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Transponder putting out fluctuating altitude

RONSIM

Well Known Member
ATC says Mode C intermittent from actual altitude (5000) to 1700. Owner (says) pressure altitude readout on G327 is steady. Flightaware track log shows the intermittent behavior. G327, encoder through AFS 5500T, uavionix echo ADS-B. First leg of trip was steady, flight home next day was squirrley.

Any ideas?

Ron
 
Unsteady altitude

As the pressure altitude readout on G327 is steady, my best guess would be
a bad crimp, loose or bent pin in a D-sub.

Good Luck
 
Was it one ATC facility? Or more than one? ATC secondary RADAR isn't exactly the same freq from one facility to the next. GTX-327 may need a tune up.
also, the coax and antenna could be suspect. If the coax is old, replace it. If it were me, I would swap out the antenna, temporarily if a spare is available
 
could be an issue with the echo's sniffer. If I remember correctly, the ADS-b signal include both the baro and GPS altitude; Controller typically only sees the baro alt. So eventhough the transponder is sending altitude data, the ground station is ignoring it and only reading the ADSb signal. Seems doubtfull that the ground station is observing both and resolving conflicts, beyond determining whether or not a specific aircaft has just transponder data or both ads-b and transponder. As long as it sees a valid ADS-b signal, it ignores the transponder data. This is a guess, not a fact.

so, the transponder sees and transmits the correct baro altitude, but the echo is having trouble reading the RF transmission (this is what a sniffer does) and gets inconsistent data and re transmits it without realizing it.

If you are using a serial mux instead of the sniffer, I would expect a failure of the mux. A loose connector should produces errors and not simply a wrong altitude. If using a sniffer, installing the mux will provide more reliable data.

I would pull up the echo config tool in flight and see if you can read the baro altitude.

Larry
 
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Good suggestions -- I will fly the airplane tomorrow (7A) to see if I can verify a few points.

Ron
 
Update

Pulled TXP, checked pins, used contact cleaner, etc. --- checked antenna connections, cable.

Flew this morning, checked with ATC, checked Flightaware readout, monitored altitude readout on iPad, and had another guy in -7A following and monitoring altitude readout. No issues ----

If you would like, the N no. is 237RV, and you can look at the Flightaware track info on this past Monday, from KSSI to KCLW and see the wild swings on the data.

Anyway, OK, for now.

Ron


Merry Christmas!
 
My airplane did the exact same thing on my last flight. Echo UAT sniffing a GTX 327. Have you seen any recurrence of this? No other anomalies noted on the flight displays or the altitude display on the transponder. The only thing I noticed was that other adsb traffic had large altitude fluctuations. I was talking to ATC inside Bravo airspace and they didn’t say anything so I’m not sure I what they were seeing, but I’m assuming it wasn’t these crazy altitude deviations.
 

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Data Source?

Flyhud - The bottom points of your graph look like legit altitudes, and the top points look suspiciously like something is getting pegged out.

What was the source of this graph data?
Was anything flagged in the PAPRR for that flight?
 
If the 327 is showing/reading the correct alt then the 327 is transmitting the correct altitude, so you can look elsewhere.
 
Flyhud - The bottom points of your graph look like legit altitudes, and the top points look suspiciously like something is getting pegged out.

What was the source of this graph data?
Was anything flagged in the PAPRR for that flight?


This graph was a flightaware graph attached is the papr report for this flight, looks like a baro alt failure.
 

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Your altitude encoder "system" is the problem. The transponder only transmits what it gets from the altitude encoder. ADSB repeats whatever the transponder sends out.

This exact problem happened to a Grumman Cheetah here. They replaced the old altitude encoder and the problem was solved.
 
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The system gets altitude data from a Garmin GSU 25, so that’s probably the best place to start.
 
Mode 3A problem too

ATC wasn't complaining = GTX327 probably ok.

I think you can scratch the FlightAware record from consideration. FlightAware showed fully half of altitude reports were way off - this probably indicates that FlightAware wasn't receiving ADSB and had resorted to Multilateration. Verify by clicking on FlightAware's Track Log for that flight and reviewing "Reporting Facility".

The PAPRR showed failure of Mode 3A (squawk errors) as well as BAlt. Multiple sniffing errors? Check the uAvionix website to see if you need to crank up Monitor Threshold a tiny bit.
 
I think you can scratch the FlightAware record from consideration. FlightAware showed fully half of altitude reports were way off - this probably indicates that FlightAware wasn't receiving ADSB and had resorted to Multilateration. Verify by clicking on FlightAware's Track Log for that flight and reviewing "Reporting Facility".

FWIW: FlightAware reports GPS altitude and/or uncompensated Barometric altitude which only matches your ATC flight altitude when 29.92 is your altimeter setting. Therefore FlightAware might show you being way off, with everything working correctly.
 
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To follow up on my adsb issue, I found the sma antenna connector that is attached to the pcb on the Echo UAT unit had a broken solder joint(s). This happened most likely as a result of overtightening this fragile connector. Fortunately I had a spare unit which I swapped out, reprogrammed and tested. Flew this morning and got a clean PAPR. Thanks to all who contributed helpful responses.
 
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