AXP340/322 ADS-B Transponder - Nerd question: ADS-B emergency flags

Print Page | Close Window

Nerd question: ADS-B emergency flags

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=1790
Printed Date: 26 Mar 2026 at 2:12pm
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: Nerd question: ADS-B emergency flags

Posted By: paulr
Subject: Nerd question: ADS-B emergency flags
Date Posted: 28 Jun 2019 at 6:54am

Last night our local flying club had its quarterly safety meeting. One of the speakers is part of the team equipping US Army fixed-wing aircraft with ADS-B, and he dropped a few factoids in his presentation that I didn't know before.

One was that the standard set of ADS-B Out parameters (there are 19 of 'em in case you were curious) includes an "emergency" flag. This may, but does not have to be, set by the transmitter when an emergency is indicated. See https://www.law.cornell.edu/cfr/text/14/91.227" rel="nofollow - FAR 91.227(d)(9) . This is formally known as the "Emergency / Priority Status Selection" or EPSS, and it uses a type code of 28, so I'll just call it TC28.

What I'm curious about is which GA transponders actually transmit this flag. I haven't found a regulatory requirement that the transponder emit a TC28 message when the pilot selects an emergency squawk code. Does the AXP series do this?

Replies:

Posted By: paulr
Date Posted: 02 Jul 2019 at 4:17am

The requirement to do this is covered both in the FAR and in AMOC. So I am going to guesstimate that the AXP340 meets this requirement.. but I'm sure not gonna test it on my own!

14 CFR § 91.227 - Automatic Dependent Surveillance-Broadcast (ADS-B) Out equipment performance requirements

...

(d) Minimum Broadcast Message Element Set for ADS-B Out. Each aircraft must broadcast the following information, as defined in TSO-C166b or TSO-C154c. The pilot must enter information for message elements listed in paragraphs (d)(7) through (d)(10) of this section during the appropriate phase of flight.

...

(9) An indication if the flightcrew has identified an emergency, radio communication failure, or unlawful interference;

And the AMOC:

AC 20-165B 3.7.3.2 Emergency Status

The installation must provide a means for the pilot to enter the emergency status of the aircraft. Although TSO-C166b and TSO-C154c identify multiple emergency codes, only the codes for general emergency, no communications, and unlawful interference are required to be available for broadcast. It is acceptable to base the ADS-B emergency status on the emergency status code input into the transponder (that is, Mode 3/A codes 7500, 7600, and 7700). Refer to section 3.7.3.5 of this AC for information on single point of entry of the emergency status.

Posted By: AviSteve
Date Posted: 03 Jul 2019 at 3:46am

Yes, AXP340 is compliant with DO-260B, which specifies "The Extended Squitter Aircraft Status Message (TYPE Code=28)"

-------------
Steve Lindsley
Avidyne Engineering

Posted By: paulr
Date Posted: 03 Jul 2019 at 3:57am

Thanks, Steve. The speaker painted this capability as optional, not a requirement, which I think is incorrect based on my reading. He also said a couple of other things that turned out upon research to be wrong too, so I guess I won't be asking him for any more avionics advice.