The Mode C Veil is a ring of airspace extending 30 nautical miles outward from certain major airports in the United States, stretching from the surface up to 10,000 feet above mean sea level. Any aircraft flying inside this ring generally needs a working transponder with altitude-reporting capability and ADS-B Out equipment. The name comes from “Mode C,” the transponder function that automatically broadcasts your aircraft’s altitude to air traffic control.
Where the Mode C Veil Exists
Mode C Veils surround the primary airports associated with Class B airspace, the busy terminal areas around cities like Seattle, Phoenix, Cleveland, Houston, and San Diego. The full list is found in Appendix D, Section 1 of 14 CFR Part 91, and it covers roughly three dozen of the country’s busiest airports. The veil itself is much larger than the Class B airspace it surrounds. Class B airspace has a layered, upside-down wedding cake shape that typically extends 10 to 20 nautical miles from the airport. The Mode C Veil, by contrast, is a simple 30-nautical-mile circle drawn around that same airport, reaching all the way to the ground.
This matters because you can be well outside the controlled Class B airspace, flying over a rural area or a small uncontrolled airport, and still be inside the Mode C Veil. If you are, your transponder and ADS-B equipment requirements still apply.
Why It Exists
The veil gives air traffic controllers a buffer zone of altitude data around the busiest airports. When every aircraft inside that 30-mile ring is broadcasting its altitude, controllers and collision-avoidance systems can track traffic that might eventually enter the Class B airspace itself. Without this requirement, aircraft could approach a major terminal area invisible to radar altitude readouts, creating a significant safety gap in some of the most congested airspace in the country.
Equipment You Need Inside the Veil
To fly inside a Mode C Veil, your aircraft needs two things. First, a working transponder capable of Mode A (which transmits an identification code assigned by ATC) and Mode C (which automatically reports your pressure altitude in 100-foot increments). A Mode S transponder, which provides additional data, also satisfies this requirement.
Second, since January 1, 2020, you also need ADS-B Out equipment. This is a system that broadcasts your position, altitude, and other flight data using GPS. For aircraft operating below 18,000 feet within the U.S., this can be either a 1090ES system or a UAT (Universal Access Transceiver) system. The FAA requires that these meet specific performance standards, commonly referred to as “Version 2” equipment. An older transponder alone is no longer enough.
Both systems must be turned on and functioning. The regulations require that any time you’re in the veil (or any controlled airspace, for that matter), your transponder operates on the correct code and your altitude reporting is active.
How to Identify It on a Chart
On a VFR sectional chart, the Mode C Veil appears as a thin magenta circle centered on the primary Class B airport. It is easy to confuse with the Class B airspace boundaries themselves, which are depicted with solid blue lines in a tiered pattern. The Mode C Veil ring is a single, continuous circle at the 30-nautical-mile radius. Terminal area charts show the same boundary. If you’re flight planning near any Class B airport, look for that outer magenta ring to know where Mode C requirements begin.
Exceptions to the Rule
A few categories of aircraft can fly inside the Mode C Veil without a transponder or ADS-B Out. These include gliders, balloons, and aircraft that were never originally certificated with an engine-driven electrical system (and haven’t had one installed since). Many older taildraggers and some ultralight-style aircraft fall into this last category.
Even with this exception, these aircraft still cannot enter Class A, Class B, or Class C airspace. They must also stay below the ceiling of any Class B or Class C airspace designated for that airport, or below 10,000 feet, whichever is lower. In practice, this means an aircraft without an electrical system can transit the outer portions of a Mode C Veil at lower altitudes, but it cannot fly through the core terminal airspace.
Flying in the Veil Without Equipment
If your transponder or ADS-B equipment fails, or if your aircraft simply isn’t equipped, you can still request authorization from ATC to enter the Mode C Veil. The regulation uses the phrase “unless otherwise authorized or directed by ATC,” which means controllers have the discretion to allow you in. In practice, you would contact the approach control facility responsible for that airspace and request a deviation. Having a specific route and destination in mind helps. Some pilots operating out of small airports located inside a Mode C Veil make these requests routinely when equipment is down for maintenance.
This is not a blanket pass. Controllers can deny the request based on traffic volume, and you should expect to follow specific instructions, including a particular route or altitude, if they do approve it.
Mode C Veil vs. Class B Airspace
The Mode C Veil and Class B airspace overlap but are not the same thing. Class B airspace requires an explicit ATC clearance to enter (“cleared into the Bravo”). The Mode C Veil does not require a clearance to enter, only that your equipment is working. You can fly freely inside the veil without talking to ATC, as long as you stay out of the Class B, Class C, and Class D airspace boundaries within it and have the right equipment operating.
This distinction trips up student pilots. The veil is an equipment mandate, not an entry restriction. You are free to fly through it on a VFR flight without contacting anyone, provided your transponder and ADS-B are on and you avoid the controlled airspace that sits inside it. The Class B airspace, on the other hand, is a locked door that requires a verbal clearance from a controller before you cross the boundary.