Navomatic 300A and GPS

[rozenfeld57mh]

Hello,

I have a Cessna 172 with a working Navomatic 300A autopilot. Does anyone know if there is a way to get an old analog Navomatic 300A autopilot to follow a Garmin 430 GPS ? Right now it follows heading bug on my heading indicator only. None of the other functions seem to work. Any help would be greatly appreciated.

 

[gismo]

Hello,

I have a Cessna 172 with a working Navomatic 300A autopilot. Does anyone know if there is a way to get an old analog Navomatic 300A autopilot to follow a Garmin 430 GPS ? Right now it follows heading bug on my heading indicator only. None of the other functions seem to work. Any help would be greatly appreciated.

IIRC a Navomatic 300 can track a VOR and if so there shouldn't be any technical problem switching the analog output from any GPS in place of the VOR CDI signal and getting the AP to track that. That said there could be some STC issues to overcome. Another approach would be GPSS. A DAC GDC-31 substitutes bank commands from a 430 for the heading bug.

 

[rozenfeld57mh]

Thank you for reply.

I haven't been able to get that Navomatic to track VOR/CDI signal at all. I have tried on multiple occasions without any success so I had given up on that. But GPSS sounds like it would be a good option. Are there any STC issues with that?

 

[Marauder]

Thank you for reply.

I haven't been able to get that Navomatic to track VOR/CDI signal at all. I have tried on multiple occasions without any success so I had given up on that. But GPSS sounds like it would be a good option. Are there any STC issues with that?

I am not sure what standalone GPSS you could add but one option is the Aspen PFD Pro. If you add an Aspen, it will interface with most older avionics and according to their website will interface with the Arc 300 series. But going this route will mean spending money on an older AP.

Are you planning on keeping the plane for a bit?

 

[rozenfeld57mh]

IIRC a Navomatic 300 can track a VOR and if so there shouldn't be any technical problem switching the analog output from any GPS in place of the VOR CDI signal and getting the AP to track that. That said there could be some STC issues to overcome. Another approach would be GPSS. A DAC GDC-31 substitutes bank commands from a 430 for the heading bug.

I just checked and that DAC GDC-31 is not STCed to work with that autopilot. Getting a field approval is nearly impossible for the autopilots, no one wants to sign off on those according to my Avionics guy.

 

[poadeleted20]

I haven't been able to get that Navomatic to track VOR/CDI signal at all. I have tried on multiple occasions without any success so I had given up on that.

Are you saying your autopilot doesn't even track VOR signals via the CDI? If so, whatever problem is preventing that is also preventing it from tracking GPS courses via the CDI. When you're in NAV mode, the only thing it's looking at is needle position, not the external signal from the VOR or GPS, so the problem would lie either in the CDI, wiring from there to the autopilot, or the autopilot itself. There is no reason I can think of for the autopilot to follow the needle when the needle is driven by the VOR but not when the needle is driven by the GPS unless something is miswired somewhere.

 

[rozenfeld57mh]

Are you saying your autopilot doesn't even track VOR signals via the CDI? If so, whatever problem is preventing that is also preventing it from tracking GPS courses via the CDI. When you're in NAV mode, the only thing it's looking at is needle position, not the external signal from the VOR or GPS, so the problem would lie either in the CDI, wiring from there to the autopilot, or the autopilot itself. There is no reason I can think of for the autopilot to follow the needle when the needle is driven by the VOR but not when the needle is driven by the GPS unless something is miswired somewhere.

Yes, this is exactly what I am saying. All it does is track heading bug. Maybe the NAV part of autopilot is not functioning somehow but there are not many shops that can check to see if it is functioning correctly.

 

[poadeleted20]

Yes, this is exactly what I am saying. All it does is track heading bug. Maybe the NAV part of autopilot is not functioning somehow but there are not many shops that can check to see if it is functioning correctly.

Nevertheless, the problem is not specific to the GPS, but rather to the nav tracking mode of the autopilot. Getting that fixed would probably be a lot cheaper than installing a roll steering module, although you'd probably really like a GPSS module regardless. If so, fixing the nav tracker would probably be in the noise level while they're installing the GPSS module. The only other alternatives are living with nothing but heading mode, or dumping the Nav-O-Matic completely and installing a new autopilot like an S-Tec 20.

 

[Clark1961]

Yes, this is exactly what I am saying. All it does is track heading bug. Maybe the NAV part of autopilot is not functioning somehow but there are not many shops that can check to see if it is functioning correctly.

Call Bob Ferguson at Autopilots Central. He'll probably be able to tell you what is broke, how to get it fixed, and how much it'll cost to fix it at their shop. When they fix it, likely it'll stay fixed.

 

[rozenfeld57mh]

I did some research today and here is what I found out:

1. Navomatic 300A needs to use heading bug even when in NAV mode. Apparently there is wind correction logic in its analog brain which relies on it. Since I wasn't using it right, I will have to test fly it again and see if it works.

2. GPSS module that is STCed with Navomatic 300A is Ikarus GPSS roll steering assist module (SAM) http://www.icarusavionics.com/. It will not work with the Navomatic 200A. Another device that will work is GDC31 Roll Steering Converter. Contrary to what I thought it is relatively easy to get a field approval for its installation. Larger Avionics shops will install it for you and have field approval before you bring your plane in for installation.

The options above are between $2k - $3k and will provide true GPS flight plan tracking by your old autopilot. The next least expensive option takes you into $12k range installed: Aspen EFD100 PFD.

I am posting all this here in case anyone else runs into the same issue. Navomatics were pretty popular in 70s and there are still places that repair and sell their head end units.

 

[poadeleted20]

1. Navomatic 300A needs to use heading bug even when in NAV mode. Apparently there is wind correction logic in its analog brain which relies on it. Since I wasn't using it right, I will have to test fly it again and see if it works.

That's the same as the Century II/III (and Piper equivalents) -- you have to set the heading bug to the approximate heading that will track the course (not the actual course, or you'll end up paralleling to the downwind side n a big crosswind). That's covered in the AFM Supplement for the Nav-O-Matics (and the Century units, too).

 

[wabower]

+1. If it's fixable, Bob can fix it.

Call Bob Ferguson at Autopilots Central. He'll probably be able to tell you what is broke, how to get it fixed, and how much it'll cost to fix it at their shop. When they fix it, likely it'll stay fixed.

 

[John Jenks]

Good advice can be found from a White paper from Avionics West of Tennessee