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.