14 CFR §135.143

1. (a)No person may operate an aircraft under this part unless that aircraft and its equipment meet the applicable regulations of this chapter.
2. (b)Except as provided in § 135.179, no person may operate an aircraft under this part unless the required instruments and equipment in it have been approved and are in an operable condition.
3. (c)ATC transponder equipment installed within the time periods indicated below must meet the performance and environmental requirements of the following TSO's:
   1. (1)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
      1. (ii)The appropriate class of TSO-C112 (Mode S).
   2. (2)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—
      1. (i)Temporary installation of TSO-C74b or TSO-C74c substitute equipment, as appropriate, during maintenance of the permanent equipment;
      2. (ii)Reinstallation of equipment after temporary removal for maintenance; or
      3. (iii)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.