While the TT21 is certified to TSO C166B, the TT21 is a Class 2 transponder and therefore does not meet the power output requirements for FAA transponder and ADS-B regulations. Currently, the FAA requirement for United States airspace is that ADS-B Out is in conjunction with a Class 1 transponder. Other ADS-B environments (ex. Europe and Australia) do not require a Class 1 transponder.

It is recommended that TT21 owners transition to the higher power, Class 1 TT22 to fully comply with the FAA ADS-B performance requirements.