AXP340/322 ADS-B Transponder - Wrong transponder number after landing and t/o

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Wrong transponder number after landing and t/o

Printed From: Avidyne
Category: Avidyne General
Forum Name: AXP340/322 ADS-B Transponder
Forum Description: Topics on the Avidyne AXP340/322 ADS-B Transponder
URL: https://avidynelive.com/forum_posts.asp?TID=2646
Printed Date: 26 Mar 2026 at 2:12pm
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: Wrong transponder number after landing and t/o

Posted By: _phm_
Subject: Wrong transponder number after landing and t/o
Date Posted: 30 Apr 2024 at 6:45am

New owner of an AXP322.

Flew to an uncontrolled strip a couple of days ago. Enroute got a squawk code from ATC.

After landing, the transponder correctly switched to 2000 (Brazilian vfr general code).

A couple of days later, before taking off, I checked the transponder on the ground and it was correctly showing 2000.

Took off and, when reaching class C airspace, got assigned a new code. To my surprise, when I went into the transponder page to set the new code it was showing the previous code from a couple of days ago !!!

Any thoughts about this weird behavior?

Replies:

Posted By: MysticCobra
Date Posted: 03 May 2024 at 5:52am

Huh. The AXP322 Pilot Guide is unfortunately not posted on the Avidyne website Documentation page. (I wonder why that is?) I was hoping it would shed light on expected behavior.

The AXP340 (panel mounted transponder) does not change the transponder code based on squat switch--it only disables ALT and ON mode while on the ground. The manual is not clear about how it behaves on takeoff; I presume it returns to the last mode it was in if the pilot hasn't changed anything. (I don't have a squat switch in my plane.)

Posted By: _phm_
Date Posted: 03 May 2024 at 4:28pm

I don't have a squat switch either.

As far as I know, even without this, the AXP322 nicely transitions back to the general VFR code when it senses that you are back on the ground, while also switching into GND mode.

Posted By: MysticCobra
Date Posted: 04 May 2024 at 4:37am

Hm. I wonder what data it uses to make that transition in lieu of a squat swich?

I was going to look in the pilot guide for information, but I couldn't find it. As an owner, you presumably have one. What does it say about how the AXP322 is supposed to behave, or what controls you have to adjust its behavior?

Posted By: _phm_
Date Posted: 04 May 2024 at 1:32pm

I only have the "AXP322 MODE S REMOTE TRANSPONDER INSTALLATION MANUAL", which says nothing about this behavior.

After a couple of landings where I noticed that the transponder went automatically back into GND mode, showing the default VFR code, I infer that its behavior is, based on the aircraft velocity, switch between GND <> ALT <> GND accordingly.

Posted By: nrproces
Date Posted: 05 May 2024 at 5:40am

I have a 340 in my baby, that is the way it has worked (going to 1200) since it was installed, perhaps it works the same way?

this is from the AXP340 Mode S Transponder Pilot guide in the post below.

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Sauce

Posted By: nrproces
Date Posted: 05 May 2024 at 5:48am

Aircraft installations that include a ground/air state input (e.g. gear squat switch or wired discrete from a GPS/Nav/Com such as the IFD 540) will automatically select GND on landing or while taxiing and will automatically select ALT when airborne. In these installations, the pilot selectable states are ALT-ON-SBY when in air and GND-SBY when on-ground

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Sauce

Posted By: _phm_
Date Posted: 05 May 2024 at 6:00am

This is my setup - the AXP322 is connected to my IFD440, and I see the GND<>ALT<>GND transitions working well

The issue is the transponder remembering the last squak code after landing and taking off again.

If this is really what happened, and I am quite sure it was, this behavior is wrong.