ATC Transponder and Altitude reporting equipment
(Federal Aviation Regulation Part 91 Section 215)
All airspace: U.S.-registered civil aircraft. For operations not conducted under
part 121 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).
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
All aircraft. In Class A, Class B, and Class C airspace areas;
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;
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 --
Outside any Class A, Class B, or Class C airspace area; and
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
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
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 -- --
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
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.
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 §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.
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:
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.
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.
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.