Brittain Accutrak II with GPS175??

[Twing207]

I have a Brittain accutrak in my M20C which drives the PC. Currently it couples to the VOR or a garmin 155. I’d like to upgrade the garmin to a GPS 175 and I’m wondering if the Accutrak will work with that. Anyone have any experience with this? I don’t want to install a new autopilot yet.

[Hank]

I don't see why it won't work. My AccuTrak II works with my G430W.

The only new autopilot that I'm aware of for our C models is either the $Garmin or at the other end of the spectrum the Aerocruz 100 that @cliffy installed. 

I'd be interested to hear if there are any other options for our Vintage birds.

[Vance Harral]

Nav tracking in Brittain autopilots relies on a +/- 5V DC differential input that signals course deviation.  The installation manual specifies this input should be driven by the output of an "omni indicator".  In the era when Brittain was still supporting products, that phrase was generally understood to be a CDI or HSI that produced an analog output signal indicating deviation.  For example, in the King KI-208/209 installation manual it's the "lateral deviation" signal output by the unit.

Modern (and not-so-modern) GPS navigators typically implement a +/- 5V DC analog lateral course deviation output (note: you may have to enable this output in the configuration settings).  The Garmin 175 has such an output.  It is designed to drive older HSI/CDI instruments that rely on analog inputs, not autopilots.  But it coincidentally happens to be the same signal the Brittain autopilot needs for nav tracking.  The electrical engineer in me thinks one should probably install some sort of isolator/repeater if you connect it this way, since Garmin never tested it with the load presented by a Brittain autopilot.  But in practice, it's probably fine.

So... If your Garmin 155 is presently driving an "omni indicator" (CDI or HSI), which you plan to retain in your Garmin 175 installation, and which implements analog deviation outputs that are connected to your autopilot, then you wouldn't have to change any of the connections to the autopilot.  You'd only have to interface the 175 to your existing CDI/HSI, and there is essentially no question about the legality of such an installation.

If your Garmin 155 has its analog outputs wired directly to the autopilot rather than through a CDI, you'll be able to do the same with the 175, and its even easier than connecting via a CDI/HSI.  But whether this new installation (and your existing installation) are "legal" depends on whether the person doing the work interprets the connectivity diagrams in the Brittain and Garmin manuals as restrictive, or merely representational.  There has been some arguing about this in the past on Mooneyspace and elsewhere, but all of it (including posts by me) is bloviation.  The only person whose opinion matters in practice, is your installer's  I bring it up only because you might want to ask your installer about this as part of the investigation process, before you put down a deposit.  You don't want to wind up with your airplane in their shop, torn up and halfway through the work, and only then be told, "Uh, we can't legally hook up to your autopilot".  Sadly, you can't necessarily rely on the shop to study this kind of issue up front.  A lot of avionics shops don't really study the work of a particular installation until they actually have the airplane apart.

[DCarlton]

20 hours ago, Vance Harral said:

Nav tracking in Brittain autopilots relies on a +/- 5V DC differential input that signals course deviation.  The installation manual specifies this input should be driven by the output of an "omni indicator".  In the era when Brittain was still supporting products, that phrase was generally understood to be a CDI or HSI that produced an analog output signal indicating deviation.  For example, in the King KI-208/209 installation manual it's the "lateral deviation" signal output by the unit.

Modern (and not-so-modern) GPS navigators typically implement a +/- 5V DC analog lateral course deviation output (note: you may have to enable this output in the configuration settings).  The Garmin 175 has such an output.  It is designed to drive older HSI/CDI instruments that rely on analog inputs, not autopilots.  But it coincidentally happens to be the same signal the Brittain autopilot needs for nav tracking.  The electrical engineer in me thinks one should probably install some sort of isolator/repeater if you connect it this way, since Garmin never tested it with the load presented by a Brittain autopilot.  But in practice, it's probably fine.

So... If your Garmin 155 is presently driving an "omni indicator" (CDI or HSI), which you plan to retain in your Garmin 175 installation, and which implements analog deviation outputs that are connected to your autopilot, then you wouldn't have to change any of the connections to the autopilot.  You'd only have to interface the 175 to your existing CDI/HSI, and there is essentially no question about the legality of such an installation.

If your Garmin 155 has its analog outputs wired directly to the autopilot rather than through a CDI, you'll be able to do the same with the 175, and its even easier than connecting via a CDI/HSI.  But whether this new installation (and your existing installation) are "legal" depends on whether the person doing the work interprets the connectivity diagrams in the Brittain and Garmin manuals as restrictive, or merely representational.  There has been some arguing about this in the past on Mooneyspace and elsewhere, but all of it (including posts by me) is bloviation.  The only person whose opinion matters in practice, is your installer's  I bring it up only because you might want to ask your installer about this as part of the investigation process, before you put down a deposit.  You don't want to wind up with your airplane in their shop, torn up and halfway through the work, and only then be told, "Uh, we can't legally hook up to your autopilot".  Sadly, you can't necessarily rely on the shop to study this kind of issue up front.  A lot of avionics shops don't really study the work of a particular installation until they actually have the airplane apart.

Would the output of a DG w/heading bug also be considered a "omni indicator".  I believe it's also +/- 5-6 volts.  Confirming the DG driven version and the CDI/HSI driver version would function off of similar signals? 

[Vance Harral]

6 hours ago, DCarlton said:

Would the output of a DG w/heading bug also be considered a "omni indicator".

No, an "omni indicator" doesn't have anything to do with heading in this context.  You're asking about autopilot heading functionality, which is independent of autopilot nav tracking functionality.

A Brittain "AccuTrak" unit is not capable of following a heading from a DG bug or any other heading source, it only follows courses.  However, a Brittain "AccuFlite" unit is capable of following a heading, and some airplanes have both the AccuTrack and the AccuFlite, with the latter interfaced to a DG or HSI with heading bug.  There are other Brittain autopilots where a single unit performs both heading and nav tracking (e.g. the B-5 in our airplane), but the AccuTrack/AccuFlite are separate units.

I don't know much about the AccuFlite unit.  In particular, I don't know what type of signal the AccuFlite wants to see from the DG/HSI.  I vaguely recall there are a couple of different "vintage" signal protcols for heading - one DC-based and one AC-based.

6 hours ago, DCarlton said:

Confirming the DG driven version and the CDI/HSI driver version would function off of similar signals? 

Maybe, but I wouldn't count on it without further research.  Again, I think some heading interfaces use AC signaling.

 

[Mellow_Mooney]

Hey how did this turn out? Considering doing the same in my 67F and have the gps 175 unit and a GI275
 

While we’re here… anybody have an accuflight or altitude hold unit/components? 
 

Happy Holidays! 

[jeremyc209]

I just had a GNX375 installed and connected to the Accutrak. I'm in the California fog we've had for weeks and haven't had a chance to test it longer than just flipping on track for a couple minutes and it seems to work.

I suspect I need to adjust the Accutrak using the factory installation/maintenance instructions once the weather improves. The gain seems too high, and I doubt my Accutrak had been used in a long time as the previous old VFR GPS was removed many years ago and the PC was inop when I bought the plane. So it's probably a good idea to give it a reset.

Once that is working I will have the Accuflite installed. I'm considering a GDC31 roll steering converter for that as it seems like all the paperwork would be straightforward and they are readily available on eBay.

 

[cliffy]

12 hours ago, jeremyc209 said:

I just had a GNX375 installed and connected to the Accutrak. I'm in the California fog we've had for weeks and haven't had a chance to test it longer than just flipping on track for a couple minutes and it seems to work.

I suspect I need to adjust the Accutrak using the factory installation/maintenance instructions once the weather improves. The gain seems too high, and I doubt my Accutrak had been used in a long time as the previous old VFR GPS was removed many years ago and the PC was inop when I bought the plane. So it's probably a good idea to give it a reset.

Once that is working I will have the Accuflite installed. I'm considering a GDC31 roll steering converter for that as it seems like all the paperwork would be straightforward and they are readily available on eBay.

 

That's the route I was on (Accutrak-Accuflite) for my C until I looked at the economics of the Aercruze 100.

I'm now very glad I went that route. It will GPSS  off a Garmin GPS navigator and no more huff and puff bellows to worry about.

[McMooney]

yes, it'll work as long as they connect it to the same cdi or to a gpss converter, mine followed the pink line like it was on rails before i removed it.

[jeremyc209]

9 hours ago, cliffy said:

That's the route I was on (Accutrak-Accuflite) for my C until I looked at the economics of the Aercruze 100.

I'm now very glad I went that route. It will GPSS  off a Garmin GPS navigator and no more huff and puff bellows to worry about.

I'll eventually upgrade but so far my all in cost on the Accutrak (came with the plane inop) + Accuflite is about $600. That includes repairing the PC system. 

 

4 hours ago, McMooney said:

yes, it'll work as long as they connect it to the same cdi or to a gpss converter, mine followed the pink line like it was on rails before i removed it.

@McMooney I know setting the GPS to .3nm scaling is needed, did you do any other work to your Accutrak? Did you have the GPSS converter or just GPS/CDI and the Accutrak?

[McMooney]

2 hours ago, jeremyc209 said:

I'll eventually upgrade but so far my all in cost on the Accutrak (came with the plane inop) + Accuflite is about $600. That includes repairing the PC system. 

 

@McMooney I know setting the GPS to .3nm scaling is needed, did you do any other work to your Accutrak? Did you have the GPSS converter or just GPS/CDI and the Accutrak?

I had both, i didn't change anything, it was already connected to the cdi/converter when i got it.  

okay full confession, the gpss converter was flakey, i belive something was wrong with it but when nit worked it was great.  the cdi deviations are what i used most, it was wonderful.

i'm sad to replace it, PC being always on made flying easy.  system even worked without vacuum/engine power, long as the engine was windmilling, all good.

I'm not sure why whoever owns brittain canned the system,  the thing is super reliable, inexpensive and could've been made to do everything the garmin/aerocruze/stc does now.    Hmm, now that i think about it, the inexpensive part is probably what got the system canned

 

 

Edited December 26, 2025 by McMooney

[Mellow_Mooney]

Well now everyone has me thinking about GPSS converters! Anybody have a good one working in their bird that they'd recommend? Do I really need one? Is the GDC31 a good route to take or is there a better one?

 

Also - I have a GPS 175 and GI275 in my 67 M20F. Does anyone know if these eliminate the need for a GPSS converter? (sorry for the novice questions - I don't know enough about GPSS converters or my system integration with autopilot yet). 

 

I was able to track town an accutrak and accuflight. Will be installing them in the coming months hopefully. Crossing fingers to find altitude hold soon but that seems like it'll be a labor of time and love. 

[Vance Harral]

On 12/26/2025 at 10:07 AM, McMooney said:

Hmm, now that i think about it, the inexpensive part is probably what got the system canned

No need for conspiracy theories, no one "canned" any Brittain autopilot.  The principal driver behind Brittain autopilots in this century, Jerry Walters, passed away in 2018 (I believe somewhat unexpectedly).  He was actively working on drawings to legally approve connecting Brittain components the the Garmin GAD29 at that time, but these did not reach completion status with the FAA, and that effectively ended further development of the product line.  His sister, Cecilia, made a good-faith attempt to sustain and sell the business, and in fact made some posts here as @CSmith.  But as I understand it, no buyer with sufficient technical and monetary resources came forth, and Cecilia was unable to keep the business operating in a meaningful way.  The Brittain web site is still up, and Cecilia may even still answer the phone.  But no parts or service are available, at least that's my understanding as of today.  @Kevin Westbrook probably knows the whole story, but not sure if he's interested in detailing it here.

[Vance Harral]

55 minutes ago, Mellow_Mooney said:

I have a GPS 175 and GI275 in my 67 M20F. Does anyone know if these eliminate the need for a GPSS converter?

The GI-275 has an output pair that can produce the AC heading error signals the Accutrack expects, so it's likely you can connect them without a GPSS converter, either directly or via an audio transformer.  But there is no drawing from either Garmin or Brittain which documents this, and thus no particular guarantee from professionals that it will work.  I suppose someone who has done so might chime in with empirical data from their own installation.

As I mentioned upthread, it's important to have an up-front conversation with your avionics installer about what they require in the way of documentation to connect modern avionics to an orphaned autopilot.  Not every shop is willing to install and sign off on something without manufacturer drawings, just because it seems like it ought to work.  Obviously you have a lot more control over this if you have the training and credentials to act as your own installer. 

[McMooney]

so t

6 hours ago, Vance Harral said:

The GI-275 has an output pair that can produce the AC heading error signals the Accutrack expects, so it's likely you can connect them without a GPSS converter, either directly or via an audio transformer.  But there is no drawing from either Garmin or Brittain which documents this, and thus no particular guarantee from professionals that it will work.  I suppose someone who has done so might chime in with empirical data from their own installation.

As I mentioned upthread, it's important to have an up-front conversation with your avionics installer about what they require in the way of documentation to connect modern avionics to an orphaned autopilot.  Not every shop is willing to install and sign off on something without manufacturer drawings, just because it seems like it ought to work.  Obviously you have a lot more control over this if you have the training and credentials to act as your own installer. 

I had a switch on the panel orginally it was nav1 / nav 2,  the avionics guys would just connect the new gps units to the switch NOT the brittain.  somehow the "Switch" was okay

[MB65E]

It Shouldn’t need GPSS steering as the G175 should already have that in the software?? 
Some G430’s originally had the capability to steer. Steering to me by a Garmin unit is the capability for the unit to draw the curved magenta line to the course line. Other units draw a direct line. I’m sure the 175 draws the curved line. A log entry might look like  
“Installed Accutrak IAW factory drawings.”
(Potentially it was already installed) “Connected analog lateral signal from existing Nav head.”

Turning the PC/AP on and off doesn’t change. 

The “Auto Pilot” ie the PC system was factory installed. The nav head and CDI were previously installed, even by a Garmin STC. Enabled the analog signal and connecting it to an input I interpret as a minor modification. 
No mechanical changes to the existing auto pilot were made. 

-Matt

[Mellow_Mooney]

This makes sense, thanks all. Looks like I will be avoiding a gpss converter and saving some money. 
 

Appreciate the help/info!

[jeremyc209]

Accutrak + GPS alone can't do GPSS from everything I've read and it doesn't work that way on my GNX375. It can track after getting on course then turning on Accutrak, but can't intercept etc.

It's my understanding you need the Accuflite + roll steering converter + GPS. Brittain has the aforementioned preliminary drawings for connecting the G5 + GAD29 to the Accuflite for GPSS as well.

[Vance Harral]

19 minutes ago, jeremyc209 said:

It's my understanding you need the Accuflite + roll steering converter + GPS

Yes, but if you have a GI-275 (or G5 and GAD29), that effectively provides an integrated roll steering converter.  That's the question @Mellow_Mooney was asking.

Again, I think it's "likely" a GI-275 -> Brittain connection will work, though it's not guaranteed.  In a production environment, nerds like me would be responsible for checking impedence matching and other signal integrity subtleties of the connection.  As a reminder, those preliminary drawings from Jerry which proposed connecting a GAD29 to Brittain hardware did so via a 1:1 audio transformer rather than direct wiring.  My guess is he did this because he wasn't 100% sure the load presented by the Brittain hardware was load and signal-integrity compatible with the output of the GAD29, even though it's likely to be fine.

[jeremyc209]

Here are the drawings for reference. GAD29 with and without transformers, and the GDC31

B-12 to Garmin G5 Rev A.pdf B-12 to GDC31 Roll Steering Converter [GPSS].pdf B-12 to Garmin G5 No Transformers.pdf

[McMooney]

12 hours ago, Vance Harral said:

Yes, but if you have a GI-275 (or G5 and GAD29), that effectively provides an integrated roll steering converter.  That's the question @Mellow_Mooney was asking.

Again, I think it's "likely" a GI-275 -> Brittain connection will work, though it's not guaranteed.  In a production environment, nerds like me would be responsible for checking impedence matching and other signal integrity subtleties of the connection.  As a reminder, those preliminary drawings from Jerry which proposed connecting a GAD29 to Brittain hardware did so via a 1:1 audio transformer rather than direct wiring.  My guess is he did this because he wasn't 100% sure the load presented by the Brittain hardware was load and signal-integrity compatible with the output of the GAD29, even though it's likely to be fine.

it will most likely work, the problem is getting an installer to actually do it.  basically, if it isn't on a drawing, avionics guys won't touch it