14 CFR § 121.345

No person may operate an airplane unless it is equipped with radio equipment required for the kind of operation being conducted.

Where two independent (separate and complete) radio systems are required by §§ 121.347 and 121.349, each system must have an independent antenna installation except that, where rigidly supported nonwire antennas or other antenna installations of equivalent reliability are used, only one antenna is required.

ATC transponder equipment installed within the time periods indicated below must meet the performance and environmental requirements of the following TSO's:

Through January 1, 1992: (i) Any class of TSO-C74b or any class of TSO-C74c as appropriate, provided that the equipment was manufactured before January 1, 1990; or

The appropriate class of TSO-C112 (Mode S).

After January 1, 1992: The appropriate class of TSO-C112 (Mode S). For purposes of paragraph (c) (2) of this section, “installation” does not include—

Temporary installation of TSO-C74b or TSO-C74c substitute equipment, as appropriate, during maintenance of the permanent equipment;

Reinstallation of equipment after temporary removal for maintenance; or

For fleet operations, installation of equipment in a fleet aircraft after removal of the equipment for maintenance from another aircraft in the same operator's fleet.