Help with intermittent altitude info Garmin GTX 320A Uavionics Echo UAT with SKYFX, Dynon D-180 EFIS

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KeithO

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So, yesterday afternoon was the final flight home of the Arion Lightning I purchased. Obviously the first long cross country under my ownership. I hired a local CFI to do the flight since my wife is still a student pilot. Overall everything went well on the flight. Performance was as expected, there were pretty severe headwinds on the way back to Michigan given that the winds had reversed from southerlies to northerlies bringing with them some cool air from the polar regions.

When the flight approached Toledo Class C airspace, the controller reported that the altimeter data on ADSB was very intermittant and to switch to mode A instead of mode C. On departure from Parkersburg WV and on previous test flights at that location ATC had never made any comment regarding problems with the reported altitude. Since I am not personally familiar with any of the avionics or ADSB hardware, I have been studying the manuals of the now approximately 15 year old transponder and the much newer ADSB system.

It seems to me that the Garmin transponder, while a mode C device, does not have a built in altitude encoder, it relies on an external encoding altimeter. It appears to me that the Dynon is the only instrument (other than a backup ASI) to be connected to the static port system. The pilot did not report any anomalies with the altimeter on the Dynon, so that suggests there must be a problem with the serial communication from the Dynon to the transponder, or elsewhere in the chain to the Echo so that the ADS-B transmitter is not getting good altimeter data.

I went to the FAA website and pulled a report on the ADS-B data for the flight and it reported a 70% error rate on the altimeter data. Some of the 70% could have been from the portion of the flight from Toledo to Jackson after the controller had him turn mode C off, but still there is obviously a problem. Has anyone had something similar happen ? I have a scope so I can look what the signals look like at the output of the Dynon and at the input on the Transponder. Obviously any loose connections would be the first catch - if any. I have to wonder if there is a way to read the Dynon output into a terminal on a laptop and see if it is broadcasting garbage ???
Thanks for the input...
 

TFF

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Class D airport don’t care because they are not radar service, they don’t return information about ADSB. Essentially not their job so they don’t look. Class C screams ADSB. It’s expected to work. On another forum someone had issues with their Uavionics and in the end it was wiring. I believe it had to connect to the transponder power wires because they communicate through them. If wired separately, they have a hard time communicating.
 

Daleandee

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The Garmin GTX 320A transponder does not have a built in encoder and an encoder of some sort is required. Perhaps the Dynon is providing that information if it has the optional encoder (if I understand the system you are using correctly).

I'm using the same transponder & EchoUAT setup that you have with the exception of having a Trans-Cal altitude encoder installed that gives the pressure altitude information to the transponder. The Echo unit reads the transponder output. As TFF noted make certain that the transponder and EchoUAT units are wired so that they have a common path as that is how the signal is read by the Echo unit.

Seems you are the new owner of this plane. There are settings that can be adjusted on the Echo unit using a smart phone. I would recommend calling uAvionix before getting in too deep on that. Here is a link for changing the transponder threshold settings:


I'm certain you know this but you can get a free Public ADS-B Performance Report from the FAA by email. It don't take long to do this and the report has some great information as to which parameters (if any) are at fault. I do a couple of reports a year and one for my yearly inspection records.


FWIW, my Garmin & Echo setup has been flawless from the start with no changes needed after initial install.
 
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KeithO

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Flight Track Log ✈ N213RC 17-Jun-2022 (KPKB - FlightAware

Clearly altitude info is missing for a large % of the flight. It seems that one of the easiest tests is to have a portable adsb receiver and monitor the output of the system. I will have to evaluate the wiring of the Echo vs 320A to see how the power has been connected in case that is the weak link as has been suggested. I may also need to clean up the DC bus which could potentially be noisy, some have connected a 25 000 ufarad capacitor from B+ to ground to absorb transients.
 

rv7charlie

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I just did the calibration on my Echo to fix that problem. It's unlikely that the problem is the encoder in the Dynon; it's probably the sensitivity setting in the Echo. Get the Echo doc in hand, load their app on your phone, and then call uavionix. They can walk you through the adjustment.
Edit, just realized Daleandee told you the same thing.
 

FinnFlyer

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... I may also need to clean up the DC bus which could potentially be noisy, some have connected a 25 000 ufarad capacitor from B+ to ground to absorb transients.
I think the Echo sniffs the altitude info from the transponder from the power line. So "cleaning it up" may be counter-productive,

Finn
 

rv7charlie

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Finn has it right. The uAvionix tech compared the adjustment to adjusting the squelch on your comm radio or intercom. too sensitive & the Echo gets swamped with noise; not sensitive enough, and it can't hear anything.

BTW, they cost me a lot of work, before giving me the correct answer. I've got a Garmin 327 that had an older grey code encoder attached (around 10% Balt failures), and they immediately blamed the encoder. So I pulled the encoder out, and added a serial input to the 327, reprogrammed it for serial data, and AFS 4500 EFIS serial encoder data. Then I got 100% Balt failures. Called them back, and they said, 'Oh, you need to adjust the sensitivity setting.' Duh.
 

KeithO

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Jackson, MI
The Jabiru voltage regulator is a known source of RFI. Thats where the cap would be. Ideally the Echo would have its power supply connected in parallel with the transponder which I need to check.

I think the Echo sniffs the altitude info from the transponder from the power line. So "cleaning it up" may be counter-productive,

Finn
 
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