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The Pilots Desk
US-FAA14 CFR 91.215

ATC transponder and altitude reporting equipment and use

Read the official rule

In plain English

A Mode C (altitude-reporting) transponder is required in the busiest airspace: Class A, B and C, within and above Class B and C, and generally at and above 10,000 ft MSL (excluding the airspace below 2,500 ft AGL). ADS-B Out is required in the same airspace under 91.225.

Key points

  • Required in/above Class B and C, in Class A.
  • Required at and above 10,000 ft MSL (except below 2,500 ft AGL).
  • The "Mode C veil" extends 30 NM around the primary Class B airport.
  • Pair with ADS-B Out (91.225) for the same airspace.

Common pitfalls

  • Forgetting the 30 NM Mode C veil even when below the Class B shelf.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*

This is an original plain-English explanation for training and reference, not legal advice and not for navigation. Always rely on the current official rule linked above. Last reviewed June 7, 2026.