TT21 Class 2 Mode S Transponder

Part No: 00710-00

Estimated Availability: October 2021
Shipping calculated at checkout.

Trig TT21 Mode S Transponder

Note: The TT21 does not meet FAA Approval for ADS-B Out in the USA. For USA aircraft, please order the Trig TT22.

Trig makes the world’s smallest and lightest Mode S certified GA transponders. The TT21 Trig compact transponder can fit into to the tightest panel space. This allows general aviation, light sport and glider pilots to fit other instrumentation or displays that otherwise might not have been possible. The TT21 is a class 2 transponder and our most popular compact transponder model.

  • Compact size - takes minimal panel space, fits in a standard instrument hole
  • Integrated altitude encoder
  • Certified for IFR / VFR flight
  • Certified class 2 transponder 130 watts nominal output
  • Low weight – 440g (0.8 lbs)
  • Low power consumption
  • 1090ES ADS-B Out capable
  • EASA ETSO and FAA TSO Certification – includes TSO C166b the latest ADS-B standard
  • Unbeatable value and quality – designed and manufactured in the U.K.
  • Two year worldwide warranty

The TT21 is a two part transponder with a compact control head; this includes an inbuilt altitude encoder - saving further space and expense. A separate hardware box can be installed anywhere in the airframe. The mounted control head is only 54mm (2 3/25”) deep and can be fitted in panel using a 57mm (2 ¼”) round hole or smaller compact mount, the unit ships with the necessary fittings for both install options. The TT21 unit is highly efficient and is ideal for use via an auxiliary battery in gliders, balloons and foot launched flight.

Trig’s TT21 is a popular compact transponder with 130 watts nominal power output. This is a class 2 transponder which meets international standards for 1090ES ADS-B Out (TSO C166b). The TT21 is ideal for use in flight where a light weight Mode S solution is required and is suitable for IFR and VFR use.

Older mode A and C transponders are now being replaced by Mode S, technology which is optimised for tomorrow’s airspace requirements. The TT21 transponder is easy to operate and with a crisp, bright display it is also easy to read in all lighting conditions. The transponder control head is splash proof too so it’s ideal for open cockpit and seaplane operations.

(Please note in the U.S. to comply with the FAA’s 2020 mandate for ADS-B Out a certified Class 1 transponder is required after 2020, the Trig TT22 matches this requirement being a higher powered Class 1 device).

All Trig products come with a two year warranty, supported by our global network of Approved Trig Dealers and Approved Trig Service Centers in the U.S. and Australia.



  • Smallest & lightest Class 2 Mode S Transponder
  • Designed for Light Aviation
  • For a Higher Performance Aircraft Option, View TT22 
  • Class 2 Transponder is suitable for aircraft cruising slower than 175 knots, and operating below 15,00 feet, VFR or IFR.
  • Remote Mount 2 Part Transponder System: panel controller & transponder box 
  • 130 Watts nominal output
  • Control includes an integrated altitude encoder, up to 35,000 ft
  • Total weight just over 1 lb
  • Minimal Panel Space, can be installed in standard 2¼" (57mm) cut-out or compact mounting just 42mm high
  • Simple Installation
  • Controller acts as a configuration module for the system - easily swap transponder box without reprogramming
  • Fully certified and compliant with all transponder requirements TSO approved for European & FAA registered aircraft
  • Includes TIS (Traffic Information Service) functionality
  • NOTE: While the TT21 is capable of 1090ES ADS-B Out with the appropriate GPS, it is NOT an acceptable ADS-B transponder for certified aircraft because of its Class 2 status. (Due to current FAA regulations)


Specification TT21 Transponder TT22 Transponder
Type Transponder Class 2 Mode S level 2els ADS-B Class B0 Transponder Class 1 Mode S level 2els
ADS-B Class B1S
Certification ETSO C88A, C112C, C166A
TSO C88b, C112c, C166b, approved for IFR and VFR flight
Compliance ED-73C, DO-160F, DO-178B Level B, DO-254
Level C, DO-260B, DO-181D
Supply voltage (DC) 9-33 V
Typical consumption (at 14v) idle: 0.15 A active: 0.28 A idle: 0.15A active: 0.34 A
Nominal Transmitter Power 130 W at connector 250 W at connector
Operating temperature for the transponder -40°C to +70°C
for the controller -25°C to +70°C
Cooling requirement no fan required
Weight (hardware back box) 440g / 0.8 lbs
Weight (TC20 control head) 90g / 0.11 lbs
Dimensions (mm) controller: H 44 x W 63 x L 54 mm
transponder in tray: H 48 x W 68 x L 160 mm
Dimensions (inches) controller: H 1.8 x W 2.4 x L 2.1 inches
transponder in tray: H 1.8 x W 2.6 x L 6.2 inches





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TT21 Class 2 Mode S Transponder


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