14.mode-c-veil-and-transponder-requirements. Mode C Veil and Transponder Requirements
The Mode C veil is the airspace within 30 nautical miles of an airport listed in 14 CFR Part 91, Appendix D, Section 1 (primarily the busiest Class B primary airports). Within this veil, from the surface up to 10,000 feet MSL, all aircraft must be equipped with an operable Mode C transponder (or Mode S) that reports pressure altitude, and—since January 1, 2020—with ADS-B Out that meets the performance requirements of 14 CFR 91.225 and 91.227.
The transponder and ADS-B Out requirements apply not only within the Mode C veil but also in several other defined volumes of airspace. Per 14 CFR 91.215 and 91.225, an operable Mode C transponder with altitude encoding and ADS-B Out are required in:
- All Class A, Class B, and Class C airspace.
- Class E airspace at or above 10,000 feet MSL, excluding airspace at and below 2,500 feet AGL.
- Within 30 NM of a Class B primary airport (the Mode C veil), from the surface to 10,000 feet MSL.
- Above the ceiling and within the lateral boundaries of Class B or Class C airspace up to 10,000 feet MSL.
- Class E airspace over the Gulf of Mexico, at and above 3,000 feet MSL, within 12 NM of the U.S. coast (ADS-B Out only).
Exception for aircraft not originally certificated with an engine-driven electrical system: Aircraft such as gliders, balloons, and certain antique airplanes (e.g., a Piper J-3 Cub without an electrical system) are exempt from the Mode C and ADS-B Out requirement except in Class A and Class B airspace itself. They may operate within the 30 NM Mode C veil—but outside the lateral limits of the Class B—without a transponder, provided they remain below the Class B floor or below 10,000 feet MSL.
Transponder codes. The transponder squawk code is a 4-digit octal number (0000–7777), yielding 4,096 possible codes. ATC assigns a discrete code for traffic identification. Pilots must memorize three emergency codes:
- 7500 — hijacking or unlawful interference.
- 7600 — two-way radio communications failure (lost comm).
- 7700 — general emergency.
Under VFR with no ATC assignment, pilots squawk 1200. Gliders squawk 1202 when not in contact with ATC.
Operating the transponder. Position the transponder in the ALT (Mode C) position before taxiing onto an active runway and throughout flight, unless ATC directs otherwise. Use STBY during taxi to avoid cluttering ground radar with low-altitude returns at busy fields. The IDENT button is pressed only when ATC requests "ident"; this momentarily highlights the target on the controller's scope.
ADS-B Out integration. ADS-B Out broadcasts the aircraft's GPS-derived position, altitude, velocity, and identification once per second. Two link types are approved: 1090ES (1090 MHz Extended Squitter), required for operations above 18,000 feet MSL and used internationally; and 978 UAT (Universal Access Transceiver on 978 MHz), usable only below 18,000 feet MSL within the United States. Both satisfy the 91.225 rule-airspace requirement.
Deviations and inoperative equipment. 14 CFR 91.215(d) permits ATC to authorize a deviation from the transponder requirement, and 91.225(g) permits a deviation from the ADS-B Out requirement, when equipment is inoperative or not installed. Requests for a deviation generally must be submitted at least one hour before the proposed operation via the FAA's ADS-B Deviation Authorization Preflight Tool (ADAPT) for ADS-B; for transponder-only deviations, the request is made directly to the controlling ATC facility. ATC may approve or deny based on workload and traffic.
Example. A student departing a Class D field located 18 NM from a Class B primary airport is operating inside the Mode C veil. Even if the flight remains in Class D and Class E airspace and never enters Class B, the aircraft must have an operable Mode C transponder and ADS-B Out (unless it qualifies for the no-electrical-system exception). If the encoder fails in flight, the pilot may continue to the destination and request a deviation from ATC for any subsequent flight.
Pretransponder checks. Before flight, verify (1) the transponder code is set correctly, (2) the altitude reporting (Mode C) is functional—ATC can confirm during a radio check by comparing the Mode C readout to the pilot-reported altitude (must agree within ±300 feet), and (3) ADS-B Out is broadcasting properly. Periodic transponder tests and inspections are required every 24 calendar months under 14 CFR 91.413, and ADS-B Out installation must meet the performance standards of 14 CFR 91.227.