Definition
A configuration of ground-based lights extending outward from the runway threshold along the extended runway centerline, designed to give pilots transitioning from instrument flight to visual flight the visual references needed to align with and land on the runway. ALS configurations vary in length, intensity, and complexity (for example, ALSF-1, ALSF-2, MALSR, SSALR, MALS, SSALS, ODALS), and the type of system installed at a runway can affect the lowest authorized landing minimums for an instrument approach.
Plain English
A line of bright lights stretching out from the start of the runway, leading you in. When breaking out of cloud near minimums, these lights are often the first thing you see and they show you exactly where the runway is.
Context Anchor
Seen in instrument approach procedures, airport lighting descriptions, and inoperative equipment notes that may change landing minimums.
Why Pilots Care
An inoperative ALS raises the required visibility or ceiling for the approach, potentially forcing a missed approach.
Grounding Statement
In low visibility, the approach lights may be the first runway-related lights you see before the runway itself becomes clear.
Intuition Check
Do not read this as any light you happen to see while approaching an airport. An Approach Light System is a specific installed light pattern aligned with the runway approach end.
Example Sentence 1
Just above minimums, the captain called the approach light system in sight and continued the descent toward the runway.
Example Sentence 2
As the Approach Light System came into view, the runway environment became identifiable and the landing continued.