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Garmin GTX 327 not displaying Flight Level

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Piper J3

Well Known Member
I bought my RV-12 flying from the original builder and now discover that the GTX 327 transponder is not displaying Flight Level in VFR ALT mode. I assume the altitude information is derived from the Dynon D-180 and maybe the two units are just not ?talking? to one another. Hopeful thinking on my part. Does anyone know how I can check if either or both units are set up correctly?
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No knowledge of a D-180, but the 327 has two com ports. You might need to pull it to see which port is wired up and then check the configuration.
 
Suggest you ask on the Dynon support forum. They're very responsive.

You are correct that the D180 can be configured as the altitude encoder for the GTX327 so it should go quick. (I have D100 and GTX327)
 
Thanks guys. I have pulled both install manuals and will investigate further tomorrow. Hopefully just a handshaking problem which can be fixed in setup parameters.
 
Has the plane ever had a transponder check it passed (check those logbooks)? If so, it used to work. If not, well, you should theoretically turn off the transponder and go get a check done once you figure it out on the ground.

The D180 should be set to 4 and the 327 to Icarus.
 
Encoder?

I had this problem on my 327. Is ATC getting an altitude readout? They were not getting an altitude readout on mine and it was the encoder. I don't know if the Dynon is providing your encoder data or if you have an independent encoder like I have. I'm assuming your encoder is putting out squak numbers.
 
ATC won't get a Mode C altitude reply if the altitude is not shown in the 327.
You can also change the FL readout to feet, makes more sense as most RV's don't do flight levels.
 
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I changed D-180 output to #4 and now I get Pressure Altitude displayed on the 327. I have the D-180 set to send altitude as 10's instead of 100's. I didn't do any setup on the 327. I'm not sure if Pressure Altitude is displaying FL or feet. Next is to break open the Garmin 327 install manual and do some studying.

Another question - if I set D-180 to 29.92" what should the 327 read for altitude?

Anyway, thanks for all the help here on VAF. I'm really glad the two pieces of equipment are communicating. Which the wife and I could settle our differences this easy...
 
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The Dynon and 327 will match at 29.92 ONLY.

Not following you exactly here. If I put Dynon D-180 on 29.92 in the setup menu then whatever altitude shows on the EFIS should also be displayed on the Xponder - correct?
 
I believe that the altimeter setting is unrelated to the encoder output to the transponder. Correct me if wrong.
 
AVweb
Because the transponder reports pressure altitude, the altimeter setting that you dial into your altimeter's Kollsman window has absolutely no effect on your Mode C altitude reports.
ASKaCFI
Although an encoding altimeter has both a indicator to the pilot that is adjustable to the local barometric pressure, it also has an encoder that uses 29.92 as its reference for the encoded altitude. So all encoders send exactly the same pressure altitude to the radar site. The software at the radar site corrects the pressure altitudes to an equivalent altitude.
 
Not following you exactly here. If I put Dynon D-180 on 29.92 in the setup menu then whatever altitude shows on the EFIS should also be displayed on the Xponder - correct?

Correct, your Dynon alt and the 327 will match only when the baro is set 29.92

I believe that the altimeter setting is unrelated to the encoder output to the transponder. Correct me if wrong.

Correct again, the encoder output is always ref'd to 29.92 and is unaffected by the baro set, hence the reason it only will match the alt when set to 29.92.
 
I believe that I now have the GTX 327 working correctly and receiving altitude data from the D-180 EFIS. The 327 is displaying Pressure Altitude in FL instead of feet. I have read the install and operator manuals and don?t see how to configure display for feet. Any help appreciated?

Next step is to have avionics shop certify the transponder.
 
Looking at the manual, the GTX-327 only displays pressure altitude, not actual altitude. The D-180 probably does not send the barometer setting or corrected altitude to the transponder.
 
Looking at the manual, the GTX-327 only displays pressure altitude, not actual altitude. The D-180 probably does not send the barometer setting or corrected altitude to the transponder.

I'm beginning to believe that is the case except for this pic on the cover of the 327 Pilot's Guide which shows feet and not FL...
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It always displays pressure altitude (as Walt and others have attested)

You can have the pressure altitude displayed as Flight Level or Feet. For me, displaying FL is a convenient way to remind me that it's NOT baro-corrected.

Of course, that big old PRESSURE ALTITUDE displayed above could be my reminder :)
 
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Look for "format" in the setup. To get to the setup menu, hold down the enter key and then power up the unit.


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I asked on another forum and learned:

The Transponder always transmits Pressure Alt, and the scope on the ground adjusts for the baro. So, as Walt said, 29.92 they match, other settings your transponder is showing what it's sending, not what your Altimeter says.
 
Got everything working Honky-Dory. 29.92 on the Dynon D-180 show Pressure Altitude within 10' of matching.

Now I will schedule transponder certification.

Thanks to everyone for helping educate this old bird...
 
Garmin 327

I found my old post
Well a long story short, it was a entry in the settings menu that was missing.
Now for a short story long the 327 was never the problem. A month ago my battery voltage ran down and I lost my #2 GRT and I had to copy all the info on GRT #1 to #2. Pin 19 on the 327 is fed by pin 5 of GRT 2 so in the settings menu I had to add "fuel air data z format" and add "9600" for the port rate and my alt on the 327 just popped up. THANKS FOR ALL THE HELP
 
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