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FAR 91.215 states that a mode C transponder is required for...

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

My flight instructor and I disagree on how this regulation is interpreted. He says that I need a transponder to fly underneath class C. I understand it to mean that I only need and transponder if I am both above and within the lateral boundaries (i.e. overflying the airspace).

Which is the correct interpretation?

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  • $\begingroup$ Based on the quote you provided, grammatically you are correct. Inclusion of the work "above", plus use of "and" rather than "or" supports this. I will be interested in what others have to say... $\endgroup$
    – Michael Hall
    Jan 9, 2019 at 22:37
  • $\begingroup$ No, you don't. The class B mode c ring is overriding, however. If you are under c but are somehow within a mode c ring, you need the transponder. $\endgroup$
    – acpilot
    Jan 11, 2019 at 1:31

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You are correct. You do not need a transponder to operate under Class C airspace if there are no other overlapping airspace areas where a transponder is required. In fact, if you read a little further in that reg, you'll see that is one of the places where aircraft built with no electrical system are allowed to fly at all!

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The reg. states that IN or ABOVE CLASS C airspace when class C IS active a MODEC transponder is required, if CLASS C is not active you are in CLASS E airspace and a transponder is NOT required below 10.000 ft MSL, Unless you are in the MODE C VEIL.

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    $\begingroup$ Do you have sources for your answer? $\endgroup$
    – dalearn
    Jun 26, 2020 at 22:07
  • $\begingroup$ §91.215 (b)(4). The answer is correct $\endgroup$
    – Romeo_4808N
    Jun 26, 2020 at 22:56
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This is dependent on where you are and the specifics of the airspace and onboard equipment.

If your aircraft was certificated with an operative engine driven electrical system, you are allowed to fly under a Class C shelf without an operative transponder. See §91.215(b)(4).

If your aircraft was not certificated with an operative engine-driven electrical system AND you are operating within the Mode C veil of an airport described in Part 91, Appendix D(1), you may conduct operations within the Mode C veil, provided they are conducted

  • outside of the Class B and C airspace.
  • Below the ceiling of the class B or C airspace, or 10,000 ft MSL, whichever is higher.

See §91.215(b)(3).

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  • $\begingroup$ Deleting previous comment-- the point was simply that while the two bullet points in this answer do mirror the language of the FARs, there is some ambiguity around what they actually mean. For example, if you are a mile away from the closest part of the Class B/Class C airspace, measured horizontally, do you still have to stay below the ceiling of that airspace? For more, see related question aviation.stackexchange.com/questions/65930/… . $\endgroup$
    – quiet flyer
    Feb 12 at 17:51

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