Why does part (b) list individual sections where transponder operations are required when part (c) states that transponder operation is required in all controlled airspace? All sections listed in part (b) are part of controlled airspace anyhow? Similarly, the AIM says that transponder operation is required the moment the aircraft starts moving until the time it stops moving (a simplified interpretation).

Am I missing some subtleties?


CFR §91.215:

Part (b) contains sections that specify precisely where transponder operation is required. For example:

(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;

(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) (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

(5) (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.

Then part (c) says:

(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, unless otherwise directed by ATC when transmitting would jeopardize the safe execution of air traffic control functions.

The AIM says:

AIM Section 1:

4−1−20. Transponder Operation a. General

  1. 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 at all airports, any time the aircraft is positioned on any portion of the airport movement area. This includes all defined taxiways and runways.

  2. Transponder and ADS­B Operations While Airborne. Unless otherwise requested by ATC, aircraft equipped with an ATC transponder maintained in accordance with 14 CFR Section 91.413 MUST operate with this equipment on the appropriate Mode 3/A code, or other code as assigned by ATC, and with altitude reporting enabled whenever in controlled airspace.

  • $\begingroup$ I know I’ve seen this question before, but I can’t find it. Short version is that B is negative (you can’t fly here if you don’t have it) whereas C is positive (once you have it, you must use it everywhere). $\endgroup$
    – StephenS
    Dec 14, 2020 at 3:52
  • 1
    $\begingroup$ Aircrafts are not even required to have electrical systems for Day VFR in some airspaces.... $\endgroup$
    – Dave
    Dec 14, 2020 at 4:21

1 Answer 1


I think what you're missing is that not all aircraft have transponders. The key words from 91.215(c) are (emphasis mine):

[...] each person operating an aircraft equipped with an operable ATC transponder maintained [...]

To put it another way, (b) says "in order to fly in these areas you must have a transponder". Whereas (c) says "if you have a transponder, this is when it needs to be on".


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