Unreadable radio transmissions/DA40NG

[Sindre]

Hi! I'm with a flying club in Norway operating two Diamond DA40 NGs. One of the planes has recently developed a radio problem, where the transmissions from the aircraft are reported as barely readable or totally unreadable. The intercom still works fine for all inside the plane, but ATC on several airports and other planes reports the transmissions as garbled.

Both we and our mechanics are at a loss at what to do, not at least because the problem is intermittent. Both COM 1 and 2 are affected.

We have tried:
- Switching headsets.
- Switching pilots.
- Switching seats, i.e. transmitting from both left and right positions.
- Transmitting on COM 1 with Avionics master OFF.
- Transmitting with engine both running and off.
- With and without other electronic equipment on and off, e.g. bluetooth connections to headsets, ipads not in flight mode etc.
Neither of these has provided any results.

On some startups the radios seems to work fine, or at least OK: it seems entirely random, but now seems to have increased to a point where we have had to ground the airplane until we can find a solution.

We've recently moved to 8.33 kHz channel spacing in our local area, and while this might have something to do with it, our other DA40 from 2015 does not show any of the symptoms at all. The radios in the plane has been on 8.33 spacing for several years due to flying to other airports and abroad, without issue.

The plane is from 2017, equipped with G1000 NXI and the GMA 1347 audio panel. I'm not entirely sure exactly which Garmin Integrated Avionics versions we have. The plane has recently had its 800 hour service (on this particular engine, the aircraft itself has more hours), but neither our mechanics nor Garmin could come up with a solution. Apparently it's a known fault on several DA40s, but as we are the sole operators of this type in our part of the country, we don't have many people to ask for advice.

Does anyone have any suggestions as to where we can start our troubleshooting?

[DaveS1900]

It might be a grounding problem with the antennas or ground problem with the coax cable. Have you checked continuity with the avionics ground on those items?

Dave S.
Irwin Ohio

[chili4way]

It sounds like your DA40NG still has the NXi "Phase 1" avionics (the GMA1347 is the tell). Have you considered upgrading to the Phase 2 version, which features the GIA64s replacing the GIA63Ws and the GMA1360 BT audio panel replacing the GMA1347? This would update both the audio panel and the NAV/COM radios.

[Sindre]

Thank you for your suggestions!

We do have the Phase 1 avionics, and maybe an upgrade would be useful — though I suspect it wouldn't be cheap! Our other DA40 is in fact a few years older, with just the base G1000, and still works rather well.

Yesterday we measured the emitted frequency of the radios, which was found to be well within spec.

On closer inspection we've noticed that both the antenna bases have cracks in them: a person familiar with radios and installation mentioned water ingress, and that moisture might cause grounding problems — like you suggest, Dave. Does that sound reasonable?



The aircraft are usually hangared, but are of course washed and will fly though the occasional shower — in addition to the fact that it rains about 2/3 of the year here, so it can get rather damp!

[DaveS1900]

Sindre,
I am an amatuer radio hobbyist. My experience is that problems with the antenna ground and/or wire shielding ground can cause transmission problems and be hard to isolate and be intermittent. Yes, moisture is a problem, causing corrosion on the wire grounding terminals and/or creating a short between the hot side and the ground. I would get an amatuer radio guy from you area to help you trouble shoot the problem. They are usually friendly people willing to help.

Dave S.
Irwin Ohio

[DaveS1900]

Sindre,
By the way I was in Norway 2 years ago. I was visiting 3rd cousins in Vikebygd. My great grandmother and grandfather emigrated from Norway in the late 1800's. We loved visiting Norway, had a great time.
Dave S
Irwin Ohio

[Sindre]

Thank you so much for your help! I'll ask one of my colleagues, an ardent amateur radio operator, for help later this week! We'll have a look to see if it's possible to disconnect the antenna cables from the IAUs and measure the cables individually.

How fascinating! I'm glad to hear you enjoyed your stay! There are a few places named Vikebygd in Norway, but two of them are very close to where the club is based — about 20 minutes flying, in fact

[DaveS1900]

Glad you have a friend that can help!

When we visited our Norway relatives, we drove South from Bergen, near Haugesund.

Dave S.
Irwin Ohio

[Sindre]

Thank you all for your suggestions and help.

A few days ago we measured the COM1 antenna with an instrument, with the help of a radio amateur. According to him the antenna is working well. We suspect the COM2 antenna is all right as well, but we haven't measured it, since it would mean stripping much of the interior installations in the avionics bay. We also couldn't see any obvious faults in the cabling or hardware.

Further testing today suggests that microphone placement makes a difference in perceived distortion in the transmissions. While correct microphone distance has always been important, the transmission errors we are experiencing now with this aircraft is new.

Listening to the transmitted audio it sounds like the audio is 'clipped', i.e. the amplitude reaches a max level and the audio is distorted. Placing the microphone further from the mouth seems to lower the perceived clipping, but does of course also lower the transmitted volume.

Might microphone gain be the culprit?

I have looked at every config-page of our G1000 NXi-installation, but I can't find any settings for adjusting the microphone gain. Some posts on this forum and manuals for the G1000 suggest such an option exists, but I don't seem to find it. Does anyone know if this is only visible on some software versions/installations?

(I know Vikebygd near Haugesund well — it is a lovely place! I cross it regularly on my flights.)

[smoss]

There is a mic gain setting on most decent headsets now, usually under a tiny screw near the mic itself. Adjusting this has been helpful to get a normal level of intercom chat without clipping for me. There is also a manual squelch setting on the audio panel of the G1000 where you can theoretically adjust it manually (but might be only for intercom), but on mine if I try it, no matter where I set it for my copilot, their speech becomes entirely muted. There is some documentation in the G1000 manual:
5.5 INTERCOM SQUELCH CONTROL
Select manual squelch for intercom audio by pressing
the MAN SQ key to light the annunciator.
Pressing the small VOL/SQ knob now toggles between
volume and squelch adjustment by lighting VOL
or SQ respectively.