a pilot, I can go from -100% to 100% using the hat switch. The AP cannot do that, however and caps at about 65%.

I’m not aware of any setting that limits the trim available to the autopilot.

Hello @SWSAlexVletsas

The trim effectiveness parameters don’t impact its position.

What I meant is that if you have an asymmetric trim limit configuration like 10° up and 6° down for example, trim position will show 60% when reaching the down limit.
But that also applies to the pilot, it’s not specific to the autopilot.

I’m not aware of any autopilot specific limitation either.
Did you try to adjust your PIDs?


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Continuing the discussion from Is there a limit to autopilot trim? :

I checked the trim settings and the trims are set to allow full trimming, but he autopilot still won’t trim past 63%. Is there a hardcoded limit to it?

The only non-zero value is “neutral_trim” which is at 11.75%, the take-off setting.

Hello @SWSAlexVletsas

I have extended the period before a topic is closed when a post is marked as solution to 1 month.

I identified the cause. I did some testing with your PC-12 and found out the trim for AI/AP is limited to a [-1;1] radians range.
Your aircraft has the up and down trim limit set to 90°, that’s where the 63% limit comes from.
Are the elevator trim tabs of the PC-12 actually able to deflect by 90°?


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No, the reason we put it up to 90 degrees is to get the correct trim rate (42 seconds end-to-end IRL).
There was a parametre to adjust trim rate but it no longer seems to work.

Default Input Events rely on ELEV_TRIM_DN and ELEV_TRIM_UP key events that have a fixed increment.
You probably want to override this behaviour to get the trim rate you expect.