(a) All airspace: U.S.-registered civil aircraft. For operations not
  conducted under part 121, 127 or 135 of this chapter, ATC transponder
  equipment installed must meet the performance and environmental requirements
  of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with
  altitude reporting capability) as appropriate, or the appropriate class of
  TSO-C112 (Mode S).
    (b) All airspace. Unless otherwise authorized or directed by ATC, no person
  may operate an aircraft in the airspace described in paragraphs (b)(1)
  through (b)(5) of this section, unless that aircraft is equipped with an
  operable coded radar beacon transponder having either Mode 3/A 4096 code
  capability, replying to Mode 3/A interrogations with the code specified by
  ATC, or a Mode S capability, replying to Mode 3/A interrogations with the
  code specified by ATC and intermode and Mode S interrogations in accordance
  with the applicable provisions specified in TSO C-112, and that aircraft is
  equipped with automatic pressure altitude reporting equipment having a Mode C
  capability that automatically replies to Mode C interrogations by
  transmitting pressure altitude information in 100-foot increments. This
  requirement applies--
    (1) All aircraft. In Class A, Class B, and Class C airspace areas;
    (2) All aircraft. In all airspace within 30 nautical miles of an airport
  listed in appendix D, section 1 of this part from the surface upward to
  10,000 feet MSL;
    (3) Notwithstanding paragraph (b)(2) of this section, any aircraft which
  was not originally certificated with an engine-driven electrical system or
  which has not subsequently been certified with such a system installed,
  balloon or glider may conduct operations in the airspace within 30 nautical
  miles of an airport listed in appendix D, section 1 of this part provided
  such operations are conducted--
    (i) Outside any Class A, Class B, or Class C airspace area; and
    (ii) Below the altitude of the ceiling of a Class B or Class C airspace
  area designated for an airport or 10,000 feet MSL, whichever is lower; and
    (4) All aircraft in all airspace above the ceiling and within the lateral
  boundaries of a Class B or Class C airspace area designated for an airport
  upward to 10,000 feet MSL; and
    (5) All aircraft except any aircraft which was not originally certificated
  with an engine-driven electrical system or which has not subsequently been
  certified with such a system installed, balloon, or glider----
    (i) In all airspace of the 48 contiguous states and the District of
  Columbia at and above 10,000 feet MSL, excluding the airspace at and below
  2,500 feet above the surface; and
    (ii) In the airspace from the surface to 10,000 feet MSL within a 10-
  nautical-mile radius of any airport listed in appendix D, section 2 of this
  part, excluding the airspace below 1,200 feet outside of the lateral
  boundaries of the surface area of the airspace designated for that airport.
    (c) Transponder-on operation. While in the airspace as specified in
  paragraph (b) of this section or in all controlled airspace, each person
  operating an aircraft equipped with an operable ATC transponder maintained in
  accordance with Sec. 91.413 of this part shall operate the transponder,
  including Mode C equipment if installed, and shall reply on the appropriate
  code or as assigned by ATC.
    (d) ATC authorized deviations. Requests for ATC authorized deviations
  must be made to the ATC facility having jurisdiction over the concerned
  airspace within the time periods specified as follows:
    (1) For operation of an aircraft with an operating transponder but without
  operating automatic pressure altitude reporting equipment having a Mode C
  capability, the request may be made at any time.
    (2) For operation of an aircraft with an inoperative transponder to the
  airport of ultimate destination, including any intermediate stops, or to
  proceed to a place where suitable repairs can be made or both, the request
  may be made at any time.
    (3) For operation of an aircraft that is not equipped with a transponder,
  the request must be made at least one hour before the proposed operation.