Definition
A configuration of high-intensity lights arranged in a specific pattern extending outward from the runway threshold along the extended runway centerline, designed to provide visual guidance to pilots transitioning from instrument flight to visual flight during the final segment of an instrument approach. Different ALS configurations exist (such as ALSF-1, ALSF-2, MALSR, MALSF, SSALR, SSALF, ODALS, and others), and the type installed at a runway affects the visibility minimums a pilot may use on an instrument approach. When all or part of the system is inoperative, published landing minimums are increased.
Plain English
A pattern of bright lights leading up to the runway that helps a pilot find the runway when breaking out of clouds or low visibility on an instrument approach. The exact pattern depends on the runway, and if the lights are broken, the pilot has to be able to see farther before they're allowed to land.
Context Anchor
Seen on instrument approach charts, airport diagrams, and in discussions of inoperative components because the system may affect the visibility required to use an approach.
Derivation
Approach comes from an old French word meaning “to come near.” In aviation, the approach is the final part of coming near the runway to land. That helps explain why an approach lighting system is placed before the runway, where it guides the airplane as it gets close to landing.
Why Pilots Care
An inoperative approach lighting system raises landing minimums and may require use of an alternate airport.
Grounding Statement
Picture flying toward a runway at night: before the runway itself is easy to see, the approach lighting system may appear as a bright path showing where the runway begins and how it is lined up.
Intuition Check
Do not read this as “any lights you see while approaching an airport.” It means a specific installed light pattern leading to the landing runway.
Example Sentence 1
Because the approach lighting system was reported inoperative, the crew added the required visibility increment to the published minimums before starting the approach.
Example Sentence 2
With the approach lighting system out of service the crew added one hundred feet to the published decision altitude.