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.

The first fixed runway lighting possibly appeared in 1930 at Cleveland Municipal Airport (now known as Cleveland Hopkins International Airport) in Cleveland, Ohio. But it was suitable only clear night or days.

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 earliest approach lighting systems were far removed from the current sophisticated generation of ALS's and were developed before/during World War II. They were commonly referred to as the Flare Path, the name being derived from the fact often flares were burned alongside the active runway to provide the pilot with an illuminated reference for approach and landing where an electrical system had not yet been installed. During the war the UK became a huge bomber base, and during England's famous fog at night a flare path was not enough even with a radar direct ground-controlled approach system that became available later in the war. The British were the most concerned because their night bombers were flying bombing missions every night year round, where the US day bombers operated when there was clear weather over the target. The British eventually developed a working system towards the end of the war that burned the fog off the area of the runway called FIDO that stood for Fog Investigation and Dispersal Operation. FIDO worked and saved a lot of bomber crew lives, but due to its high cost (ie $4000 dollars a day) and inability to disperse fog during heavy rains, was not suitable for civilian airline operation. After the war the US Navy and United Airlines worked together on various methods at the US Navy's Landing Aids Experimental Station located at the Arcata, Californian 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 US 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 -- ie then called Strobeacon lights. The first large commercial airport to have installed a strobe light ASL visual approach path was New York cities New York International Airport. Soon other large airports had strobe light ASL systems installed.

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 uncontrolled airports, Pilot Controlled Lighting may be installed which 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. In the event of radio failure, the control tower can communicate with the aircraft via aviation light signals.

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.