Sluggish Autopilot - how to troubleshoot?

[Larry Biever]

I've recently installed a GTN 650 with a Aspen and a Stec 40 auto pilot. All are new. I'm unsure if my autopilot is working properly when trying to track a VOR. It seems very slow to turn to track and is also very slow to react when the needle starts to leave center. I'm wondering if someone has experience with this configuration and can provide suggestions on how to check to see if it is working properly and what my expectations should be? I've been with my friends in their planes and their systems react quite quickly and mine is very slow compared to theirs. The do have different systems so I'm not sure if it is a good comparision. My system works very well when using the GPS - is fast to react and holds very accurately.

Thanks
Larry

[Zach Grant L1011jock]

Unfortunately that is normal for the Stec when tracking nav. It will keep turning until the left right signal goes the other way. There is no "centering" feed to the nav tracker. You must be on course and have a steady state needle before engaging, and then it works OK, but it has no target to turn to for an intercept. Using GPSS adds a computer that does have a centering tarhget course, and then it will intercept and not overshoot the course.
-Zach

[James Turner]

Hi Larry,

I am thinking of installing an Stec 30 but just can't believe how pricey this equipment is and how poorly it seems to perform as compared to the non-certified (non) options. I'm sorry to hear that you are having issues; on previous larger aircraft that I have flown I know how annoying it can be to watch the autopilot chase a track...

I don't want to hijack you thread however, if you have the inclination I'd be interested to hear more details of your installation; time, costs, difficulty etc. FWIW I have just installed the Aspen/CNX80 set up with no autopilot... Love the set up but kind of wish I'd sprung for the autopilot...

Cheers,

James.

[Larry Biever]

Thanks Zach. From what you are saying it sounds like it is working properly. It even seems to have difficulty holding the track on the VOR once I have it centred - but I'll try it on a glideslope and see what it does.

[Larry Biever]

Hi James,

This is a long story. I started out wanting to upgrade my plane to basic IFR and my indicators were non serviceable so I was planning on doing the upgrade with used avionics. I was thinking of getting a used 430WASS with no glass and no auto pilot. I knew I wanted to change the shotgun panel to the new Ron and John’s STC’d one. My AME, Mark, started looking for a used 430 WASS and found they were almost impossible to find and we would likely need to buy new. I looked at the price of a new one and found that for not a lot more, I could have the GTN 650, and be using something that is current. About the same time I went for a flight with my boss in his Cessna 340. We were in the soup and I quickly realized how much easier life would be with an auto pilot. Right after the flight I phoned Mark and said I need a basic autopilot that would be a wing leveler and fly a heading, but could be upgradeable. Now knowing that I wanted a autopilot and a GTN 650 he asked what my future plans would be. I said that someday I would like GPSS steering. At that point Mark brought up installing an Aspen. He said that if I went with an Apen I wouldn’t need to buy a new DG, and that it would come with GPSS steering. We crunched the numbers, and once again for a bit more I was at the point of having everything (those 2 items were almost half way to an Aspen). Mark had installed many Aspens at his previous job, so he is very familiar with them and held them with high regard. We also installed everything with a Fast Stack Wiring hub. This made the install easier for Mark since it is all prewired, eliminated a bunch of wiring at his shop, and will also make future changes easier – its more plug and play instead of rewiring every time you make a change. My plane was down for about 3 weeks but was well worth it. Costs.... well, there is no way to justify it other than the love of a Comanche, but about the same as the plane was worth before I started.

[James Turner]

Hi Larry,

Thanks for that. Similar story for me except that my aircraft was down for 4 months!..

I looked for a used 430W but the availability and price turned me in the direction of the CNX80/GNS480. I'm glad it did as I love the unit and as an ex Airbus pilot I find it works very much like an airliner FMS. Same decision process with the Aspen as you went through. Can I ask where you mounted the RSM? We had some problems with magnetic interference with the elevator trim cables; when you moved the trim the change in magnetic field affected the RSM... I went for the JPI 930 also. Lot of problems with this unit all to do with the poor quality of the JPI supplied wiring harness...

Hope you get some resolution to the tracking issues.

Cheers,

James.

[Larry Biever]

We mounted the RSM on the pilot side of centre behind the cabin. We did a lot of checking with the trim cables because we had heard of issues, but Mark found the "sweet spot" and it is working great.

Thanks
Larry

[James Turner]

Hi Larry,

Interesting; that's pretty much where I had to move mine from. We simply couldn't get it magnetically clean enough with the trim cables running directly underneath (even after degaussing all the cables).

If you have the inclination sometime try entering the diagnostic screen (press menu, go to the last page, press and hold menu, line select one, and line select two for 3 seconds; then go to the last screen). Whilst on the diagnostics page wind the elevator trim from full down to full up and watch the MAG GAUSS values. Do they change?

Cheers,

James.

[Larry Biever]

I'll give that a try. I'll have to measure, but I think we are about a foot or so to the left of centre. It looks off centred, but that's where Mark said he had to mount them so they worked properly.