 Class C airspace is defined around airports of moderate importance; airports with regular commercial passenger jet service of 100 passengers per flight or more are typically Class C. The FAA requirements for Class C airspace status are an operational control tower, a radar-controlled approach system, and a minimum number of IFR approaches conducted per year.The airspace class designation is in effect only during the hours of tower and approach operation at the primary airport; the airspace reverts to Class D if approach control is not operating, and to class E or G if the tower is closed.The vertical boundary is usually above the airport surface.