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Recycling an RBI

Hello avionic- and electronic wizzards.

We had a nice "little" Bendix T-12C ADF installed, witch decided to retire this summer and serves as part of our "private avionic museum" (i.e. as a bookend). However, the problem was the receiver not so much the RBI. Having a Garmin 100 (I know, some may see it as another bookend, but it works well and reliable) with the possibility of an NMEA output, I thought eventually one could recycle the RBI and display Off Track Error or the turn to the next waypoint (If I ever bother to program a route into the 100, but then any NMEA could be used as Input).

As I am not a stranger to learning, I'd rather like to learn about controlling the RBI from a book rather than trial and error or taking apart something I'm not sure to get back together again. Has anyone a recommendation on literature on the issue?

Since ADFs aren't en vogue anymore in VFR aviation, I figure many RBIs dusting in the corner of a bookshelf leaving an empty hole in the panel could be reanimated. Perhaps this box could turn into a piece of "open source avionics"?

Not sure, though, what EASA thinks of panel mount GPS displays, but since it would be VFR only, I wouldn't expect too much problems from this direction.

Any thoughts? Recommendations?

Thanks, Malte

PS: Sorry, somehow my keyboard swallowed an "n" in the title. Edit doesn't allow me to add it myself.

Aufwind GmbH
EKPB, Germany

Sounds interesting - certainly it should be possible to concoct something or other more or less useful - but it is unclear what could be achieved. In a certified plane, such a contraption could never be used or even installed legally, I should think? And in non-certified planes, one prefers LED-bars or even LCD screens. Less weight, better readability, look more fancy (remember the bit about making an impression on passengers).

Kindly elaborate? But I am afraid the RBI would best serve in a flight-simmers pseudo-cockpit.

EBZH Kiewit, Belgium

Do you mean connecting the RBI (which IIRC is just an instrument with 1 needle) to the GPS so it shows the bearing to the next GPS flight plan waypoint?

Or do you actually have a CDI (an instrument with a deviation bar, like for tracking VORs)? You can certainly connect a CDI to a GPS, and in fact that is normally a requirement for a BRNAV approvable IFR GPS installation.

Shoreham EGKA, United Kingdom

I do have a CDI, and I have a Crosspointer (both King) but that would require emulating a complete VOR, wouldn't it? I figured those converters weren't that easy to replicate, I was going to start with the RBI and then see where it leads me.

As I see it, many VFR planes still have an ADF installed and the minute the receiver refuses to work, the ADF gets thrown out because in times of multiple GPS usage no one needs an ADF in a VFR cockpit any more. Anyway, I like it and I'd rather have a simple RBI installed instead of a blank plate covering a hole in the panel. If you have a Garmin 100 (or similar), it indeed would give you surplus information, aswell as with the GPSMAP 195. Or if you have installed one of those first moving maps, you can actually have the map displayed and information about track / bearing.

The Idea was to just let the RBI display the angle between track and bearing. Within x NM (or Minutes) of reaching the Waypoint, a single LED-Button would go on and you would press the button to have the next waypoint referenced on the RBI, thus giving you a heads up tor a heading change when reaching the waypoint. I personally would find that useful.

I certainly wouldn't expect people to buy those RBI converters en mass, thus the open source thinking. Giving the stuff (schematics, calculations, etc) for free and having people join development, if they so choose. If they don't would be fine with me, too.

I was under the impression, those panel mount VFR GPS weren't certified either, but I have not done much searching on that matter. I thought there would always be some way. How goes that saying? If you want to archive something, you'll find ways, and if you don't, you'll find reasons :-)

best regards,

PS: Thanks for the correction of the title!

Aufwind GmbH
EKPB, Germany

The Idea was to just let the RBI display the angle between track and bearing

I think that is the same thing as pointing to the next waypoint, isn't it?

The "bearing" is your current GPS track.

This is non-trivial but entirely possible. You would need to decode the data stream coming out of the GPS, which is probably something similar to that documented for a GNS box.

Google is your friend as usual and here is the IM for the GNS430W. See page 129. I remember digging around this for the SN3500 project 2 years ago.

The GPS100 IM is here and while there is no data format spec in there, page 12 shows the RS232 output on pin 13, and looking at the devices shown it will be something pretty standard.

Here is more.

Then you have to extract the current leg track and convert it into the sin/cos signals (or something similar) for the RBI. There are all kinds of converters out there for stuff like that - rarely used nowadays - but I have never heard of one which will do just this. You might have to build something. Or maybe go RS232 to ARINC429 to sin/cos.

Regarding "paperwork" I doubt it will be legal to install anything homemade. Everything installed on an aircraft with an ICAO CofA needs to be supported by Approved Data, which can be a TSO, a PMA, or something supported by a (in the FAA world) a DER 8110-3 design package (EASA 21 over here), which is loads of €€€€€...

Shoreham EGKA, United Kingdom

Hi, thanks for the input.

The GPS-side of the device wouldn't be a great problem, since the NMEA output is well documented. I just didn't find much literature on operating an RMI, as I have just basic knowledge of electronics.

Anyhow, I guess I have to study the regulations on this first, I suppose.


Aufwind GmbH
EKPB, Germany

An RMI is a different device again.

The compass card rotates, and for that it needs an input from a slaved compass system e.g. a KCS-55, which is a KG-102A directional gyro, a KMT-112 fluxgate magnetometer, a KI-525 HSI, a KA-51B slaving accessory... The result is wonderful and I would never want to go back to the old DI which drifts all over the place and has to be periodically re-set from the liquid compass (which is usually crap anyway, even if one can read it in any turbulence) but it's not a cheap system. There is a solid state equivalent from Sandel, which is nice but not cheap either.

The most common RMI is a KI-229 which in addition to the sin/cos inputs can accept a composite signal (which is the demodulated VOR signal, and is wholly in the audio range) for the pointer rotation. That may or may not be easier to synthesise in software.

I have a KI-229 and a spare and will soon be selling both, following the install of a 2nd EHSI on the RHS.

It's a fun project but I wouldn't take it on myself

Shoreham EGKA, United Kingdom

Sorry, I wasn't too concentrated. The instrument is still an RBI, perfectly sufficient for the VFR flying I do with this plane.

I can clearly see the advantages of your Sandel installation, and If European IR gets easier, there will be much discussion about exchanging a/c for a more IR capable type. The old 35 (clean 35 or up to the G, which are MOGAS capable) spring to mind. But until then, there's much water under the bridge. The RBI "project" is just for fun, indeed.

Aufwind GmbH
EKPB, Germany


If of any interest, I have cancelled the complete recycling-old-avioncs project for good. I have (finally) a smartphone since last friday and invested less than 10 Euro for a working ADF and VOR/HSI/DME (with slant range calculation) emulation on the phone. No problems with EASA stuff, great readability and usability for my VFR-Flying (plus, in future I couldn't care less if the onboard ADF or VOR works, when I have to teach radio nav. The Apps will do quite right).

I have tried moving map software, like VFRnav, but at least with an Android Phone, the implemented features seems a little odd for me (Like: North-Up only, no CDI, no HSI, but writing down takeoff and landing times and showing traffic circuits... ).

I don't know how links to apps are handled in this forum, but since I have no connection to the developers:

Aircraft ADF

Aircraft VOR

Radio Nav Aids

Aufwind GmbH
EKPB, Germany
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