The TT21 is a two part transponder system, which includes an integrated altitude encoder. In design and technology the TT21 is identical to the TT22; the only difference is that the transmitted output power of the TT21 is lower, qualifying the unit as a Class 2 transponder. Class 2 transponders may be used on aircraft cruising slower than 175 knots, and operating below 15,000 feet, IFR or VFR.
The front panel controller is smaller than a standard instrument, and the remote transponder is not much bigger than a pack of cards. The TT21 can fit into the smallest of aircraft– and yet it is fully certified and compliant with all the transponder requirements.
The TT21 can be mounted in a standard 57 mm (2¼ inch) instrument hole, or where space is really tight, in a compact mounting just 42 mm high.
The TT21 is TSO approved for fitting to European and FAA registered aircraft.
