There are many airports that have control towers that do not operate 24 hours a day. When airport services change, the overlying airspace can also change and this can catch a pilot in the wrong place at the wrong time.
How it works:
Airports with operating control towers are classified as Class D airspace. When the control tower is open the Class D is “in effect.” Class D rules require the pilot to communicate with the tower controller for takeoff, and landing at the airport, as well as transition through the area. But when the control tower closes and the controllers go home for the day, the airspace can no longer be Class D. Usually the airspace “demotes” down to a Class G, which is uncontrolled airspace at what now is an uncontrolled airport. Pilots flying to the airport after the control tower closes now must use uncontrolled airport procedures for arrival and departure.
How you work with it:
When the tower is closed what radio frequency should be used? Usually the control tower frequency is used even though the tower is closed. To be sure, find the frequency on the sectional chart beside a blue circle that has the letter C inside. This indicates the Common Traffic Advisory Frequency (CTAF) and this frequency will always be used for air traffic communications. At airports with a part-time control tower, the CTAF frequency will have a star by it to indicate the tower does not operate 24 hours. When the tower is open the tower frequency is the CTAF. In this case pilots should call and address “Tower” but when the tower is closed pilots pilot should address “Traffic” on the CTAF. The Class D open and closed times are published in the Airport/Facility Directory.
Why it is VERY important:
It would pay to know these times when approaching an airport with a part-time Class D tower. What if a pilot flew to an airport that had a closed control tower and entered the traffic pattern as if it was an uncontrolled airport. Then without the pilot knowing it, the control tower opens and the airspace instantly becomes Class D. The pilot lands only to discover the airport is now controlled and the landing was made without the required landing clearance!
Remember to check when the airspace will move *before* you takeoff — and set your watch.