Radio equipment

This regulation establishes radio equipment requirements for Part 121 air carrier operations. Aircraft must carry the radio equipment appropriate for their specific type of operation.

When regulations require two independent radio systems (as specified in §§121.347 and 121.349), each system needs its own antenna installation. However, if you use rigidly supported non-wire antennas or similarly reliable designs, a single antenna suffices.

For ATC transponders, the regulation sets technology standards based on installation dates. Equipment installed after January 1, 1992, must meet Mode S transponder standards (TSO-C112). The rule clarifies that "installation" doesn't include temporary equipment swaps during maintenance, reinstalling equipment after maintenance, or moving equipment between aircraft in the same operator's fleet for maintenance purposes.

These requirements ensure air carriers maintain reliable communication and transponder capabilities, with flexibility for routine maintenance activities that don't constitute new installations requiring upgraded equipment.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*