ILS guidance through the final approach zone
Instrument Landing System (ILS) provides precision guidance during low-visibility approaches using two radio beams: the localizer guides the aircraft left-right toward the runway centerline, while the glide slope guides the descent at the correct angle (typically 3 degrees). As the aircraft descends and approaches the runway, these signals grow stronger, and the pilot or autopilot adjusts the aircraft's path to stay centered on both beams.
ILS is extraordinarily precise, allowing safe approaches in visibility as low as 200 feet horizontally and 50 feet vertically in Category III operations. The system essentially provides a radio highway from the general approach pattern directly to the runway threshold, eliminating the need for pilots to circle visually searching for the runway in fog or heavy weather.
Categories of precision and automation levels
ILS approaches are categorized by minimum visibility and decision height. Category I requires at least 200-foot visibility and 200-foot decision height; Category II requires 100-foot visibility and 100-foot decision height; Category III (rare, requiring special certification) drops to 50 feet or less. Category III approaches are fully automatic, with the aircraft's autopilot following the ILS beams all the way to touchdown without any visual reference from the pilot.
Automatic landing capabilities depend on infrastructure: only major airports install the most stringent ILS categories, while smaller regional airports may have basic ILS or rely on simpler approaches. Modern aircraft can execute Category II and III approaches, but most airlines restrict their use to emergency situations because they require additional crew training and ground equipment maintenance. The system fundamentally transformed aviation in severe weather, reducing delays and cancellations while improving safety when visibility drops below what humans can navigate visually.