Final answer:
The localizer is a component of the Instrument Landing System that emits a vertical beam along a runway's centerline, aiding pilots in aligning with the runway during an instrument approach.
Step-by-step explanation:
The part of the Instrument Landing System (ILS) that sends out a vertical beam along the centre line of the runway is called the localizer. This component is essential for providing lateral guidance to pilots during an instrument approach. The localizer signal helps the pilot to keep the aircraft aligned with the runway's centreline, ensuring a safe and accurate approach in conditions where visibility is poor or during precision landings. Along with the localizer, the ILS also comprises the glide slope, which provides vertical guidance to maintain the correct descent path.