Autopilot - can’t catch heading

[Nikhiln25]

I bought my plane last year. It has a navomatic 300 heading hold autopilot connected to my aspen. Since I’ve had the plane I’ve had this issue with the autopilot and I don’t use it as a result. I’m considering getting it fixed but wondering if this is a simple fix or going to cost me a decent bit


I will have the plane straight and level and heading bugged to current course. I will turn the auto pilot on and the plane will start making small corrections to keep the heading.. they will then become ever larger corrections, where it is overshooting the heading in both direction and then correcting. So the plane is effectively acting as a pendulum, it corrects to the left and over corrected, then it corrects to the right and over corrects by a larger amount, then to the left then to the right. I have it on video but unable to upload because of size restriction, but it just. Starts swaying to the left then the right. Curious if anyone has had an issue like this and knows what could be causing it

 

[Hang 4]

One possibility. There is slack in the servo cable. AP does a correction, nothing happens, so it corrects more. That then turns into increasing oscillations.
There are others, but those are beyond my knowledge of your AP.

 

[flyingcheesehead]

One possibility. There is slack in the servo cable. AP does a correction, nothing happens, so it corrects more. That then turns into increasing oscillations.
There are others, but those are beyond my knowledge of your AP.

This. S-TEC hands out (or at least used to) an autopilot troubleshooting list on a business card at Oshkosh. The first thing on both pitch and roll troubleshooting lists is to check the aircraft rigging. They told me they get a LOT of units shipped back to them that test out just fine on the bench...

That said, @Nikhiln25, if you get the aircraft rigged and this still happens, I wouldn't bother spending money on an old autopilot. I had to put 5 figures into my autopilot to get/keep it running before the GFC500 came out; I wish I'd had the option of doing something new! The GFC500 is a fantastic autopilot, way more capable than was even available at all until it came out. Well worth the coin if you're going to be keeping your airplane for a while.

 

[Dan Thomas]

Way too many of the airplanes I encountered for the first time in the shop had their rigging all off. Cable tensions nowhere near correct (sometimes no tension at all), surface travels screwed up. Control system pulleys and bellcranks seized or sticky. Once everything was lubed and rerigged, the owner was amazed at the crisp, easy, "like-new" handling.

 

[Pinecone]

Way too many of the airplanes I encountered for the first time in the shop had their rigging all off. Cable tensions nowhere near correct (sometimes no tension at all), surface travels screwed up. Control system pulleys and bellcranks seized or sticky. Once everything was lubed and rerigged, the owner was amazed at the crisp, easy, "like-new" handling.

I have heard the same thing from several avionics shops.

 

[Nikhiln25]

Way too many of the airplanes I encountered for the first time in the shop had their rigging all off. Cable tensions nowhere near correct (sometimes no tension at all), surface travels screwed up. Control system pulleys and bellcranks seized or sticky. Once everything was lubed and rerigged, the owner was amazed at the crisp, easy, "like-new" handling.

I’m getting this addressed on Monday. Hopeful it fixed the problem