What frequency should you advise ATC prior to entering and after exiting an ILS Critical Area?

The correct frequency to advise Air Traffic Control (ATC) prior to entering and after exiting an Instrument Landing System (ILS) Critical Area is either 121.7 MHz or 127.15 MHz. These frequencies are specifically designated for communications related to the airside operations at airports, particularly in relation to situations where aircraft and vehicles might be interacting with critical navigation signals such as ILS.

When entering or exiting an ILS Critical Area, it is important to communicate your intentions on the appropriate tower frequency to ensure safety and coordination around the critical area, where interference could potentially cause navigation errors for aircraft. The selected frequencies of 121.7 MHz or 127.15 MHz are commonly used in airport operations for this purpose.

Advising ATC on these frequencies allows for timely updates on your position and movements, which helps maintain organized and safe airspace management in the vicinity of an active runway or landing approach corridor. Proper communication is vital to prevent conflicts with aircraft using the ILS approach.