 Class A airspace was formerly known as Positive Control Airspace (PCA).Class B airspace is defined around key airport traffic areas, usually airspace surrounding the busiest airports in the US according to the number of IFR operations and passengers served.The exact shape of the airspace varies from one class B area to another, but in most cases it has the shape of an inverted wedding cake, with a series of circular "shelves" of airspace of several thousand feet in thickness centered on a specific airport.