Which communication frequency is used when approaching an airport without an operating control tower?

When approaching an airport that does not have an operating control tower, the Common Traffic Advisory Frequency (CTAF) is used for communication. The CTAF operates as a dedicated frequency that pilots use to announce their intentions, position, and any other relevant information to ensure safe coordination among aircraft in and around the airport environment.

Using the CTAF allows pilots to maintain awareness of other traffic in the area and helps them manage their arrival and departure procedures safely. This is particularly important at non-towered airports where there is no air traffic control to advise on traffic or provide separation.

Other options such as ATIS (Automatic Terminal Information Service), FSS (Flight Service Station), and AWOS (Automated Weather Observing System) serve different purposes. While ATIS provides essential pre-recorded information about weather and airport conditions, it doesn't facilitate real-time communication between pilots. FSS offers flight planning assistance and weather information, and AWOS provides weather data, neither of which can replace the traffic coordination that occurs on the CTAF frequency. Therefore, the CTAF is the appropriate choice for communication at airports without control towers.