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Garmin GTN 650 / Garmin 335 Transponder combination

Our C182 has a GTN 650 fitted, and we need a new transponder. A Garmin 335 appears to be an obvious choice, not the least as it fits in the panel slot of an antique TDR-950.

However, I can’t pin our supplier down on whether the altitude encoder requirement could be met from the 650 feed (which provides WAAS GPS).

I’m waiting for Garmin support to reply, but need a rather urgent answer. Is anyone clued-up on this set-up?

Swanborough Farm (UK), Shoreham EGKA, Soysambu (Kenya), Kenya

Mode C requires barometry, doesn’t it?

If you used GPS altitude, how would it be corrected to 1013.25?

EGKB Biggin Hill

It could be a different angle.

I don’t know whether the GTN has a baro input but the older IFR GPSs did (for RAIM checking mainly) and if the GPS re-outputs that pressure altitude on ARINC429, that could be fed to the GTX335, possibly…

For sure an altitude encoder will be needed somewhere… It can also be in the form of an encoding altimeter e.g. a KEA130A.

Administrator
Shoreham EGKA, United Kingdom

Mode C might well do, Timothy.

But the 335 is Mode S, and if it gets its WAAS GPS feed from the 650, I can’t see it needs an altitude encoder in addition.

Or have I got that wrong, and an altitude encoder has a barometric feed?

Last Edited by 2greens1red at 21 Mar 15:05
Swanborough Farm (UK), Shoreham EGKA, Soysambu (Kenya), Kenya

You absolutely need the static vent connection, to an altitude encoder.

The output of that encoder can be ARINC429 digital (2 wires plus ground), or Gray code binary (10 or 11 wires, plus ground).

Picture of an encoder (an old style).

Administrator
Shoreham EGKA, United Kingdom

The GAE12 altitude encoder for the GTX 335 is minuscule (it fits on the back of the transponder’s tray and takes the place of the configuration module) and compared to the cost of the rest of your transponder upgrade, costs peanuts – and you’re going to need a barometric source, GPS alone won’t do (you’ll have to get barometric information from somewhere).

The GTX3x5 installation manual can be found here – http://joesrv12.com/Garmin/GTX%20345%20TSO%20190-01499-02_4.pdf – this’ll tell you what it will support.

local copy

Andreas IOM

Many thanks for the feedback – I’ll immerse myself in the manual.

I had another question, related to the same kit.

It’s possible to use the 650’s display to select a remotely installed 335. Is the only reason for that to save panel space?

I can see several disadvantages with having transponder display and approach info (for example) displayed on the same screen.

Swanborough Farm (UK), Shoreham EGKA, Soysambu (Kenya), Kenya

Peter wrote:

I don’t know whether the GTN has a baro input but the older IFR GPSs did (for RAIM checking mainly) and if the GPS re-outputs that pressure altitude on ARINC429, that could be fed to the GTX335, possibly…

The GTN doesn’t have a baro input, nor a parallell encoder input. When we installed a GTN650, we connected RS232 wires both directions between the transponder (a Trig TT31) and the GTN. The encoder was connected to the parallell encoder input on the transponder and the wire to the GTN forwarded pressure altitude. The wire from the GTN sent GPS data for ADS-B out.

ESKC (Uppsala/Sundbro), Sweden

When you use a GAE12 you can throw away the exsisting blind encoder
GAE12 sends pressure altitude to the transponder (NOT baro alt)
And yes a GTX335R can remotely controlled from 650
A few customers of my do have it, and it seems working alright

Last Edited by Maurice at 21 Mar 21:33
IAS Hangar One
EHTE, Netherlands

There might be a certification issue, BTW. For example the KFC225 autopilot is certified to accept altitude data from a KEA130A and probably little else. So if you put in a GAE12, you end up with two sources of pressure altitude in your aircraft, which could give interesting results, which could be quite irritating too… since they won’t exactly match.

Administrator
Shoreham EGKA, United Kingdom
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