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Baro Altitude Failure - GTX 345/GRT HX

M

mic2377

Active Member
I have had a persistent issue with Baro Alt after a transponder failure and subsequent repair. The aircraft is a RV-7A with a GRT HX dual EFIS, Garmin 530W, Tru-Trak 385, and GTX-345.

To begin, I experienced a complete GTX-345 transponder failure in flight this last year. This resulted in a complete loss of ADS-B In/out, and necessitated removal and factory service of the transponder by Garmin. Upon reinstallation it appeared to have no issues, with restoration of ADS-B in/out and an appropriate pressure altitude being displayed on the GTX. I did notice however that the autopilot (coupled to the 530W) was not receiving altitude guidance from the EFIS, although it would track GPS course. I don't think I connected that this would indicate a altitude source fault at the time.

I am a VFR pilot, and at that time did not realize I could verify ADS-B performance at that time with a PAPR report.

I then relocated to the aircraft a much busier Class D airport. It was noted by ATC that while ADS-B in/out performance would give my position, I was missing Baro Alt output. I then checked the PAPR report, and it demonstrated a 100% Baro Alt fault. The aircraft was due for its pitot/static check as well as transponder check. When this was conducted, faults were noted when undergoing this. On my next flight outbound, it unfortunately again demonstrated a Baro Alt fault, which could be seen on the PAPR as well the Flightaware tracing. However the Baro Alt fault corrected itself on the return flight, with an accurate altitude tracing.

I am unfortunately not the original builder of the RV, although I have done some very significant maintenance (engine removal and IRAN for a case seam oil leak, tire/brake work, baffle repairs, etc). I have not done any avionics work to it, and will admit that I have little to no familiarity with this.

In terms of troubleshooting further, my understanding is that altitude out signal is from the GRT EFIS via serial to the Tru-trak and GTX, in lieu of a stand-alone encoder. I am not really sure where to go from here in terms of next steps, although I suspect this is either an issue with the GRT or the wiring from which it provides altitude out. My home avionics shop would do the pitot/static/transponder check, but they really only work on certified aircraft and were not helpful beyond this. If not, would anyone be able to suggest an avionics shop in OH that would be able to assist with this?
 
I presume the efis altimeter continues to work properly? Odds are there is a wiring problem. General rule: check easy stuff first, even if unlikely.
1. Get under the panel and look for any loose, dangling, broken wires. Long shot but easy. If you have a wiring schematic you can pay special attention to the RS232 wire sending data to the transponder. This is unlikely to be the problem but easy to look. Also look at the wires to the 530W.
2. Buy a can of spray electronic contact cleaner. Unplug all the d-Sub connectors on the back of the efis, 530W, and transponder, spray male and female pins with the cleaner, re-assemble. (Again, if you can locate the specific connectors that carry the ADSB data, check them first.) And look for any bent pins.
3. Pull the transponder from its rack, spray the pins on the back, reassemble while still slightly wet so the mating pins get some cleaner too. But while it is out, check to see if any pins got bent in the recent service. Use a flashlight to look into the back of the rack, too, for bent pins.

That’s the easy stuff. Anything more, you’ll need to get familiar with the wiring. Do you have schematics?
 
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Go into your HX menu and see what serial pin sends the baro alt to the transponder. Then check for continuity along that wire. Because you said it fixed itself, that points to a likely contact failure at one of the pins. Check that or simply replace that wire.
 
I believe the GTX-345 should already be the latest firmware.

I also checked all of the GRT HX settings. I unfortunately was unable to find a schematic, but will keep looking. It seems that the serial 1 output drives the Trutrak, serial 5 input is from the Garmin 530W, and I am not sure the serial for the altitude output to the GTX.

I booted everything up on the ground. The GTX did provide appropriate pressure altitude, however a thunderstorm rolled in and I couldn't fly to verify that ATC was receiving anything after the intermittent function prior.

Next plan is to pull the EFIS, check wiring, and clean the connectors/pins. Really appreciate everyone's help. I am mechanically capable, and know how to use the avionics... but the setup and troubleshooting is a whole different ballgame.
 
A quick update - I was doing some additional work to the aircraft to update the EIS due to an erroneous warning, as well as fix a minor oil leak (darn rocker drain tubes!).

Day 1 - prior to pulling the GRT's from the panel, I ran the engine without the cowling, and also started up the EFIS. Neither of the EFIS units (it is a dual AHRS/dual EFIS setup) would align the horizon despite multiple restart attempts. The pressure ALT out to the GTX was rock solid.

Day 2 - upon powering up, no issues with EFIS aligning this time. I then pulled EFIS units. There did not appear to be a loose connector or obvious wiring issue. I unfortunately ran out of time and daylight to pull and clean the connectors.

If I can't detect a wiring fault and cleaning/replacing the D-sub connectors is unhelpful, my next step will be reaching out to GRT and potentially sending in the HX for service. The aircraft has already had a HX failure about 5 years ago that required complete replacement.
 
mic2377 said:
A quick update - I was doing some additional work to the aircraft to update the EIS due to an erroneous warning, as well as fix a minor oil leak (darn rocker drain tubes!).

Day 1 - prior to pulling the GRT's from the panel, I ran the engine without the cowling, and also started up the EFIS. Neither of the EFIS units (it is a dual AHRS/dual EFIS setup) would align the horizon despite multiple restart attempts. The pressure ALT out to the GTX was rock solid.

Day 2 - upon powering up, no issues with EFIS aligning this time. I then pulled EFIS units. There did not appear to be a loose connector or obvious wiring issue. I unfortunately ran out of time and daylight to pull and clean the connectors.

If I can't detect a wiring fault and cleaning/replacing the D-sub connectors is unhelpful, my next step will be reaching out to GRT and potentially sending in the HX for service. The aircraft has already had a HX failure about 5 years ago that required complete replacement.
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I would stay on wiring first. The serial port you are looking for will be configured as something with "air Data" in the setting and it will be on the output side. Once you have identified the port number, Go to the GRT manual and find the pin configuration for the two serial connectors To identify the pin associated with air data output. Then go to the GTX and do the same thing. Once you have the specific two pins and the wire that goes between them, you should be able to do a bunch of testing to confirm the integrity of that connection. You'll need to wiggle the wires around and do various things to help identify an intermittent issue.

Repairs are expensive and take time, so I would absolutely confirm your wiring is solid before going to that step. Your recent experience certainly could indicate that something is failing internally. However, it only happened once and we are not certain the Baro problem is internal., so I would still stick with wiring. Also, a failure to align is more likely to be the AHRS than the display unit. If the AHRS was the source of the altitude problem, your EFIS would not show an altitude. So an intermittent wiring issue is the more likely cause in my opinion.
 
I pulled both EFIS units and removed/reinserted both D-sub connectors on each unit, after cleaning the connectors with contact cleaner.

After doing so, I certainly see how an intermittent connection between the EFIS and output wiring could be the culprit. The D-sub connections seemed to have minimal insertion force, much less than I would have expected based on the typical RS232 plug. I have a much higher suspicion that one of the pins or wiring associated with the D-sub connector is the cause.

I did not interrogate the D-sub to transponder wiring yet. I am hoping that the cleaning will do the job - it is not very well labeled and is a mess behind the panel.
 
mic2377 said:
I pulled both EFIS units and removed/reinserted both D-sub connectors on each unit, after cleaning the connectors with contact cleaner.

After doing so, I certainly see how an intermittent connection between the EFIS and output wiring could be the culprit. The D-sub connections seemed to have minimal insertion force, much less than I would have expected based on the typical RS232 plug. I have a much higher suspicion that one of the pins or wiring associated with the D-sub connector is the cause.

I did not interrogate the D-sub to transponder wiring yet. I am hoping that the cleaning will do the job - it is not very well labeled and is a mess behind the panel.
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The d-sub connectors are also held together with screws on either end, right?
Is it possible someone used ‘high density’ male pins and ‘low density’ female pins? That could account for the ease of putting them together.
 
So to provide closure to this whole situation - it looks like removing and cleaning the D sub connections did the trick. Last PAPR and the tracking on Flightaware show a consistent altitude track in addition to ADSB X/Y location. I am hoping that this is the end of this problem.

I do think the slightly loose pins were likely the cause. I felt one of the connectors was not as snug as it should be. It the problem persists I will replace the wiring with different pins (hopefully not).
 
mic2377 said:
So to provide closure to this whole situation - it looks like removing and cleaning the D sub connections did the trick. Last PAPR and the tracking on Flightaware show a consistent altitude track in addition to ADSB X/Y location. I am hoping that this is the end of this problem.

I do think the slightly loose pins were likely the cause. I felt one of the connectors was not as snug as it should be. It the problem persists I will replace the wiring with different pins (hopefully not).
Click to expand...
Before you do all of that, make SURE that the two screws that hold the dsub connector in, are nice and SNUG (wiggle the cable just a little bit as you are pressing it in.)

And if the screws are the regular ones that require a flat screwdriver, do your best to find THUMSCREWS to replace them! You will thank me later. :) :)

If the EFIS is aligning and it adjusts correctly when you adjust "BARO", then again, it is likely wiring (something somewhat loose, from over the years).
 
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