Mode C veil

Mode C veil refers to a kind of airspace which currently surrounds all primary Class B airports within the United States. This airspace extends horizontally to a circle of 30 NM radius centered on the airport, and extends vertically from the surface up to 10,000 feet MSL.^[1] The name refers to the mode of transponder operation which is required within this airspace — that is, with very limited exceptions, all aircraft operating within this airspace must have an altitude-reporting Mode C transponder in operation. An additional requirement for the transponder to have ADS-B Out became effective January 1, 2020.^[2]

As of August 2017^[update], all 37 Class B airports in the United States^[3] have Mode C veils centered on them.^[4] Prior to November 2014, two Class B airports did not have a Mode C veil (at least de jure): William P. Hobby Airport in Houston^[5] and Marine Corps Air Station Miramar in San Diego.^[6]

Mode C veils were implemented after the collision of Aeroméxico Flight 498 and a Piper Archer on August 31, 1986, within the terminal control area of Los Angeles airport.