Where is a transponder required in aviation?

A Mode C transponder gives air traffic control (ATC) your altitude and an identification code. Specifically, a Mode C transponder is required if you wish to operate in Class A, B, or C airspace, at an altitude of over 10,000' MSL, or within a 30-nautical mile radius of the primary airport in Class B airspace.



Under current FAA 14 CFR § 91.215 regulations in 2026, a transponder with altitude reporting (Mode C or S) is required in several specific airspaces across the United States. This includes Class A, B, and C airspace at all altitudes, as well as within a 30-nautical-mile radius of the primary airport in Class B airspace (the "Mode C Veil"). Furthermore, a transponder is mandatory for any flight above 10,000 feet MSL, excluding the airspace at or below 2,500 feet AGL. It is also required above and within the lateral boundaries of Class B and C airspace up to 10,000 feet. In 2026, most of these requirements are paired with ADS-B Out mandates, which require aircraft to broadcast their position and velocity. Exceptions are limited to aircraft without electrical systems, such as certain gliders or balloons, provided they remain outside controlled Class B or C areas and below the 10,000-foot threshold.

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A Primary Radar System. Typically located on the control tower or other strategic location on the airport, the Primary Radar antenna is able to detect and display aircraft that are not equipped with or have malfunctioning transponders or ADS-B.

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From the 1940s to the early 1960s, ATC only had primary radar available. Over half a century later, when an aircraft's transponder fails, modern radar controllers are trained to use some “back to basics” primary radar techniques to work around the problem and identify aircraft.

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Transponder
  • Transponder A: sends back a transponder code to ATC.
  • Transponder C: sends a transponder code plus altitude information to ATC.
  • Transponder S: sends a transponder code plus altitude information to ATC, and receives and reports information to other transponders of the same type.


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Class G airspace (uncontrolled) is that portion of airspace that has not been designated as Class A, Class B, Class C, Class D, or Class E airspace. Rules governing VFR flight have been adopted to assist the pilot in meeting the responsibility to see and avoid other aircraft.

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