Approach lighting system

An approach lighting system (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.

Operation

The required minimum visibilities 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.