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  #11  
Old 05-24-2016, 07:40 PM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by Walt View Post
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?
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  #12  
Old 05-24-2016, 08:03 PM
Mich48041 Mich48041 is offline
 
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I believe that the altimeter setting is unrelated to the encoder output to the transponder. Correct me if wrong.
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  #13  
Old 05-24-2016, 08:32 PM
Mich48041 Mich48041 is offline
 
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AVweb
Quote:
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
Quote:
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.
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  #14  
Old 05-24-2016, 08:37 PM
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Walt Walt is offline
 
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Quote:
Originally Posted by Piper J3 View Post
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

Quote:
Originally Posted by Mich48041 View Post
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.
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  #15  
Old 05-25-2016, 02:34 AM
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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.
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Jim Stricker
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 622

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
  #16  
Old 05-25-2016, 06:17 AM
Mich48041 Mich48041 is offline
 
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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.
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  #17  
Old 05-25-2016, 06:27 AM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by Mich48041 View Post
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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Jim Stricker
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 622

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
  #18  
Old 05-25-2016, 07:31 AM
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rolivi rolivi is offline
 
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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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Last edited by rolivi : 05-25-2016 at 07:34 AM.
  #19  
Old 05-25-2016, 08:06 AM
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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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  #20  
Old 05-25-2016, 10:59 AM
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rolivi rolivi is offline
 
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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.
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