Mode C veil

Mode C veil refers to a kind of airspace which surrounds most primary Class B airports within 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. The name refers to the mode of transponder operation which is required within this airspace (i.e., with very limited exceptions, all aircraft operating within this airspace must have an altitude reporting Mode C transponder in operation). All 37 existing (as of August 2016) Class B airports in the United States, have Mode C veil centered on them. This was not the case for many years, whereas two Class B airports did not have it (at least de jure): Houston Hobby Airport and Miramar Naval Air Station.