Requirements are discussed in the AIM 4-1-20 Transponder Operation. The requirements for transponder operation are covered in FAR § 91.215 for general operation and FAR § 99.13 for ADIZ operation. The Washington DC ADIZ isn’t included in this part, it is handled outside of the FARs as TFRs. There are multiple TFRs that apply to the ADIZ. They require a transponder.
AOPA link has a tutorial on operations within it as does the FAA. All flights within the ADIZ must have an operating transponder and must be squawking specific codes.
§ 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, 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 §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.
§ 99.13 Transponder-on requirements.
(a) Aircraft transponder-on operation. Each person operating an aircraft into or out of the United States into, within, or across an ADIZ designated in subpart B of this part, if that aircraft is equipped with an operable radar beacon transponder, shall operate the transponder, including altitude encoding equipment if installed, and shall reply on the appropriate code or as assigned by ATC.
(b) ATC transponder equipment and use. Effective September 7, 1990, unless otherwise authorized by ATC, no person may operate a civil aircraft into or out of the United States into, within, or across the contiguous U.S. ADIZ designated in subpart B of this part unless that aircraft is equipped with a coded radar beacon transponder.
(c) ATC transponder and altitude reporting equipment and use. Effective December 30, 1990, unless otherwise authorized by ATC, no person may operate a civil aircraft into or out of the United States into, within, or across the contiguous U.S. ADIZ unless that aircraft is equipped with a coded radar beacon transponder and automatic pressure altitude reporting equipment having altitude reporting capability that automatically replies to interrogations by transmitting pressure altitude information in 100-foot increments.
(d) Paragraphs (b) and (c) of this section do not apply to the operation of an aircraft which was not originally certificated with an engine-driven electrical system and which has not subsequently been certified with such a system installed, a balloon, or a glider.
I can’t find TSO-C74a, probably because it is so old. However, TSO-C74b states that all models manufactured “on or after January 26, 1973” must be Mode C capable. This means that they can be switched to report just the transponder code and no altitude—Mode A (technically 3/A). The spec requires that they transmit a “framing pulse” when in code only mode. This allows interrogation by TCAS. The spec also mandates the familiar modes that we see on transponders today.
…controls must be provided to accomplish the following functions:
- a. Selection of reply codes.
- b. Selection of “standby” condition.
- c. Selection of Modes 3/A and C combined.
- d. Activation of identification feature.
- e. Removal of all information pulses on the Mode C reply. [I don’t know what this part means. JS]
ATC authorized deviations can be made by approach control or the tower when in flight. On the ground the appropriate Flight Service Center can be contacted by telephone.
The area referred to in airspace within 30 nautical miles of an airport listed in appendix D, section 1 is know as the Mode C veil. It is a ring centered at one or more airports in Class B airspace and extending for 30 nm. All Class B airspaces have at least one Mode C veil where transponder use is required. Some, like Washington and New York, have more than one ring. It is marked on the sectional chart as a magenta ring with the notation “MODE C” on outside of the ring and “30 NM” on inside at several locations on the ring.
§91.215 (b) (5) (ii) refers to airports listed in Appendix D, section 2. This section is currently empty.
AIM 4−1−20. Transponder Operation
3. Transponder and ADS-B operations on the ground.
Civil and military aircraft should operate with the transponder in the altitude reporting mode (consult the aircraft’s flight manual to determine the specific transponder position to enable altitude reporting) and ADS-B Out transmissions enabled (if equipped) at all airports, any time the aircraft is positioned on any portion of an airport movement area. This includes all defined taxiways and runways.
I leave my Garmin transponder on and in the ALT mode at all times. I do need to remember to switch it to 1200 after landing if I have been flying on a code.
John Collins clarified whether it is possible to fly with an inoperative transponder. If “the altimeter, static system, and encoding altimeter were checked within the previous 24 months and were found in compliance, thus satisfying 91.411(a)(1). Subsequent to that time, the encoder failed.… In the mean time the aircraft can be operated if it is in compliance with “91.213 Inoperative instruments and equipment”, “91.215 ATC transponder and altitude reporting equipment and use”, and “91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.”
Transponder requirements apply to the airspace not the type of flight. So, somewhat counterintuitively, you can file and fly an IFR flight if you don’t have a working transponder as long as you stay out of the airspace where a transponder is required (or comply with the no-transponder requirements of the FARs).
If your Mode C is inoperative, you will be asked to give altitude from time to time. If both are inoperative, you will be asked to give position reports and may be asked to perform maneuvers so that ATC can positively identify the aircraft.
|Class A, B, C||All|
|Above the ceiling of Class B or C and within lateral boundaries||Below 10,000′ MSL|
|Within 30 nm of at least one airport in Class B||Below 10,000′ MSL|
|Within US||Above 10,000′ MSL and above 2,500′ AGL|
|Controlled Airspace–if equipped and maintained.||All|
|DC Special Flight Rules Area||All|
* Exceptions exist for aircraft not originally certified with an electrical system, balloons, and gliders.