G3x autopilot drifting

[Michael Henning]

G3X with GPS175 and Garmin AP servos. Sometimes the plane will drift to the right of course. By a full deflection of the cdi needle. My question is why? All of the Nav points are correct, and it only happens occasionally.
Thanks in advance.

[Longez]

Originally Posted by Michael Henning

G3X with GPS175 and Garmin AP servos. Sometimes the plane will drift to the right of course. By a full deflection of the cdi needle. My question is why? All of the Nav points are correct, and it only happens occasionally.
Thanks in advance.

Mike,

Are you logging data to an SD card in one of your G3X displays while flying? If so, G3Xpert@garmin.com should be able to easily tell you why there is a lateral deviation on some flights.

There are only a couple of reasons why this could happen.

Steve

[lr172]

How do you address this when it happens? What do the status bars say? You sure the AP is not disconnecting? sometimes if you accidentally bump the trim switch, the AP shuts off off. Have you looked at the display to comfirm it is not going into roll mode, which holds wings level vs NAV which tracks a course.

Larry

[bhester]

On the way home from a 2.3 hr calm flight a week ago mine was drifting off to the left. It did it a few times, never before. I turn the autopilot off and adjusted my manual aileron trim and the reengaged my autopilot. Worked great the rest of the trip. I'm thinking I need to increase my roll torque from 15% to 20%
I did not have much of a crosswind.

Does this sound right?

[g3xpert]

Like Steve mentioned drop us a 1hz Flight Data Log to G3Xpert@Garmin.com. These are generated as long as an SD card is in your display during the flight. If you didn't have an SD card in the display during the flight, take a short test flight to gather some data when you are able to, and send us the resulting file.

Low roll servo torque can cause this type of behavior but if we have a FDL to take a look at, we can take some of the guess work out of the process for you.

Thanks,

Justin

[mturnerb]

It will be interesting to hear from Garmin experts here. I would assume that Garmin's design would include either disconnecting or giving some kind of warning if the autopilot is unable to hold course due to some kind of mechanical issue such as a torque setting.

I wonder if OP's issue might be either roll mode as Larry mentioned or heading mode which could drift off course over time (however would tend to go one way or another, not same direction all the time).

[g3xpert]

Originally Posted by bhester

On the way home from a 2.3 hr calm flight a week ago mine was drifting off to the left. It did it a few times, never before. I turn the autopilot off and adjusted my manual aileron trim and the reengaged my autopilot. Worked great the rest of the trip. I'm thinking I need to increase my roll torque from 15% to 20%
I did not have much of a crosswind.

Does this sound right?

Hey Bobby,

Yes it is very possible that you're roll servo torque is just a tad low. You can raise the servo torque for the roll axis, but you will want to do some on ground testing just to ensure that the servo torque can still be overcome by the pilot. If you want to send a 1hz FDL over as well, we can take a look at it to help determine if this is the issue or not for the same reason, to take some of the guess work out of it.

Thanks,

Justin

[mburch]

I often see these autopilots installed with the maximum torque limit set too low, which can result in poor autopilot performance. Remember what it says in the installation manual:

The Max Torque setting should be set high enough to not slip during in flight air loading, but the torque setting should be low enough for the pilot to override the autopilot if required.

You want this number to be as high as possible, up to the point where you can still safely overpower it by manipulating the flight controls.

[bhester]

After looking at my file GarminG3Xperts agreed that increasing my roll torque should fix the drift problem.
Thanks guys!

[Michael Henning]

Hi Justin,
I did have a data card in there, and will send you the info. Autopilot was not disengaged for sure.