MGL Avionics User Forum

I recently decided to make some fine adjustments to my autopilot to improve the response a little, before doing so I spent some time going through the autopilot setup manual to refresh my memory but I couldn't seem to find any info or reference to the "autotrim" functions and on whether the boxes "Bank autotrim" and "Pitch autotrim" in the setup menu should be ticked on or left unticked Should I tick them both as on? and if I do tick them what exactly does autotrim function do! My aircraft has a mechanical manual trim system set up so is not connected to the iEFIS at all.

I'm sure I'm missing something obvious

Thanks, Don.

Your autopilot must not only feed your servos but have feedback from the servos to the autopilot. If you don't have this feed back (such as from Ray Allen servos - which is what I use) you will not have autotrim for which the check-box must be checked to use.

For my aircraft I found that the autotrim only kicked in when the plane really needed trimming ie with a lot of load so found the function to be superfluous, I unchecked the boxes. My plane is light, all carbon fibre max all up weight 750 kgs (1653 lbs).

Cheers Peter

Hi Pete,

Thanks for the reply, I’m also flying a lightweight aircraft, a Jabiru J170 with a MAUW of 600kg. I have a Challenger iEFIS with MGL servos for bank and pitch. What I’m missing is an understanding of what parameters are the autotrims trying to control? And how will it go about achieving its objectives? I have flown with both boxes ticked and unticked, I haven’t really noticed a difference in how the autopilot behaves. I did read in the manuals the torque settings must be below 80% for the autotrims to function correctly, my torque settings are both set at 50% so the autotrim functions should be working.

Cheers Don.

Presumably all of these aircraft have electric trim systems and those are connected to the autopilot system in some way. I not, how do you think the servos are going to be able to trim the aircraft? The autopilot servos just move the elevators and ailerons, they don't move the trim tabs. I suppose they could 're-datum' the null point but, without actually re-trimming, that's going to make no difference to the pilot when the autopilot is disconnected.

My aircraft has a manual elevator trim. I try to make sure it is pretty well in trim before engaging the autopilot but, due to weight changes etc during the flight, it will be slightly out of trim when I disconnect the A/P. There is no clever process whereby it can automatically trim the aircraft and, while the A/P is engaged, I think it has to just sit there and put in the appropriate signals to keep the altitude at whatever its been told to hold. Same with headings (but fewer trim changes to consider in roll).

I will be glad for Rainier to tell me it's more clever than that but I just reckon it fights to keep the heading and altitude at zero (whatever it is set to) and if that means holding a bunch of up elevator then that's what it'll do.....without trimming; as it can't.

Hi PaulSS,

I don’t disagree at all. My thinking was that the iEFIS monitors the torque being applied by the servos and based on the torque being applied it applies some degree of adjustment to the trim tabs, assuming of course the trims tabs are electrically driven and connected/controlled through the iEFIS. However my aircraft has a manual pitch trim system and no bank trim at all therefore I assumed the two boxes should remain unticked, but I couldn’t find anything in the documentation to confirm that and I wondered if maybe I was missing something.

Cheers, Don.

I don't think you've missed anything, Don. My aircraft is set up the same as yours and I've left the boxes unticked. It seems to do an okay job but definitely a bit of waffling, especially if the slip ball is out a little.

We'll wait and see if Rainier tells us to tick the boxes