Flying Down an Invisible Beam: How the ILS Lands Airplanes in Fog

On a clear day, landing an airplane is a visual act: the pilot sees the runway and flies to it. But weather does not always cooperate, and for nearly a century engineers have worked to let aircraft land safely when the crew cannot see the ground until the last moment. The most important answer is the Instrument Landing System, the ILS. An ILS is built from two radio beams broadcast from the runway environment. The localizer provides horizontal guidance, telling the pilot whether the aircraft is left or right of the runway centerline. The glideslope provides vertical guidance, defining the correct descent angle — typically about three degrees — down to the touchdown zone. In the cockpit the two beams drive a pair of crossed needles; the pilot flies to keep them centered, descending along the intersection of the two beams like a marble rolling down an invisible groove in the sky. The precision is remarkable. Category I ILS approaches guide an aircraft down to a decision height of 200 feet above the runway, at which point the pilot must see the runway environment to continue. More capable Category II and III systems, paired with appropriately equipped aircraft and trained crews, allow approaches to far lower minimums — in the most advanced cases, automatic landings in visibility so poor a pilot can barely see the wingtips. The ILS does not remove the pilot's judgment; minimums, approach categories, and the decision to land or go around all rest with the crew per the rules in 14 CFR Part 91 and the pilot in command's authority under §91.3. But it transformed aviation from a fair-weather pursuit into a system that runs reliably through the seasons, and it remains, decades after its introduction, the backbone of precision instrument approaches worldwide.