How far is the localizer effective for?

The localizer is a component of the Instrument Landing System (ILS) that provides horizontal guidance to aircraft as they approach the runway for landing. The effective range of the localizer signal is typically up to 18 nautical miles (nm) from the runway threshold.

This 18nm range is established to ensure that aircraft can receive a reliable and accurate signal as they approach the final approach course. Factors such as terrain and obstacles can affect this range, but under standard conditions, 18nm is the distance at which the localizer is designed to be effective for guiding aircraft laterally toward the runway.

Understanding the effective range of the localizer is crucial for pilots, as it helps them plan their descent and approach phase correctly, ensuring they intercept the localizer signal at the appropriate point and fly a stable approach to landing. Thus, the choice of 18nm is the correct response to the question about the localizer's effectiveness.