Final answer:
When an aerodrome traffic frequency is not published, pilots should use the Common Traffic Advisory Frequency (CTAF). The default CTAF in the United States is 122.9 MHz for non-controlled airports, which helps maintain situational awareness and flight safety.
Step-by-step explanation:
If an aerodrome traffic frequency is not published, the frequency to be used is the Common Traffic Advisory Frequency (CTAF). This is a frequency designated for the purpose of carrying out airport advisory practices while operating to or from an aerodrome. Pilots use it for announcing their positions and intentions so that other pilots can be aware of the traffic within the vicinity. The CTAF may be a UNICOM, MULTICOM, FSS, or TOWER frequency, and in cases where it is not published, the default frequency in the United States is 122.9 MHz for non-controlled airports.
It is important for pilots to monitor and communicate on the correct frequency to maintain situational awareness and ensure safety. Knowing the correct frequency when it is not published would likely come from pilot training, experience, and supplemental resources such as the Aeronautical Information Manual (AIM).