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  #1  
Old 05-21-2022, 11:55 AM
Pfatt2 Pfatt2 is offline
 
Join Date: Jan 2022
Location: fly-in at Spruce Creek (7FL6)
Posts: 22
Default Transponder Issues

I have a 6A with the EchoUAT connected to my Garmin GTX327. Also Dynon D100 with IFD440.
ATC continues to show me at the wrong altitude. An FAA check shows my ADSB reporting is correct. When I recycle the transx, it usually comes back and shows the correct altitude.
I know these ADSB products work in very mysterious ways... How can I tell if it is my transponder (is the 327 an encoding transponder or would I have a separate encoder?) or if it is the Echo?
Does anyone have a similar setup who might have some insight?
Thank you.
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  #2  
Old 05-21-2022, 12:35 PM
pa38112 pa38112 is offline
 
Join Date: Mar 2008
Location: Clarksboro, NJ
Posts: 1,043
Default

How far off is ATC seeing you. Are you sure that it is not your altimeter that is incorrect? My understanding is that if you are getting a good ADS-b performance report that your Transponder is reporting you in the same position that ATC's radar is showing you - I could be way off on that, but that is how I currently understand it.
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  #3  
Old 05-21-2022, 12:53 PM
Avanza Avanza is offline
 
Join Date: Jul 2020
Location: Vastervik Sweden
Posts: 139
Smile ADSB Altitude

As it is ADSB, the position (GPS) most likely comes from the IFD440.
There will be a difference between GPS altitude and the not so accurate pressure altimeter.

Good luck
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  #4  
Old 05-21-2022, 01:14 PM
Walt's Avatar
Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,674
Default

Quote:
Originally Posted by Pfatt2 View Post
I have a 6A with the EchoUAT connected to my Garmin GTX327. Also Dynon D100 with IFD440.
ATC continues to show me at the wrong altitude. An FAA check shows my ADSB reporting is correct. When I recycle the transx, it usually comes back and shows the correct altitude.
I know these ADSB products work in very mysterious ways... How can I tell if it is my transponder (is the 327 an encoding transponder or would I have a separate encoder?) or if it is the Echo?
Does anyone have a similar setup who might have some insight?
Thank you.
The 327 has to get it's mode C altitude data from somewhere, which would be either the Dynon or a separate encoder (the echo has no encoder).
My guess would be a separate encoder is getting flakey or needs adjustment. Part of a 91.411 cert is to verify accuracy of the encoder as it relates to the barometric altimeter.
I've tested a zillion 327's and don't recall ever seeing one report incorrect altitude unless it was receiving bad data or had a external wiring fault.
(Older 327's do suffer from frequency drift though and need that adjusted occasionally)
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Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags, MTV-9 prop
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154

Last edited by Walt : 05-21-2022 at 01:25 PM.
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  #5  
Old 05-21-2022, 01:17 PM
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Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,674
Default

Quote:
Originally Posted by Avanza View Post
As it is ADSB, the position (GPS) most likely comes from the IFD440.
There will be a difference between GPS altitude and the not so accurate pressure altimeter.

Good luck
All aircraft altitudes are based on barometric which will be different than GPS altitudes, with the correct barometric setting altimeters are quite accurate.
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Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags, MTV-9 prop
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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  #6  
Old 05-21-2022, 01:41 PM
BH1166 BH1166 is offline
 
Join Date: Nov 2017
Location: Eatonton Georgia
Posts: 708
Default

Remember the altitude you see is adjusted to local barometer pressure, what ATC sees and reports back to you is pressure altitude, same as if you rolled the baro input to 29.92. Next time ATC says they see you at x altitude, change your baro setting to 29.92, at that point you can know for sure if something is askew.

Your Dynon is likely your encoder, feeding altitude data via rs232 or gray code to the GTX327. Your position data is coming from the ifd440. You could have an separate encoder as well. The GTX config page shows how your unit is receiving altitude data. Download the manual and determine how to view, and how it is receiving altitude info, rs232 #1 or rs242 #2 or gray code.

https://chapters.eaa.org/eaa245/-/me...90A075FC0.ashx


I have GTX327 and EchoUAT in my 6a.
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  #7  
Old 05-21-2022, 01:56 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 8,242
Default So much misinformation!

1. Your altimeter displays indicated altitude. But your encoder sends pressure altitude to your transponder and adsb-out.
2. ATC computers know local altimeter settings, and re-convert pressure altitude to indicated altitude. What ATC tells you should match your altimeter.
3. Exception to rule 2: I’ve seen a few times when over the central valley I changed ‘early’ to LVK’s altimeter setting, but ATC’s computer still had me on another altimeter setting, and the controller noted the difference.
4. The rules for adsb require you to use the same encoder for both your transponder and adsb-out, so this split should not happen.
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