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EAA AirVenture Oshkosh RSS Feed Transponder Tech: Getting to 2020

By Joseph E. (Jeb) Burnside

July 31, 2013 - On January 1, 2020, aircraft operators wanting to access certain airspace in the United States must be equipped with ADS-B Out technology, according to existing regulations. The airspace in question basically is the same as where a Mode C transponder is required today. The question of when, how, and with what to update an in-service aircraft's avionics to meet the ADS-B Out mandate remains unanswered for many owners, but new transponders from the "usual suspects" of avionics manufacturers is a good place to start.

The transponder is a key component of any effort to update existing avionics to the ADS-B standard because the vast majority of in-service aircraft sport a Mode C-compliant installation. Mode C, of course, provides position information for secondary surveillance radars as well as altitude. Mode S does all that, also, but adds ADS-B Out information, including more accurate position plus the aircraft's registration number or call sign.

Importantly, the ADS-B Out requirement includes the ability to transmit navigation-system-enabled position data. As such, merely installing a Mode S transponder doesn't equate to full ADS-B Out compliance; that requires equipment that meets performance requirements of FAR 91.227; TSO'd WAAS GPS does that.

The good news? Today's Mode S transponders offer lots of capability and numerous features. Plus, many available models are a slide-in replacement for the venerable and ubiquitous Bendix/King KT-76A Mode A/C transponder. The bad news is they aren't exactly free, and ensuring their compliance with ADS-B equipment requirements likely will require additional wiring, even if you're replacing a KT-76A. Here's a rundown on four Mode S transponders on display at EAA AirVenture Oshkosh 2013 that will get operators started on their ADS-B Out-compliant panel.


Avidyne AXP340
Avidyne's AXP340 transponder is a panel-mounted Class 1 Mode S Level 2 device supporting the 1090 MHz extended squitter (ES) standard for aircraft cruising at/above FL180 after 2019. It's a slide-in replacement for existing KT-76A/KT-78A transponders, and adds capabilities like a direct-entry keypad, pressure altitude, and GPS-derived latitude/longitude readout, flight ID entry, and a one-button VFR code, plus a stopwatch/timer, flight timer, and altitude alerter. The AXP340 retails for $5,995.


Bendix/King KT-74
Like Avidyne's offering, the newly announced Bendix/King KT-74 transponder provides Class 1 Mode S Level 2 functionality and certification under TSO C112D, C166B and ETSO C112C, and C166B, meaning it complies with applicable U.S. and European standards and requirements. It also is a slide-in replacement for existing KT-76/78 installations. An eight-digit keypad, a VFR squawk button, pressure altitude display, and high power output round out the product's features. The KT-74 has a retail price of $2,999.


Garmin GTX 330/33/23 Series
Garmin International offers three different Mode S transponders, each of which delivers 250 watts of transmission power. The GTX 330 is the panel-mount version; the GTX 33 is the remote product, for use with the company's GTN 650/750 and G1000 products; and the GTX 23 also is a remote-mounted design, for installation in experimental aircraft using a G3X primary flight display. Retail pricing for the GTX 330, GTX 33, and GTX 23 ES is $4,995, $4,195 and $2,450, respectively.


Trig TT31
Trig, based in Scotland, is a relatively young company specializing in helping European aircraft operators meet that region's avionics requirements. The TT31 is an efficient Mode S transponder that also can use the same mounting tray as the KT-76A. Although it meets European requirements, it also is approved for U.S. operators, since it meets the same standards. Rather than the push-buttons becoming popular on offerings from other companies, however, Trig's TT31 uses rotary knobs and also includes a timer/stopwatch function plus an altitude monitor. Trig's TT31 retails for $3,580.


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