Instrument Landing System

An instrument landing system, or ILS, is a precision radio navigation system that guides an aircraft down to a runway in poor visibility, fog, rain or at night, when the crew cannot see the runway from a distance. It combines two radio beams: a localizer, broadcast on a frequency between 108 and 112 megahertz from an antenna array at the far end of the runway, which gives the pilot lateral guidance to stay aligned with the runway centerline, and a glideslope, broadcast between 329.15 and 335 megahertz from an antenna beside the runway, which gives vertical guidance along an approach path of about three degrees above the horizon; the pilot flies the aircraft to keep both indicators centered until reaching a decision height, at which point the runway must be in sight or the approach is abandoned. Testing of the system began in the United States in 1929 with the involvement of Jimmy Doolittle, and the first scheduled airline landing made using an ILS came on January 26, 1938, when a Pennsylvania Central Airlines Boeing 247D landed at Pittsburgh during a snowstorm. The International Civil Aviation Organization adopted the ILS as an international standard in 1950.