Autopilot bucks when on the glideslope

[awmuhs@aol.com]

My autopilot works great in level flight, climbs, and descents. the servos do not slip as it transitions to climbs and descents.

During an approach to an ILS I arm the localizer and glide slope. The autopilot intercepts the localizer and switches to localizer tracking. Tracks just fine.

I intercept the GS from below. the instant the autopilot intercepts the GS and switches to GS mode the stick starts oscillating forward and aft at about 1 second intervals. The nose of course follows along rising about 3 inches and then dropping back down. I let it go for about 15 seconds a couple times while I adjusted the trim with no improvement.

Any ideas?

Autopilot settings are:
Torque 100%
Sensitivity 13
Pitch gain 2.2
Altitude gain 1.5
Pull rate 1.0
VSI gain 1.5
G error gain 3.0
G error limit .25
Default climb 500
Default descent 500
Max AS 180
Min AS 81

Thanks, Alan Muhs

 

[led]

Hi Alan,

I would look first at reducing the G Error Gain.

Your best guide for glideslope performance is the airspeed hold test detailed in the tuning guide, so double check that after you make adjustments.

 

[awmuhs@aol.com]

I have a GTN650. It has more options for setup than the 430. I did not see any suggested settings in the manual. Do you have any suggestions?

Thanks
Alan Muhs

 

[led]

Alan,

I'm not 100% sure what you're asking for here; There are no GTN or 430 settings that are used by the autopilot. As long as your nav radio is correctly connected to SkyView and the needles are displayed, the AP will fly the guidance.

-Lawrence

 

[awmuhs@aol.com]

lawrence,

I flew the plane, ran the IAS autopilot setup, and made some of the changes you mentioned. it got better but still needs work.

I asked about the GTN setup because I thought the signal might not be coming through properly.

Thanks for your help.
Alan

 

[led]

Alan,

If you'd like, get in touch with our support folks (support@dynonavionics.com) and send us a diagnostic log; I can do more to help with that data.

-Lawrence

 

[Edwardoc]

Re: Autopilot stops tracking glideslope

Question on which file to download. On ILS final with VNAV selected, system intercepts Glideslope and VNAV turns green. At about 10-15 seconds into descent the airplane pitches slightly up and down and then ceases to track the GS. It just continues to descend at about 5-600 FPM while tracking the Localizer and drops below GS until full scale deflection. I let it go and it never makes an attempt to climb back to GS. No slippage of servos is indicated. Would it do any good to download the data from the flight and if so, which file. The Log file or the other file which is saved for the flight (can't recall what it is called) or should I download both to have you look at?

 

[Edwardoc]

Never mind I see to download the diagnostics file. I would like to send it to see if you can tell why it is doing this. Tomorrow I will go fly a different ILS to see if it does it at a different airport.

 

[Edwardoc]

Support:  I would like to send you a diagnostic log to take a look at an auto pilot problem.  Can I get a case # or a request for it.  Thanks.

 

[Dynon]

Absolutely. Submit via http://www.dynon.aero/reportlog, and make sure to put a link to this thread in the message area of the submission.

 

[Edwardoc]

I sent a zipped diagnostic file via the link. Thanks

 

[Edwardoc]

Any progress on looking at my file. I have a case number but can't find it.

 

[Edwardoc]

I received a message from Tech Support that they had my file to analyze the Auto Pilot behavior but most everyone was at Oshkosh and it would be a couple of weeks before they could get to it. I have not heard anything so was wondering if it has been lost. It did the same thing again but I did not save the flight.

 

[Dynon]

Do you still have the email we sent from technical support? If you do, can you reply to it? That's the best way for us to link up to your original submission. We should still have your log. Apologies for not replying as of yet.

 

[Edwardoc]

That is the problem. I could not find the e-mail with the case number. Was hoping you could find it somehow.

 

[Dynon]

Found your case. I'll ping our support team and help move things along.

 

[rfazio1951]

One problem I have found when intercepting the glideslop, is that the autopilot pitches the plane down. In my RV6, pitching down that much causes enough force that the pitch needs to be trimmed. What I have done is disconnect the autopilot when I first intercept the glideslope. Pitch down along the slop and trim. Then turn turn the autopilot back on. This cures the problem. If you try this and have the same results. Then it's just a trim issue. Auto trim is a possibility, or stronger pitch servo.

RF

 

[Edwardoc]

I did some tweaking to the AP setting sensitivity etc but I am now getting quite of bit of slip due to my linkage ratio.  I need to do some more testing.  In turbulence I am getting a lot of Roll slip also even at 100% setting.  So I am going to rethink my setup.