My air data computer is interfering with my transponder???

nickmatic

nickmatic

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Nickmatic
So this is an odd one. When flying now ATC is telling me my transponder is reporting an altitude about 2500 feet too high (and they haven't been too happy about it, either). I figured out, however, that if I pull the breaker on my Air Data Computer, the problem goes away...

WTH?? These 2 things aren't related at all. Anyone have any clues on how the ADC might be interfering with the transponder?

Another point of interest, the local avionics guy tried attaching a different altitude sender and the same problem occurred, so it's not my altimeter, per se. Sadly, said avionics guy has no time to work on this as he's the only one around for miles.

Last data point, the ADC is currently not working properly either. All air data data readouts (OAT, TAS, wind direction, speed) show --- when the ADC is powered on (and of course also when powered off). I spoke to Shadin and they suggested to first try debugging the temp sensor but given the transponder altitude issue, maybe the whole ADC is bad and it's going ape**** and interfering with the transponder? Not sure how that could happen but open to suggestions. I mentioned that to Shadin and they had no idea how that could happen either.

Mooney 252/Encore
Shadin ADC 200
GNS 480 GPS
GTX 33 remote transponder
 
Pitot/Static system problem/leak inside the caae? They both attach to the ADC.
 
denverpilot said:
Pitot/Static system problem/leak inside the caae? They both attach to the ADC.
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Yeah hmm, they are both attached to the static system. Good point. I suppose I can try disconnecting the static line from the ADC and capping it...

Is the ADC capable of dropping the pressure in the static line in a way that would disappear when it is powered down?
 
nickmatic said:
Yeah hmm, they are both attached to the static system. Good point. I suppose I can try disconnecting the static line from the ADC and capping it...

Is the ADC capable of dropping the pressure in the static line in a way that would disappear when it is powered down?
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The ADC may have a heated static pressure sensor. That could possibly mess with the encoder if they are in close proximity.
 
Clark1961 said:
The ADC may have a heated static pressure sensor. That could possibly mess with the encoder if they are in close proximity.
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I've never heard of that! Well they're not physically close to each other, but if they use the same static line and it's heating the air in there then the altitude would show higher. Does the Shadin ADC 200 have a heated static sensor? Maybe it's malfunctioning and overheating...?
 
nickmatic said:
I've never heard of that! Well they're not physically close to each other, but if they use the same static line and it's heating the air in there then the altitude would show higher. Does the Shadin ADC 200 have a heated static sensor? Maybe it's malfunctioning and overheating...?
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I don’t know about the Shadin. I know the Aspen is heated so I threw it out there. The Shadin should warm up when it’s turned on if it is heated. The Aspen sure gets hot.

Edit: Page 2-8 of the Shadin manual notes a warm up time so yeah, it’s heated. As I understand it the air sensors function at a known, fixed temperature.
 
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Clark1961 said:
I don’t know about the Shadin. I know the Aspen is heated so I threw it out there. The Shadin should warm up when it’s turned on if it is heated. The Aspen sure gets hot.

Edit: Page 2-8 of the Shadin manual notes a warm up time so yeah, it’s heated. As I understand it the air sensors function at a known, fixed temperature.
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Really interesting... I wonder if the temp control has broken and it's overheating, causing the static air to heat up as well. Seems possible. Disconnecting and capping the static line seems like the way to debug this.

Only point against this is that the altitude problem seems to go away really quickly when the ADC is shut off. I'd figure a significant amount of thermal mass has to cool down first.
 
nickmatic said:
Really interesting... I wonder if the temp control has broken and it's overheating, causing the static air to heat up as well. Seems possible. Disconnecting and capping the static line seems like the way to debug this.

Only point against this is that the altitude problem seems to go away really quickly when the ADC is shut off. But maybe that's expected.
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You may be aware that if the static system is opened then regulations require a leak test prior to returning to service. At least for N registered aircraft. Dunno your flag or regs so just mentioning it.
 
Ok, I spoke to Shadin to float the static heater idea by them. He said he'd never heard of that but suggested it's likely that both the ADC and the altimeter are wired to the transponder and that the transponder uses the altitude from the ADC unless it isn't receiving it and then uses the gray code altitude from the transponder as a backup. So a bad ADC would explain the issue and explain why it works immediately when you kill the power. I think this pretty clearly points to getting the ADC replaced.
 
If it was static temp rising that would increase static pressure and your reported altitude would be low, not high.

I’d say you have a static leak somewhere inside the cabin to go the other direction.

How it relates to the ADC yet, unsure, but you also have pitot side errors on the ADC as well.

You could, as you say, “cap it off” as troubleshooting but you might be chasing multiple things, and you’ll need it all re-certified in whatever configuration it ends up in anyway...

I’d find a shop (or we use a mobile guy who comes to our hangar) that will do a proper leak test on the whole system, both sides, before changing anything.

Leak may not even be in the ADC. No point in making it unairworthy trying to troubleshoot it. If the locals are busy, it’s an airplane. Fly it to someone with the pressure test gear.

Only takes about an hour to do the tests. Let them cap off the ADC if it’s gone south and make a log entry that they re-routed stuff and it all checks out in the new configuration.

It’s a Mooney. You’ll get there fast! :)
 
I didn’t see anything in the install manual that said the encoder needed to feed the ADC for anything, nor that the ADC could feed the transponder the encoder altitude in grey code, but yes, if the encoder is passing through the ADC on the way to the transponder, that would explain why the grey code might be getting mangled on the way through, if the ADC goes into “bypass” for the encoder.

My assumption would be that the blind encoder goes straight to the transponder.

Do you have a wiring diagram or know if the blind encoder goes through the ADC?
 
denverpilot said:
I didn’t see anything in the install manual that said the encoder needed to feed the ADC for anything, nor that the ADC could feed the transponder the encoder altitude in grey code, but yes, if the encoder is passing through the ADC on the way to the transponder, that would explain why the grey code might be getting mangled on the way through, if the ADC goes into “bypass” for the encoder.

My assumption would be that the blind encoder goes straight to the transponder.

Do you have a wiring diagram or know if the blind encoder goes through the ADC?
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Not sure if I explained it correctly above. The way the Shadin guy explained it I THINK is that the gray code from the altimeter and the RS-232 from the ADC both come into the GTX 33 (maybe through the GNS 480, not sure) and it can fall back to the gray code if it's not getting the RS-232 data. He stressed that it prioritizes the RS-232 over the gray code so that would make sense that once the ADC is shut down everything works.

The gray code definitely does not go through the ADC since there are no pins for that on the ADC connectors. The wiring information for everything is most likely in all my maintenance records. This stuff was installed long before I bought the plane so I'd have to dig through a several inch thick binder. I'll carve out some time for that soon.

Regarding finding a shop to do tests and/or troubleshoot, I've recently relocated to Northern Spain and it's not quite that easy, at least until I fully get my bearings. Aviation here isn't like the US, in fact, it's stuck in the stone age here in Spain. I've been hearing rumors of a shop in southern France but unable to get details just yet.

Regarding it being a static leak, I fail to see how that's possible since the problem disappears when the ADC is powered down.
 
nickmatic said:
Regarding finding a shop to do tests and/or troubleshoot, I've recently relocated to Northern Spain and it's not quite that easy, at least until I fully get my bearings. Aviation here isn't like the US, in fact, it's stuck in the stone age here in Spain. I've been hearing rumors of a shop in southern France but unable to get details just yet.

Regarding it being a static leak, I fail to see how that's possible since the problem disappears when the ADC is powered down.
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Spain. Yeah that makes it harder. Heh.

I thought you said the ADC is constantly out to lunch on airspeed and other items that would be pitot/static related?

The ADC certainly sounds like it’s unhealthy BUT, if it’s getting bad inputs... it’ll probably give bad outputs, from a systems engineering point of view. I’d want to know the inputs were right.

If the stupid static has a huge leak inside the cabin, that’s lower pressure than it should be... e.g. same problem you see in the POH for aircraft that have an alternate static source... or if you did the old “bust the glass on the VSI” trick in icing conditions. Probably not 2500’ worth of error, but you don’t know at this point.

I’m questioning “why is the ADC always out of whack”?
 
denverpilot said:
I thought you said the ADC is constantly out to lunch on airspeed and other items that would be pitot/static related?
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Just showing --- (dashes) on the GPS display for any air related items. Indeed the static system is a likely source of the issue, as is the OAT probe (that's where Shadin initially suggested I look). But given the fact that the transponder altitude is affected (and corrects when the ADC is powered down), it seems like the ADC itself has gone wonky.

I dug through my maintenance logs and was unable to find a wiring diagram. I did find the service order to install most of the new avionics but it didn't have much detail. If it's in there, it's buried.
 
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