No ALT being reported. Encoder?

TrueCourse

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TrueCourse
This may simply be an altitude encoder problem and we'll get an avionics shop involved after I check a few things, but I'll throw it on here with some background just to see if there is anything I'm missing. It's a Garmin GTX 327 xpdr in a Cherokee, a Trans-Cal encoder (which was in the airplane before the Garmin was installed), skyBeacon ADS-B out. Alt/Stat and transponder passed inspections last year.

Flew through some Class C airspace requesting flight following and ATC said there was no altitude being transmitted. This happened twice that day, so its not intermittent. I pulled the FAA's PAPR and it shows most of the ADS-B was working fine, but at the top of the page under the "Missing Elements" section it shows Mode 3A with no failures and Baro Altitude column with 100% Fail. OK, that makes sense. However, near the bottom of the page under the "Other Checks" it shows Mode 3A 100% fail. Anyone know more about this difference?

I was running skyBeacon in Anonymous mode which I don't think was an issue because, by design, after ATC gave me a squawk code they started receiving the code (after being told it was briefly one digit off?). The transponder is displaying a pressure altitude. The Reply light was occasionally displaying. Anyone have an issue using anonymous mode?

So, I'm thinking a dead encoder or other issue with the encoder. But if the encoder was dead, wouldn't the transponder not display a pressure altitude? Or, can the encoder be good, while the transponder is not delivering the altitude information?

I did read the uAvionix information about Resolve Failing Mode 3A on PAPR and the Transponder Threshold Adjustment, but that appears to be related to the squawk code in general, not necessarily the Mode C readout. Next flight I'll use the Monitor function to see if it is reporting as it should. This was the first time flying this airplane through Class C so don't know how long its not been working.
 
If your GTX-327 is showing pressure altitude fine then there I'd say the encoder is fine.
 
If your GTX-327 is showing pressure altitude fine then there I'd say the encoder is fine.


I concur. I got told a couple times last year the same thing. It seemed I lost mode c reporting for a few minutes at a time. When I did the ads-b install, I found the antenna was broken and the rod literally fell it when I touched it. That might’ve been unrelated but was suspicious.
 
I wonder if it is an issue with the anonymous mode? I know, per the rules, you're not supposed to use anonymous mode when requesting ATC services, and when squawking a discrete code it's supposed to automatically disable the anonymous mode. Maybe it's a software bug in the SkyBeacon? You could try again without starting out in anonymous mode.
 
I wonder if it is an issue with the anonymous mode? I know, per the rules, you're not supposed to use anonymous mode when requesting ATC services, and when squawking a discrete code it's supposed to automatically disable the anonymous mode. Maybe it's a software bug in the SkyBeacon? You could try again without starting out in anonymous mode.

Since the anonymous has a mystical connotation I was thinking the same thing at first. I'll experiment with that. After the flight I thought maybe I should have said, "let me turn off my ADS-B to see if you get the Mode C" but not sure how that would have gone over. It's strange how they word it, "When enabled, the operator will not be eligible to receive ATC services" instead of "If you leave it on 1200 you won't be eligible...".

As a side note, the skyBeacon has the ability through an Annunciator LED on the fin to inform the pilot in flight of its operating status. But on the low wing installations you can't see that LED. So, fly to a destination that has Class B or C and you might not find out until you get all the way there that your ADS-B is not working. Ooops.
 
How old is your coax? And is it RG400?
 
Your transponder is not physically connected to skybeacon. Your transponder/encoder should be sending altitude regardless of skybeacon.

Skybeacon transmits on 978 mhz...your transponder transmits on 1090mhz.

Like a previous poster suggested, you should see the pressure altitude shown on the screen of your GTX327 if it is working properly.

Also, you mentioned the transponder antenna fell apart recently...if this is the case, you might have a damaged output module in your transponder.
 
See that the antenna is clean, not all greasy and dirty. If it's the plastic blade, look for cracks.
 
How old is your coax? And is it RG400?

Don't know yet on both counts. The GNS430W and GTX327 install is five years old, don't know yet what changes they made to wiring. I'm pretty sure the encoder was in before that install, but I'd have to check the logs again to be sure.
 
I wonder if it is an issue with the anonymous mode? I
The only thing anonymous mode does differently is transmit a "random" number in place of your coded N number in the ADS-B out message string. Won't affect what your transponder does or does not transmit.
 
Your transponder is not physically connected to skybeacon. Your transponder/encoder should be sending altitude regardless of skybeacon.

Skybeacon transmits on 978 mhz...your transponder transmits on 1090mhz.

Like a previous poster suggested, you should see the pressure altitude shown on the screen of your GTX327 if it is working properly.

Also, you mentioned the transponder antenna fell apart recently...if this is the case, you might have a damaged output module in your transponder.

Correct, the transponder isn't physically connected to the skyBeacon, but my understanding is the skyBeacon interacts with the existing transponder. So, does the skyBeacon retransmit the barometric altimeter it reads from the transponder (when its interrogated)? I know there are angular and barometric components of its altitude reporting capability, but not fully sure how this little wingtip creature works.

I didn't mention the transponder antenna fell apart. That was someone else.
 
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See that the antenna is clean, not all greasy and dirty. If it's the plastic blade, look for cracks.

For the transponder antenna, if ATC is getting the squawk code I can't see this being an issue. For the blade on the skyBeacon not sure that matters either since all the other components of ADS-B transmitting seems to be working according to the report I pulled.
 
The TransCals are antiques, but on the other hand, mine chugs right along.

Wiring is often suspect.

The question is really whether the thing is not responding to mode C interrogations or it is responding with invalid answers.
 
So, does the skyBeacon retransmit the barometric altimeter it reads from the transponder (when its interrogated)?
Yes, it gets your squawk code and altitude by looking at the electrical noise on the power circuit generated by the transponder as it transmits, then the sky bacon rebroadcasts the numbers.
 
"The TransCals are antiques, but on the other hand, mine chugs right along.

Wiring is often suspect.

The question is really whether the thing is not responding to mode C interrogations or it is responding with invalid answers."

It sure still looks shiny! I did read about Garmin not recommending the use of Narco and King adapters with the GTX-327 but I have no idea if that is part of the picture yet. Well, the only error ATC mentioned early on was that my squawk was transmitting one digit off what I set, but that only lasted a moment or two and he never said that again. And the mode C was not intermittent. Just not there at all.
 
For the transponder antenna, if ATC is getting the squawk code I can't see this being an issue. For the blade on the skyBeacon not sure that matters either since all the other components of ADS-B transmitting seems to be working according to the report I pulled.
I have had flight school airplanes lose the Mode C (altitude) but the Mode A (squawk) is fine. It was an avionics tech that told me to clean off that antenna, and I have fixed several that way.
 
OK, I’ll consider that too than. Thanks.
 
I have had flight school airplanes lose the Mode C (altitude) but the Mode A (squawk) is fine. It was an avionics tech that told me to clean off that antenna, and I have fixed several that way.
Odd. There is no difference in the reply transmission (other than the numerical value) of mode A and C. You know it's a mode C reply because you got it when you sent a mode C interrogation.

The only difference in the interrogations is a longer delay (21us vs 8us) between P1 and P3.
 
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Maybe the tech fixed it and sent student and CFI to “fix it” because he wanted antenna cleaned. :)
 
This is probably a dumb question, but have you been pushing the "ALT" button on the GTX 327 after punching in the squawk code?

Mode Selection Keys
OFF  Powers off the GTX 327. Pressing STBY, ON, or ALT key powers on the transponder displaying the last active identification code.

STBY  Selects the standby mode. When in standby mode,
the transponder will not reply to any interrogations.

ON  Selects Mode A. In this mode, the transponder replies to interrogations, as indicated by the Reply Symbol ( ). Replies do not include altitude information.

ALT  Selects Mode A and Mode C. In ALT mode, the transponder replies to identification and altitude interrogations as indicated by the Reply Symbol ( ). Replies to altitude interrogations include the standard pressure altitude received from an external altitude source, which is not adjusted for barometric pressure. The ALT mode may be selected in aircraft not equipped with an optional altitude encoder; however, the reply signal will not include altitude information.
Any time the function ON or ALT is selected the transponder becomes an active part of the Air Traffic Control Radar Beacon System (ATCRBS). The transponder also responds to interrogations from TCAS equipped aircraft.
 
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Its in ALT mode already, the whole time, so I don't need to change it to ON mode. It was cycled to STBY and back to ALT one time and that didn't help.
 
When I was looking at the GTX327 RS-232 INPUT/OUTPUT setup page I came across the following and want to verify it is correct.

RS-232 INPUT...................OUTPUT
CHNL 1: OFF....................OFF
CHNL 2: GPS

For review, the plane has a GNS430W and there is a Trans-Cal Encoder. The 327 is displaying altitude. I believe this is all good, but wanted to verify it. Haven't had the chance to fly again or do any other troubleshooting yet.
 
This may simply be an altitude encoder problem and we'll get an avionics shop involved after I check a few things, but I'll throw it on here with some background just to see if there is anything I'm missing. It's a Garmin GTX 327 xpdr in a Cherokee, a Trans-Cal encoder (which was in the airplane before the Garmin was installed), skyBeacon ADS-B out. Alt/Stat and transponder passed inspections last year.

Flew through some Class C airspace requesting flight following and ATC said there was no altitude being transmitted. This happened twice that day, so its not intermittent. I pulled the FAA's PAPR and it shows most of the ADS-B was working fine, but at the top of the page under the "Missing Elements" section it shows Mode 3A with no failures and Baro Altitude column with 100% Fail. OK, that makes sense. However, near the bottom of the page under the "Other Checks" it shows Mode 3A 100% fail. Anyone know more about this difference?

I was running skyBeacon in Anonymous mode which I don't think was an issue because, by design, after ATC gave me a squawk code they started receiving the code (after being told it was briefly one digit off?). The transponder is displaying a pressure altitude. The Reply light was occasionally displaying. Anyone have an issue using anonymous mode?

So, I'm thinking a dead encoder or other issue with the encoder. But if the encoder was dead, wouldn't the transponder not display a pressure altitude? Or, can the encoder be good, while the transponder is not delivering the altitude information?

I did read the uAvionix information about Resolve Failing Mode 3A on PAPR and the Transponder Threshold Adjustment, but that appears to be related to the squawk code in general, not necessarily the Mode C readout. Next flight I'll use the Monitor function to see if it is reporting as it should. This was the first time flying this airplane through Class C so don't know how long its not been working.
I had the same problem. Call uavionics tech. Software set up sensitivity setting needs to be changed to lower. Mine ended up at 1200.
E
 
OK. I guess that's not your problem.

Haha...but it could be mine. I was trouble shooting a hum and charging issue back in early April. I did cycle the 330ES, but don’t remember if I turned it on with ALT or the ON button. Issue turned to be the alternator. Ever since that flight FlightAware is not reporting my altitude correctly, and I had error messages on the ADSB report. Tomorrow, I am going to put the 330ES in ALT mode and hope it solves the mystery. What is interesting is that the 330ES is till showing correct altitude when I select that function. I will hazard a guess to say that it is showing the correct altitude from the encoder, but just not sending that info out to ATC or FlightAware.
 
Hello,

We are working a similar (weird) issue with our installation, hope this doesnt take this thread too far off track but in working our issue have come to some "truths" new to us, perhaps not to you.

Installation is a Transcal blind encoder , a KT76A transponder and a Uavionix Tail Beacon.

Tail beacon has been in the aircraft at least a year at this point and have had no issues with it or with our PAPR reports.

Last IFR Cert was previous August.

A few weeks ago ATC started telling us our Mode 3/A squawk was intermittently switching between VFR (1200) and 7770.

Further troubleshooting on the ground we noticed that the 1200 squawk changes to 7770 (intermittently) whenever we select Altitude (ALT) on the transponder and continues to intermittently switch from 1200 to 7770.

We can see the issue on the ground while observing our Tail beacon output (which by the way is being interrogated by the reply light blinking on our transponder but the closest ATCRBS radar is located (NS Mayport,FL) beyond the horizon so its not the source, must be from TCAS interrogations?).

During a test flight ATC noted , also, that when we stopped altitude squawk that there was no 7770 code being reported, we stayed on the 1200 squawk. So we have an issue being generated whenever we squawk altitude.

And to isolate we switched out our Transponder unit with an identical KT-76A (slide in replacement, we did not switch tray or wiring), and there was no change , same Mode 3/A issue occurs with either KT76A.

Only "significant" maintenance done to the aircraft just prior to this was a new aircraft battery was installed. That installation involved only firewall forward disconnect and reconnect of the battery in the engine compartment. Reason I mention this is Uavionix support (I like their support phone doesnt ring forever and not asked to leave a message, have always got a hold of help) asked if we had done any electrical work on the aircraft.

They suggested we could increase our Transponder Monitor Threshold as previously mentioned in the thread..we tried that but it was no help (we tried lower and higher settings as well from 25 all the way up to 45, continues to change Mode 3 squawk to 7770 intermittently. (all of this was on the ground).

And we think with have an Altitude encoder problem but didn't understand why that would cause our Mode 3/A to be messed up, but the nature of the Uavionix ADS-B is it detects your Mode 3/A via detection of your Squawk by "listening" to the electrical wiring to pick up mode 3/A transmissions from your transponder replies to ATC.

Current thinking is that the Transcal blind encoder is causing this issue when we squawk altitude (which turns it on) and it is causing either noise or grounding which messes up the uavionix sensing of the squawk code and just filling in as if thats what our Mode 3 is,i.e. 7770.

We couldn't understand how our altitude with ATC couldn't be messed up as well if we have a encoder problem .(but we should have understood if we had remembered that uavionix ADS-B beacons have their own Baro ALT encoder built in and thats what was being reported to ATC ) This Uavionix baro altitude syncs with ownship Mode C occasionally during changes in altitude.

Going to launch with ADS-B off and ask for a Mode C only check, if its goofy (or invalid) then its most likely the ALT Encoder (been in the plane for god knows how long) or some funky thing with wiring.

If its good Mode C check, then Uavionix issue?

Sorry for the lengthy blathering, yes I'm an engineer..

Cheers,
Tunahawk
 
Welcome with your first post. I'm the OP on this thread. Sounds like you are chasing things like I was. Your problem, which to me does sound like the encoder, is not exactly what I experienced and does have the potential of running this thread off track, but we'll live through it.

In my case, and I should have posted this earlier, was a combination of two things. I suspected at first it was the encoder, but I was proven wrong. First, an avionics shop, quickly started with connection issues and sure enough there was a poor connection at the coupler going into the transponder. This was determined quickly on the ground using test equipment and a transponder swap. The problem didn't go away. They also double checked the connection to the antenna and it was cleaned up. Departed that's shop airport thinking all was fixed. On the flight back to base I was getting more consistent reply lights from the transponder. However, I noticed on the next PAPR report, it still showed an issue. Called uAvionix and discussed with the rep and he suspected an issue with the internal baro of the SkyBeacon. They accepted the old one and once their testing proved what was suspected, provided a new SkyBeacon. All is well now. As an added bonus, the new one came with the latest software upgrade.

The one lesson learned here is that if you don't talk to ATC much and fly around with your transponder and ADS-B on, and know other traffic sees you on their ADS-B in, don't assume all is well. That other traffic may not tell you, "yes I see you, but I don't see your altitude".

Good luck in solving your issue. I'm not an engineer.
 
Roger that.. Im betting 50/50 Encoder issue.. Mode C check should tell.

One thing you brought up , about connection at the transponder , has me thinking if that may be a source of our issue perhaps a bad connection there is causing issues with mode 3/A squawk code when the altitude encode is on??

Oh well down another rabbit hole I guess

Heading out in a few , maybe it will all gel today

Cheers,
Tunahawk
 
Yesterdays Mode C check with tower while ADS-B (tail beacon / nav lights OFF) and KT-76A Transponder in ALT, Our Altitude was not being reported confirmed by Tower , also noticed (for the second time) a message on the GNS430 that there was no altitude input being received (which we expect normally when the transponder is OFF, but in this case I had it IN ALT mode ) so this helps confirm we have (most likely) an encoder problem. Inflight I turned on the tail beacon and stopped ALT squawk on the Transponder. Within a few minutes ATC told us mode C was working again, but I believe that is not quite correct what they are seeing is the Baro Alt reported by the Tail Beacon's baro sensor, nothing to do with Mode C out of the transponder, I dont know what their displays show them wrt that..

The Public ADS-B Performance Report shows nothing failed (of course only shows the portion of flight after I had turned on the Tail Beacon transceiver).
Trans-cal encoder ordered, expecting good results but prepared for more gotchas..

I think if you want a check from ATC of your Mode C , ask them that you want a readout of your Mode C Only..

Cheers,
Tunahawk
 
I experienced a similar issue a couple months ago..during flight in VFR I noticed no pressure altitude readout on my GTX327. My old ACK encoder was not working. I checked my flight on Flight Aware...showed my flight from ADS-B out but there was no altitude info...new AKG installed and problem solved.
 
Yes, it gets your squawk code and altitude by looking at the electrical noise on the power circuit generated by the transponder as it transmits, then the sky bacon rebroadcasts the numbers.
It gets the squawk code and altitude from sensing/sniffing/receiving the output of the transponder antenna. It is not through the electrical system.
 
It gets the squawk code and altitude from sensing/sniffing/receiving the output of the transponder antenna. It is not through the electrical system.
Garmin has a patent on that - uAvionix no can do.

https://www.aopa.org/news-and-media/all-news/2018/december/06/uavionix-transcoder-patent-approved
“'The power transcoder differs from other implementations in that, instead of receiving the information over the air when the transponder broadcasts, it actually detects noise from the transponder that is induced on the power wires [within the aircraft] during transmission,' said Ryan Braun, uAvionix chief operating officer."


https://patents.google.com/patent/US10156627B2/en

"Abstract

The ADS-B radio extracts Mode transponder data from parasitic oscillations on the aircraft power line induced by transmissions of ownship radar transponder reply signals. The radio is configured for replacement installation of an aircraft lighting assembly, and connection thereby to legacy onboard power sources without resorting to wireless or wired radar transponder, or pneumatic connections."

https://patentimages.storage.googleapis.com/e6/89/c6/a04b24e2ae9135/US10156627.pdf
 
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