Where a transponder with Mode C is required14 CFR 91.215 ATC transponder and altitude reporting equipment and use
An operable transponder with Mode C altitude reporting is required in Class A, B and C airspace, within 30 nautical miles of the airports listed in appendix D, section 1, from the surface to 10,000 feet MSL, above the ceiling of Class B and C airspace (within their lateral boundaries) up to 10,000 feet MSL, and at and above 10,000 feet MSL in the 48 contiguous states and DC (excluding at and below 2,500 feet above the surface).
By the ATIS Alerts team · Updated October 15, 2026 · Regulation text current as of October 14, 2026. A study aid based on the official regulation text, not legal advice or an FAA interpretation.
At a glance
- Class A, Class B and Class C airspace areas (b)(1).
- Within 30 nautical miles of an airport listed in appendix D, section 1, from the surface upward to 10,000 feet MSL (b)(2).
- Above the ceiling and within the lateral boundaries of Class B or Class C airspace, upward to 10,000 feet MSL (b)(4).
- In the 48 contiguous states and DC at and above 10,000 feet MSL, excluding the airspace at and below 2,500 feet above the surface (b)(5)(i).
- If you have an operable transponder, it must be on, including Mode C if installed, in this airspace and in all controlled airspace (c).
- Deviation requests: any time for no Mode C or an inoperative transponder; at least one hour ahead if not equipped (d).
In plain English
Section 91.215 says where an aircraft needs an operable transponder with automatic pressure altitude reporting (Mode C), unless ATC authorizes or directs otherwise. The transponder must have Mode A 4096 code or Mode S capability and reply to Mode A interrogations with the code ATC specifies, and the altitude equipment must report pressure altitude in 100-foot increments.
For a typical general aviation airplane, the airspace in (b) is: Class A, Class B and Class C airspace areas; all airspace within 30 nautical miles of an airport listed in appendix D, section 1, from the surface upward to 10,000 feet MSL (informally called the Mode C veil); all airspace above the ceiling and within the lateral boundaries of a Class B or Class C airspace area upward to 10,000 feet MSL; and 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. Paragraph (b)(5)(ii) adds the airspace from the surface to 10,000 feet MSL within 10 nautical miles of any airport listed in appendix D, section 2, excluding the airspace below 1,200 feet outside the lateral boundaries of that airport's surface area.
Some aircraft get relief. An aircraft that was not originally certificated with an engine-driven electrical system (and has not since been certified with one), a balloon or a glider may operate within the 30-nautical-mile area if it stays outside Class A, B and C airspace and below the ceiling of the Class B or C airspace or 10,000 feet MSL, whichever is lower (b)(3). The same aircraft are excluded from the requirements in (b)(5).
Paragraph (c) is the transponder-on rule: in the airspace described in (b), or in any controlled airspace, an aircraft equipped with an operable transponder maintained under 91.413 must operate it, including Mode C if installed, and reply on the appropriate code or as assigned by ATC. Paragraph (d) sets the timing for deviation requests to the ATC facility with jurisdiction: at any time for an aircraft with a working transponder but no working Mode C, at any time for an inoperative transponder going to its destination or a repair location, and at least one hour before the operation for an aircraft not equipped with a transponder.
Common mistakes
- Forgetting the 2,500-foot exception at and above 10,000 feet MSL. Airspace at and below 2,500 feet above the surface is excluded (b)(5)(i), which matters over high terrain.
- Assuming the 30-nautical-mile requirement ends at the Class B boundary. It runs from the surface to 10,000 feet MSL within 30 nautical miles of the listed airport (b)(2).
- Forgetting the one-hour notice for an aircraft with no transponder at all (d)(3).
The official text
14 CFR 91.215 as published on the eCFR, current as of October 14, 2026.
(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 § 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)
Test yourself
Practice questions from our free FAA written test prep that cite 14 CFR 91.215.
Commercial Pilot
A Mode C transponder is required within:
- A. 30 nautical miles of a Class B primary airport, from the surface to 10,000 feet MSL.
- B. 20 nautical miles of a Class B primary airport at all altitudes.
- C. 10 nautical miles of any Class C airport.
- D. 25 nautical miles of a Class B primary airport below 4,000 feet MSL.
Show answer
A. 30 nautical miles of a Class B primary airport, from the surface to 10,000 feet MSL. The 30 NM Mode C veil runs from the surface to 10,000 MSL around each Class B primary airport. Mode C is also required in and above Class C, above 10,000 MSL except within 2,500 AGL, and above Class B.
More Commercial Pilot practice →Basic Ground Instructor
An aircraft operating within 30 nautical miles of a Class B primary airport, below 10,000 feet MSL, must be equipped with:
- A. an operable transponder with altitude reporting and ADS-B Out.
- B. a two-way radio and DME.
- C. a transponder only if entering the Class B itself.
- D. no special equipment below the Class B shelves.
Show answer
A. an operable transponder with altitude reporting and ADS-B Out. This is the Mode C veil: the transponder-with-altitude and ADS-B Out requirements apply throughout the 30 NM ring from the surface up to 10,000 feet MSL, even for flights that never touch the Class B airspace itself.
More Basic Ground Instructor practice →Private Helicopter
An operable transponder with altitude reporting and ADS-B Out is required:
- A. within 30 nautical miles of a Class B primary airport, from the surface up to 10,000 feet MSL.
- B. only inside the lateral boundaries of Class B airspace itself.
- C. everywhere above 3,000 feet AGL.
- D. in Class D airspace at towered airports.
Show answer
A. within 30 nautical miles of a Class B primary airport, from the surface up to 10,000 feet MSL. That 30-NM ring is the Mode C veil, and ADS-B Out (91.225) mirrors it. The equipment is also required in and above Class B and C airspace and above 10,000 feet MSL (excluding at and below 2,500 feet AGL).
More Private Helicopter practice →
Questions pilots ask
Do I need a transponder in Class D airspace?
Class D is not in the list in 91.215(b), but a Class D airport can sit inside one of the areas that is, such as within 30 nautical miles of an appendix D, section 1 airport (b)(2). Class D is controlled airspace, and if the aircraft has an operable transponder, (c) requires it to be on in all controlled airspace.
What is the Mode C veil?
An informal name for the airspace within 30 nautical miles of an airport listed in appendix D, section 1 of Part 91, from the surface upward to 10,000 feet MSL, where a transponder with Mode C is required (91.215(b)(2)).
Do I need a transponder above 10,000 feet?
In the 48 contiguous states and the District of Columbia, yes at and above 10,000 feet MSL, excluding the airspace at and below 2,500 feet above the surface (91.215(b)(5)(i)). Aircraft not originally certificated with an engine-driven electrical system, balloons and gliders are excluded from (b)(5).
Can I fly into Class C with an inoperative transponder?
Only with an ATC authorized deviation. For an inoperative transponder going to the airport of ultimate destination or to a place where repairs can be made, the request may be made at any time (91.215(d)(2)).