G1000 COMM and Electrical Weirdness *VID*

[Vflyer]

Posted this on another forum and didn't get much feedback. Hoping someone can shed some light on to what's going on with a 2008 C172S G1000:

Turned on the master prior to start and heard a *piercing* noise of what sounded like fragmented words of the AWOS leaking over onto the CTAF frequency (CTAF was currently in the active spot).

Example: ".........WIND......AT 5........ALT.........02" only loud enough that I had to take my headset off.

I immediately tried adjusting the volumes of both the COMM1 and COMM2 radios and that seemed to help the volume aspect, but there was still this broken leak of what sounded like the AWOS coming over. This was all apparent on COMM1 and I believe on COMM2 when I isolated that, but I can't be sure. It seemed no matter what combo, there would be some strange phantom "RX" that was creeping in.

Here is a video of what I mean. I tuned COMM2 to 121.5 as there theoretically wouldn't be any radio chatter on that, but yet you can see the "RX" persistently popping up (looks like in sync with when the COMM1 awos is receiving).

It also may just be in my head, but I thought I heard a much more apparent whining noise when taxiing out and running up (higher pitch with higher rpm). I know this is indicative of something off with the alternator and ground, but at the time wasn't sure and haven't had a chance to really research it yet. This ultimately made me scrub the flight as I figured the COMM1 weirdness was potentially a symptom of a larger problem as evident by the alternator noise.

In any case both an instructor and mechanic took a look and thought it was odd as well, but I had to leave before I heard an actual diagnosis. Since then others reportd the same problem, which makes me believe its not related to a particular headset, or maybe interference from portable electronics or something. So anyway, I'm trying to just get a little background before taking the plane back up. I know small things like this may be indicative of a larger problem, so want to be prepared to talk with the mechanic if it does come up again.

Thanks!

 

[Roger Roger]

Posted this on another forum and didn't get much feedback. Hoping someone can shed some light on to what's going on with a 2008 C172S G1000:

Turned on the master prior to start and heard a *piercing* noise of what sounded like fragmented words of the AWOS leaking over onto the CTAF frequency (CTAF was currently in the active spot).

Example: ".........WIND......AT 5........ALT.........02" only loud enough that I had to take my headset off.

I immediately tried adjusting the volumes of both the COMM1 and COMM2 radios and that seemed to help the volume aspect, but there was still this broken leak of what sounded like the AWOS coming over. This was all apparent on COMM1 and I believe on COMM2 when I isolated that, but I can't be sure. It seemed no matter what combo, there would be some strange phantom "RX" that was creeping in.

Here is a video of what I mean. I tuned COMM2 to 121.5 as there theoretically wouldn't be any radio chatter on that, but yet you can see the "RX" persistently popping up (looks like in sync with when the COMM1 awos is receiving).

It also may just be in my head, but I thought I heard a much more apparent whining noise when taxiing out and running up (higher pitch with higher rpm). I know this is indicative of something off with the alternator and ground, but at the time wasn't sure and haven't had a chance to really research it yet. This ultimately made me scrub the flight as I figured the COMM1 weirdness was potentially a symptom of a larger problem as evident by the alternator noise.

In any case both an instructor and mechanic took a look and thought it was odd as well, but I had to leave before I heard an actual diagnosis. Since then others reportd the same problem, which makes me believe its not related to a particular headset, or maybe interference from portable electronics or something. So anyway, I'm trying to just get a little background before taking the plane back up. I know small things like this may be indicative of a larger problem, so want to be prepared to talk with the mechanic if it does come up again.

Thanks!

My guess? Audio panel internal issue.

 

[Cptnchia]

Possible RF filter failure?

 

[Roger Roger]

Possible RF filter failure?

Yep, could be something like that too.

 

[NovemberEcho]

Sounds like it was hacked

 

[dustoff17]

Sounds like it was hacked

This!

Any chance you were parked next to a 757 in Atlantic City when all this was going on?

Seriously, not sure I'm following: Do you have an AWOS frequency tuned into either radio? (i.e. AWOS freq in Comm 1 and receiving bleed over on Comm2 or vis a versa). OR are you stating that you don't have an AWOS freq tuned into either Comm but are receiving an AWOS transmission?

 

[denverpilot]

Start with the simple stuff. Make sure someone didn’t put the radios or the intercom in manual squelch mode.

 

[Vflyer]

This!

Any chance you were parked next to a 757 in Atlantic City when all this was going on?

Seriously, not sure I'm following: Do you have an AWOS frequency tuned into either radio? (i.e. AWOS freq in Comm 1 and receiving bleed over on Comm2 or vis a versa). OR are you stating that you don't have an AWOS freq tuned into either Comm but are receiving an AWOS transmission?

So at first I wasn't sure where the phantom "RX" was coming from, but the more I think about it, it seems like it was bleed over from the standby freq to the active. Looking at the video and COMM2, I have 121.5 in the active spot and 118.925 (awos) in the standby. You can see the "RX" popping up intermittently, but as soon as there is a pause in the cycle of the AWOS (as evident by COMM1) the "RX" on the COMM2 radio goes away; then returns when it the AWOS is transmitting again.

As I mentioned above, I heard it immediately when first turning on the master which means COMM1 would have been the only radio on. It's most likely COMM1 had the CTAF in active and the AWOS in the standby from the previous flight.

So in summary I think I can say with 80% confidence the problem is: Bleed over transmission (i.e RX) from the standby frequency onto the active frequency on both the COMM1 and COMM2 radio.

I would guess that if I put in an unused frequency like 123.45 or something in the standby frequency I wouldn't hear any words interrupting the active freq, but possibly some intermittent static.

 

[hook_dupin]

Have you tried this same thing in other airplanes? I ask because sometimes transmitters aren't that clean either -it could be something like a short between coax lines at the FBO. The Unicom and AWOS are very likely on the same antenna mast.

 

[Roger Roger]

Have you tried this same thing in other airplanes? I ask because sometimes transmitters aren't that clean either -it could be something like a short between coax lines at the FBO. The Unicom and AWOS are very likely on the same antenna mast.

I don’t run G1000s but I have had this issue (standby freq bleeding over) on a couple other radios and it always ended up being the radio internally. But yes, first steps are to try on COMM2 and on another aircraft.

 

[Vflyer]

Have you tried this same thing in other airplanes? I ask because sometimes transmitters aren't that clean either -it could be something like a short between coax lines at the FBO. The Unicom and AWOS are very likely on the same antenna mast.

Not yet, but I will tomorrow. It's funny you mention that though as the ceiling height information was missing from the automated reporting inside the FBO. I'm not sure of the relation between the sensors and the transmission, but it would be very interesting if that was the problem.

 

[Vflyer]

I don’t run G1000s but I have had this issue (standby freq bleeding over) on a couple other radios and it always ended up being the radio internally. But yes, first steps are to try on COMM2 and on another aircraft.

Do you know what the fix was out of curiosity? I'm not sure what the FBO's motivation will be to ground the plane, so was there something you did to mitigate the problem? Probably just turned the radio off I would imagine.

 

[Roger Roger]

Do you know what the fix was out of curiosity? I'm not sure what the FBO's motivation will be to ground the plane, so was there something you did to mitigate the problem? Probably just turned the radio off I would imagine.

We replaced the offending radio. There should be some troubleshooting they can do to narrow down which Garmin box is the offender (assuming it’s not a broadcast or an antenna/coax issue), but ultimately I’m guessing they’ll have to repair or replace MMDR1 GIA#1.