Definition
A configuration of lights arranged on the ground at the approach end of a runway, extending outward from the runway threshold, that provides pilots transitioning from instrument flight to visual flight with the visual cues needed to align with, descend to, and land on the runway. ALS configurations vary in length, intensity, and complexity depending on the type of instrument approach the runway supports.
Plain English
A pattern of bright lights laid out in front of the runway that helps a pilot see and line up with the runway when breaking out of cloud or low visibility on an instrument approach.
Context Anchor
Seen on instrument approach charts, airport information, and outside the cockpit when nearing a runway in low visibility.
Derivation
"Approach" here refers specifically to the final segment of an instrument approach procedure, where the aircraft is descending toward the runway. The lighting system is named for the phase of flight it supports, not the act of arriving in general.
Why Pilots Care
Enables safe transition from instruments to visual references and reduces risk of misalignment or runway excursions in marginal weather.
Grounding Statement
Picture coming out of cloud or haze and seeing a straight pattern of lights that leads your eyes toward the runway.
Intuition Check
An ALS is not the same as the lights along the runway edges. It is a separate light pattern before the runway that helps you find and align with the runway during the approach.
Example Sentence 1
Breaking out of the clouds at minimums, the pilot saw the approach lighting system and continued the descent to land.
Example Sentence 2
Runway 27R’s ALS helped the crew maintain centerline alignment through a low ceiling.