Definition
A high-intensity approach lighting system installed at the approach end of a runway served by a Category II or Category III instrument approach. It consists of a 2,400-foot pattern of steady-burning white lights extending outward from the runway threshold, a row of red side bars in the final 1,000 feet, a green threshold bar at the runway end, and a sequenced flashing white strobe ("the rabbit") that pulses rapidly toward the threshold. Its purpose is to give pilots breaking out of low cloud or poor visibility a clear visual reference to the runway environment so they can transition from instruments to a visual landing.
Plain English
A specific style of bright runway approach lights, used at airports that allow landings in very low visibility. It includes long rows of white lights leading to the runway, red bars near the threshold, a green bar at the runway edge, and a chain of strobe lights that flash one after another toward the runway, helping the pilot find the runway when breaking out of cloud.
Context Anchor
Seen on instrument approach charts, airport lighting information, and notes about inoperative approach lighting components.
Derivation
"Approach Lighting System" describes its function: lights guiding the approach to the runway. "Sequenced Flashers" refers to the strobe lights that fire one after another in sequence, creating a visual arrow pointing toward the runway. The "-2" distinguishes this configuration from ALSF-1; the two systems differ mainly in the colour and arrangement of the inner lights near the threshold.
Why Pilots Care
An inoperative ALSF-2 often raises landing minimums and may prevent completion of the approach.
Analogy
It is like a line of guide lights that points you toward a doorway in the dark, with some lights blinking in order to show the direction you should follow.
Intuition Check
Do not think of ALSF-2 as just “some flashing runway lights.” It is a specific approved lighting layout, and the lights flash in a planned sequence toward the runway.
Example Sentence 1
The ILS approach into runway 27L is served by an ALSF-2, allowing Category II minimums when all components are operational.
Example Sentence 2
Because the ALSF-2 was inoperative, the crew flew the approach to the higher published minimums.