An approach lighting system (ALS) is an array of lights that provide visual information on runway alignment, height perception, roll guidance, and horizon references for pilots when making an approach to a runway. It is designed to smooth the transition from instrument to visual flight for landing through a configuration of signal lights that start at the landing threshold and extend into the approach zone.

Depending on the type of precision approach and visibility minimums of the approach, the ALS can be in the form of medium intensity approach lighting system without sequenced flashers (MALS), medium intensity approach lighting system with sequenced flashing lights (MALSF), medium intensity approach lighting system with runway alignment indicator lights (MALSR), simplified short approach lighting system with runway alignment indicator lights (SSALR), approach lighting system with sequenced flashers II (ALSF-2), omni-directional approach lighting system (ODALS), or precision approach path indicator (PAPI) system.

The MALS is an economy type system used for non-precision approaches. It consists of a threshold light bar and seven five-light bars disposed symmetrically about the runway centerline extended, with the first bar installed 200 feet from the runway threshold, and the remaining bars extending at 200-foot intervals into the approach area at a distance of 1,400 feet from the threshold.

The MALSF comes in the same configuration with the MALS, but are equipped with three sequenced flashers located at the last three light bar stations where approach area identification is difficult at night due to surrounding lights. The MALSR consists of a MALS plus a runway alignment indicator light (RAIL) portion. The MALS portion does not use flashing lights and requires land that is 1,600 feet long by 400 feet wide. The RAIL portion is 1,000 feet in length by 25 feet in width.

MALS, MALSF and MALSR lighting systems provide early runway lineup and lead-in guidance, runway end identification and roll guidance. The SSALR is used as part of a dual-mode approach lighting system designed for category II designated runways that may have presence of category I conditions. It consists of a threshold bar and seven five-light bars that extend into the approach area a distance of 1,400 feet from the runway threshold. Two additional five-light bars form a 70-foot crossbar that is1,000 feet from the runway threshold. Five sequenced flashers located on the extended runway centerline constitute its RAIL portion. The first flasher is located 200 feet beyond the approach end of the last steady-burning light bar. Successive flashers extend at 200-foot intervals to 2,400 feet from the threshold.

SSALR is operated as a subsystem of the ALSF-2 which is used for category II and category III precision approaches. The ALSF-2 places a five-light bar (approximately 13½ feet long) at each 100-foot interval starting 100 feet across a distance of 2,400 feet. The ODALS, which provides circling, offset, and straight-in approaches to non-precision runways, uses seven omnidirectional sequenced flashing lights along runway approach centerline. The PAPI system provides approach slope guidance to assist the pilot of an aircraft in flying a stabilized approach.