Intermittent X-Ponder Altitude Issues :-(

I’m frustrated as hell with Avidyne and feel like I’m a Beta tester for them.  I installed an IFD-540/AXP-340 combo in May 2015. I a few minor glitches, but no issues.  Felt I was home free.

In November 2015 I took the plunge and installed an MLB-100.  The day I picked it up when I took off ATC reported that I was showing an altitude of 64,000’.  After landing back at my avionics shop Avidyne tech support determined that it was an MLB-100 SW issue.  The shop disabled the MLB-100, but on the way back to my home base ATC (Washington TRACON) reported that I had no Mode C altitude.  Recycled and got back home.  Flew a few times with one incident of no Mode C altitude that was fixed by recycling the transponder.  In February I had the MLB-100 patch installed along with the newest patch for the IFD-540.  Flew back home without incident.  On March 10^th was trying to get back to my home base and again ATC told me that I had no Mode C, but again I recycled and fixed the issue.

Today, March 26^th had the same issue, and recycling didn’t help.  ATC said that my altitude was going in and out.  Now I’m stuck inside the Washington, DC SFRA and have to coordinate with both ATC and Homeland Security in order to get it back to the shop.  I’m not a happy camper.  Don’t expect to spend big, big $$ to have an experimental product.

Couple of questions:

1.        Anyone else having this issue?

2.       Any ideas of what it could be?

3.       Why do I think this is an MLB-100 related issue?

4.       How do I fix it?

I’m so frustrated that I’m about to chuck my whole stack and go with Garmin and let everyone in the world know that Avidyne is not ready for prime time.

Guess the question is where does the AXP get its squawk info? Directly from the encoder or thru the MLB-100 and then to the encoder. Is this an install option so I can have the shop bypass the MLB-100?   All the problems started when the MLP-100 was installed.

I think you're doing Avidyne a disservice with your comments about them not being ready for prime time, given the experience you relate. Avidyne is doing everyone a huge favour with this forum, and Garmin sure as heck isn't taking a risk with something similar allowing everyone to spread their similar Garmin experiences for all the world to hear.

Sure, there were quite a few squawks that arose in the first year after the 540 came out, but that has really quieted down in the last 9 months. You said yourself that you had no issues with the 540/340 combo and it sounds like the current ones are MLB100-related. So throwing out all Avidyne and replacing with Garmin sounds like an over-reaction to me.

A large number of the issues that come up in this forum are installation-related. I'm not familiar with the MLB100, but other threads suggest to me that there are various ways to interconnect MLB100/AXP340/encoder/IFD540. Have you checked the installation manual and the schematics of your installation that were provided by your avionics shop? Your shop did provide you the schematics, didn't they?

Did you notice if GPS reception remained ok? The AXP340 will lose mode C if the GPS feed from the IFD540 drops. It has happened once to me. If that were to happen, it might have come back by the time you were finished recycling the xpdr causing you to think that the recycle fixed the issue.

I suggest you take Orest's advice about contacting tech support directly via phone.

I'm also having a problem with my mode-c. Got plane back from annual Wednesday and initial contact with atc they said they were not getting my mode c -altitude I guess. Next atc had me recycle the xponder and did not question me after that. Today leaving home airport atc said my altitude was showing ground altitude. I recycled and they said it was still off so I switched to mode a. I have ifd 540, axp322, Tas605 and mlb100. I am at 10.1.2 and mlb 4.10. Any ideas.

Thanks, Cajungene. I'll check the air data page.