Mode C veil refers to a kind of airspace which currently surrounds all primary Class B airports within the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

. 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. The name refers to the mode of

transponder In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a flight trans ...

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

in operation. , all 37

Class B airports The world's navigable airspace is divided into three-dimensional segments, each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization (ICAO) and described bel ...

in the United States have Mode C veils centered on them. Prior to November 2014, two Class B airports did not have a Mode C veil (at least ''

de jure In law and government, ''de jure'' ( ; , "by law") describes practices that are legally recognized, regardless of whether the practice exists in reality. In contrast, ("in fact") describes situations that exist in reality, even if not legall ...

''): William P. Hobby Airport in

Houston Houston (; ) is the List of cities in Texas by population, most populous city in Texas, the Southern United States#Major cities, most populous city in the Southern United States, the List of United States cities by population, fourth-most pop ...

and

Marine Corps Air Station Miramar Marine Corps Air Station Miramar (MCAS Miramar) , formerly Naval Auxiliary Air Station (NAAS) Miramar and Naval Air Station (NAS) Miramar, is a United States Marine Corps installation that is home to the 3rd Marine Aircraft Wing, which is the av ...

San Diego San Diego ( , ; ) is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth most populous city in the United States ...

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

References

Which had it ''de facto'', although not necessarily ''de jure'': ("the FAA plans to add the William P. Hobby Airport to the list of airports identified in Title 14 of the Code of Federal Regulations, Part 91, Appendix D, Section 1"). For many years, both of these exceptions had Mode C veils centered on them on the sectional, WAC, and Low IFR charts, which was inconsistent with CFR and as such was either a cartographic mistake, a regulatory mistake, or both. In a separate action ({{USFR, 79, 57431, published 2014-09-25 and effective 2014-11-13), the FAA updated CFR Title 14 Appendix D to Part 91, Section 1, to include both Hobby and Miramar. Air traffic control in the United States