Transponder intermittent

[Marc_B]

Have GTX330ES and GDL88, and I’ve recently been getting notified that my Mode C is intermittent.  The GTX is the oldest avionics piece in my panel but I’ve never had an issue and all the cockpit checks are good. Software most recent, LRU page on GTN shows all green. No errors logged.

I've checked antennas, connections and having trouble getting into my avionics shop as they’re backed up. Went to another shop and went through basics and nothing obvious. Garmin said it’s rare to have the CL105 antennas go bad and the intermittent faults are usually software, terrain/antenna masking, or potentially frequency drift. Shop was going to use another GTX330ES off the shelf to switch out as a step but unit needed to be sent to Garmin due to malfunction.

Anything I can or should check? Can GTX units be bench checked?  Send to Garmin for repair vs change out equipment with newer model…seems more involved than just change unit as a change would likely be trade GDL/GTX for a 345 which would need wiring and install time.

What’s the typical lifespan of a GTX 330ES?

PAPR usually has shown SDA fail and reports similar to below.

[201Mooniac]

The GTX are fairly reliable as I've heard.  What is the GPS source?  While I expect it could be bench checked, most shops won't have the data and will just recommend sending back to Garmin for a flat rate repair.  I recently did replace my GTX and GDL with a GTX 345 for compatibility with other equipment reasons and the needed new wiring was really minimal.  If your GPS source is a GTN, you just need to ad an HSDB connection from the GTX 345 to the GTN.  If you have an active traffic system attached to the GDL-88, it does get a little more involved as you need to get that wired to the GTX but in my case they reused the wiring from the GDL to the GTN (from the rear avionics bay to the front) for the TAS to the GTX.  Overall they completed it in about 16 hours.

[donkaye, MCFI]

Marc, I can't speak to the possible issues with the GTX 330EX, but I can say what I did with my GTX 330ES.  I didn't like the green color of the display, so I sold the unit and bought the GTX 335 with the bright white display and a few other niceties, which, of course, I never use.  The GTS 800 interfaces to my GDL 88 where correlation with ADS-B traffic data takes place.  In addition to saving a bunch of money, I liked the idea of having both 1090 and 978 ADS-B Out, so I kept the GDL 88.  As a result the replacement didn't take much time at all.  The labor was $1,500, but also included the wiring to upgrade the GMA 35 to the GMA 35C.

[Marc_B]

@donkaye, MCFI It’s interesting that the shop in Greeley said it should have single out (1090ES) and dual in. So when they did the upgrade they turned off UAT out.

[201Mooniac]

1 hour ago, donkaye, MCFI said:

The GTX 335 interfaces with both my GDL 88 and my Active Traffic GTS 800.

Since the GTX 335 is ADS-B out only, it doesn't interface with the GTS 800, only the GTX 345 does.  The GTX 345 has an HSDB connection with the GTS 800 for correlation between ADS-B traffic (TIS-B) and active traffic from the GTS 800.

 

Edit: To be complete, there is a serial interconnect between the GTX 335 and the GDL 88 for transponder suppresion.

[donkaye, MCFI]

2 hours ago, Marc_B said:

@donkaye, MCFI It’s interesting that the shop in Greeley said it should have single out (1090ES) and dual in. So when they did the upgrade they turned off UAT out.

The ADS-B system was updated over 5 years ago to accommodate ADS-B Out on both channels simultaneously.

[donkaye, MCFI]

1 hour ago, 201Mooniac said:

Since the GTX 335 is ADS-B out only, it doesn't interface with the GTS 800, only the GTX 345 does.  The GTX 345 has an HSDB connection with the GTS 800 for correlation between ADS-B traffic (TIS-B) and active traffic from the GTS 800.

 

Edit: To be complete, there is a serial interconnect between the GTX 335 and the GDL 88 for transponder suppresion.

I corrected my above statement to state that correlation of Active traffic from the GTS 800 and ADS-B traffic from the GDL 88 occurs within the GDL88.

[donkaye, MCFI]

I'm also posting the reference email I had with the FAA 11 years ago regarding an airplane having dual ADS-B Out capability.  It states that their system has been "adapted" to handle an aircraft with both ADS-B signals, if properly configured.

[Ragsf15e]

14 hours ago, Marc_B said:

Have GTX330ES and GDL88, and I’ve recently been getting notified that my Mode C is intermittent.  The GTX is the oldest avionics piece in my panel but I’ve never had an issue and all the cockpit checks are good. Software most recent, LRU page on GTN shows all green. No errors logged.

I've checked antennas, connections and having trouble getting into my avionics shop as they’re backed up. Went to another shop and went through basics and nothing obvious. Garmin said it’s rare to have the CL105 antennas go bad and the intermittent faults are usually software, terrain/antenna masking, or potentially frequency drift. Shop was going to use another GTX330ES off the shelf to switch out as a step but unit needed to be sent to Garmin due to malfunction.

Anything I can or should check? Can GTX units be bench checked?  Send to Garmin for repair vs change out equipment with newer model…seems more involved than just change unit as a change would likely be trade GDL/GTX for a 345 which would need wiring and install time.

What’s the typical lifespan of a GTX 330ES?

PAPR usually has shown SDA fail and reports similar to below.

What is it using as an altitude encoder?

I had this happen with a gtx-345 and it was actually the encoder.

[Marc_B]

43 minutes ago, Ragsf15e said:

What is it using as an altitude encoder?

I have it linked to GTN 750Xi; no flags on GTN or on LRU page.  Software 8.04 in the GTX330ES, and 3.42 in the GDL88.

 

[Ragsf15e]

40 minutes ago, Marc_B said:

I have it linked to GTN 750Xi; no flags on GTN or on LRU page.  Software 8.04 in the GTX330ES, and 3.42 in the GDL88.

 

Yes, but what’s its altitude source?  I don’t think a gtn can be the altitude source?  There will be some type of encoder hooked up to static air that provides it.  Could be anything from an old school grey code box or a fancy little garmin one on the back of your unit. It’s where the transponder gets the pressure altitude.  Strangely, the transponder needs a different box to do this for it. The transponder itself just takes care of sending out the different mode c signals, adsb, etc.

[Ragsf15e]

1 hour ago, Marc_B said:

I have it linked to GTN 750Xi; no flags on GTN or on LRU page.  Software 8.04 in the GTX330ES, and 3.42 in the GDL88.

 

The reason I ask about your encoding source is that they said your mode C was intermittent. The mode C is the altitude reporting portion of your transponder signal. The transponder gets its altitude from the encoder which is a separate device. 

[Marc_B]

3 hours ago, Ragsf15e said:

Yes, but what’s its altitude source?

G500TXi with integrated AHRS/ADC. 

[Ragsf15e]

5 hours ago, Marc_B said:

G500TXi with integrated AHRS/ADC. 

Strangely, my gtx345 was using a G5 as the encoder and (after several conversations), garmin admitted there is an issue with that setup that can cause intermittent mode c loss.  They even offered to send me a gae-12 encoder.  Now I doubt the same issue exists with the g500txi, but I guess my thought is that it might not necessarily be an issue with the transponder itself. Maybe a wiring issue from the g500?  It will need reasonable troubleshooting.

[Marc_B]

Talked to shop this morning. Said altitude is coming from the GTN to the xpdr so whatever source the GTN is using sends it to the transponder = altitude from the G500, but if that were to stop working the G5 would still be feeding the GTN and it would pass the altitude to the XPDR.  “I do get all the A/C information at the test set however the power level is about half of what is acceptable to pass the test.  I am thinking that most likely you have a bad GTX330ES, however, without another unit to swap out I cannot completely confirm that.”

Sounds like repair vs exchange is most economical option. GTX335 would need to tap into HSDB, remote mount GTX345 could adopt GDL88 HSDB in tail. But both of these would be more install costs plus at least double equipment cost.

Any difference between an exchange vs repair from Garmin?

Any other potential causes of intermittent reception likely caused by low output signal besides faulty unit?

Update: Garmin’s flat rate repair $1125, exchange $1350 for the GTX330ES.  Still cheaper than 335/345 + install. 

[Marc_B]

Follow-up. Interestingly enough my shop said when they swapped the GTX (with a loaner unit) output was still low. But swapping antenna fixed it. So evidently antennas do go bad…

Funny that the three things mentioned in the manual are antenna, ground and coax for intermittent or weak transmission. Duh! Ha ha ha

[Ragsf15e]

2 hours ago, Marc_B said:

Follow-up. Interestingly enough my shop said when they swapped the GTX (with a loaner unit) output was still low. But swapping antenna fixed it. So evidently antennas do go bad…

Funny that the three things mentioned in the manual are antenna, ground and coax for intermittent or weak transmission. Duh! Ha ha ha

Well, that was probably cheaper than a new unit. Sometimes they change out the antenna, but maybe it was the antenna mounting or the wiring in it was fixed during installation of the new antenna.