The National Airspace System (NAS) of the United States is a complex system of airspace that regulates the movement of aircraft in the country. The NAS is divided into several different types of airspace, each with its own rules and regulations. Class B airspace is the most restrictive type of airspace, and it encompasses major airports. Within Class B airspace, there is a inner core area that has an even higher level of restrictions. This inner core area is known as the 15 statute mile ring.
The 15 statute mile ring refers to the distance from the center point of a Class B airport's runway that has an instrument approach procedure published for it. This radius is depicted on Sectional Charts as a magenta circle with a dashed line. Aircraft operating within this area must be equipped with certain instruments and must follow specific procedures when entering or leaving the area. These procedures are designed to ensure safe separation between aircraft operating in close proximity to each other.