Federal Aviation Regulations
14 CFR § 91.215 — ATC transponder and altitude reporting equipment and use
Volume 2 · Title 14 of the Code of Federal Regulations, 2025 annual edition. U.S. Government work — public domain (17 U.S.C. §105).
The regulation, in full
Sec. 91.215 ATC transponder and altitude reporting equipment and use.
(a) 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).
(b) All airspace. Unless otherwise authorized or directed by ATC,
and except as provided in paragraph (e)(1) of this section, no person
may operate an aircraft in the airspace described in paragraphs (b)(1)
through (5) of this section, unless that aircraft is equipped with an
operable coded radar beacon transponder having either Mode A 4096 code
capability, replying to Mode A interrogations with the code specified by
ATC, or a Mode S capability, replying to Mode A interrogations with the
code specified by ATC and Mode S interrogations in accordance with the
applicable provisions specified in TSO-C112, 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. The
requirements of this paragraph (b) apply to--
(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. Except as provided in paragraph (e)(2)
of this section, 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 shall operate the transponder, including
Mode C equipment if installed, and shall reply on the appropriate code
or as assigned by ATC, unless otherwise directed by ATC when
transmitting would jeopardize the safe execution of air traffic control
functions.
(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.
(e) Unmanned aircraft. (1) The requirements of paragraph (b) of this
section do not apply to a person operating an unmanned aircraft under
this part unless the operation is conducted under a flight plan and the
person operating the unmanned aircraft maintains two-way communication
with ATC.
(2) No person may operate an unmanned aircraft under this part with
a transponder on unless:
(i) The operation is conducted under a flight plan and the person
operating the unmanned aircraft maintains two-way communication with
ATC; or
(ii) The use of a transponder is otherwise authorized by the
Administrator.
(Approved by the Office of Management and Budget under control number
2120-0005)
[Docket 18334, 54 FR 34304, Aug. 18, 1989, as amended by Amdt. 91-221,
56 FR 469, Jan. 4, 1991; Amdt. 91-227, 56 FR 65660, Dec. 17, 1991; Amdt.
91-227, 7 FR 328, Jan. 3, 1992; Amdt. 91-229, 57 FR 34618, Aug. 5, 1992;
Amdt. 91-267, 66 FR 21066, Apr. 27, 2001; Amdt. 91-355, 84 FR 34287,
July 18, 2019; Amdt. 91-361, 86 FR 4512, Jan. 15, 2021; Docket FAA-2023-
1836, Amdt. 91-371, 88 FR 71476, Oct. 17, 2023]What this means for the knowledge test
Our own study notes for this section — reviewed through our AI instructional-review system, not by a certificated flight instructor. Not an authoritative source.
A Mode C transponder and ADS-B Out are generally required in Class A, B, and C airspace, above the Class B and Class C airspace up to 10,000 feet MSL, and generally at and above 10,000 feet MSL (excluding airspace at and below 2,500 feet AGL). Requirements should always be confirmed against current regulations before flight.
14 CFR 91.215 / 91.225
Lessons that cover § 91.215
Practice questions on § 91.215 (1)
Which equipment/communication combination is required to operate in Class C airspace?
Answer Two-way radio, Mode C transponder, and ADS-B Out
Class C requires two-way communication established before entry, a Mode C transponder, and ADS-B Out.