Airports with no control towers are generally referred to as uncontrolled airports. There is a central frequency, but it is used for cooperation among pilots and information from the ground--not for instructions or clearance.
Common Traffic Advisory Frequency (CTAF) - At uncontrolled airports, aircraft cooperate by announcing their intentions and movements on a single designated frequency. If there is a part-time control tower, CTAF should be on the tower frequency. If there is a Flight Service Station, CTAF will be 123.600. If there is no FSS, CTAF will be on the Unicom frequency. If there is no Unicom and no FSS, the frequency will be 122.900. If the airport has a part-time control tower, the tower frequency will function as the CTAF.
If the airport hosts skydiving, gliding, or similar activities, you'll also hear these on the CTAF.
Unicom - Unicom has the same function at small and large airports. It's used for contact with the Fixed Base Operator, who sells fuel, supplies, and services to aircraft passing through. The only difference at uncontrolled airports is that the Unicom frequency (if it has one) also functions as the CTAF. For years all Unicom at uncontrolled airports was found on 122.700, 122.800 and 123.000. More recently, 123.050, 123.075 and 122.725, and 122.975 have become available to relieve frequency crowding. Other frequencies in the 136 MHz range may come available as well.
NOTE: If you monitor all seven Unicom frequencies, you can hear airport activity over a 30 mile or more radius. Skydiving activity (usually at 10,000-13,000 feet) can be heard over 100 miles away. Every announcement made by a pilot begins and ends with the name of the airport involved.
Clearance Delivery - Some of the busier uncontrolled airports have a remote radio outlet where aircraft on the ground can talk to Clearance Delivery at a nearby controlled airport. This airport may also be providing Approach/Departure Control for the small airport, and the Clearance Delivery frequency may simulcast Approach/Departure between uses as a Clearance Frequency. This provides us with Neat Monitoring Trick No.1.
Approach/Departure - Just because it's an uncontrolled airport doesn't mean that there's no radar service. If the uncontrolled airport is near a controlled airport with Class B or Class C airspace, that controlled airport may be providing radar service. If the smaller airport has a special frequency for Clearance Delivery, it may also be simulcasting Approach Control. (Neat Monitoring Trick No.1)
Another option is to have FAA Air Route Traffic Control Center (ARTCC) provide radar service. This is generally done on the same low altitude sector frequency used by aircraft flying IFR below 23,000 feet. If there are a number of airports needing radar service in the same sector, a discrete frequency may be provided. If Approach Control is provided by an airport with a part-time tower, the ARTCC may take over during that tower's off hours.
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