Approach lighting system

An approach lighting system, or ALS, is a lighting system installed on the approach end of an airport runway and consisting of a series of lightbars, strobe lights, or a combination of the two that extends outward from the runway end. ALS usually serves a runway that has an instrument approach procedure (IAP) associated with it and allows the pilot to visually identify the runway environment and align the aircraft with the runway upon arriving at a prescribed point on an approach.

Modern approach lighting systems are highly complex in their design and significantly enhance the safety of aircraft operations, particularly in conditions of reduced visibility.

The required minimum visibility for instrument approaches is influenced by the presence and type of approach lighting system. In the U.S., a CAT I ILS approach without approach lights will have a minimum required visibility of 3/4 mile, or 4000 foot runway visual range. With a 1400-foot or longer approach light system, the minimum potential visibility might be reduced to 1/2 mile (2400 runway visual range), and the presence of touchdown zone and centerline lights with a suitable approach light system might further reduce the visibility to 3/8 mile (1800 feet runway visual range).

The runway lighting is controlled by the air traffic control tower. At non-towered airports, pilot-controlled lighting may be installed that can be switched on by the pilot via radio. In both cases, the brightness of the lights can be adjusted for day and night operations.

Depth perception is inoperative at the distances usually involved in flying aircraft, and so the position and distance of a runway with respect to an aircraft must be judged by a pilot using only two-dimensional cues such as perspective, as well as angular size and movement within the visual field. Approach lighting systems provide additional cues that bear a known relationship to the runway itself and help pilots to judge distance and alignment for landing.

After World War II, the U.S. Navy and United Airlines worked together on various methods at the U.S. Navy's Landing Aids Experimental Station located at the Arcata, California air base, to allow aircraft to land safely at night and under zero visibility weather, whether it was rain or heavy fog. The predecessor of today's modern ALS while crude had the basics — a 3,500 foot visual approach of 38 towers, with 17 on each side, and atop each 75 foot high tower a 5000 watt natural gas light. After the U.S. Navy's development of the lighted towers it was not long before the natural gas lights were soon replaced by more efficient and brighter strobe lights, then called Strobeacon lights. The first large commercial airport to have installed a strobe light ASL visual approach path was New York City's John F. Kennedy International Airport. Soon other large airports had strobe light ASL systems installed.

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