Definition
A high-intensity approach lighting system installed on the runway approach end for Category I precision instrument runways. It consists of steady-burning white lights arranged in a 2,400-foot pattern aligned with the runway centerline, plus a sequence of high-intensity white strobe lights that flash in rapid succession from the outer end inward toward the runway threshold, giving the impression of a fast-moving ball of light traveling toward the runway. It provides the pilot with visual cues for alignment, distance from the threshold, and roll guidance during the final stages of an instrument approach.
Plain English
A long line of bright lights that leads up to the runway, with strobe lights that flash in sequence so it looks like a streak of light racing toward the runway. It helps the pilot find and line up with the runway when breaking out of cloud near the end of an instrument approach. The '-1' version is the type used for standard precision approaches.
Context Anchor
Seen on instrument approach charts, airport lighting information, and in inoperative-component notes that tell you what changes if the lighting system is not working.
Derivation
The name describes the system itself: it lights the approach to the runway, and the strobes flash in a timed sequence rather than all at once. The '-1' simply distinguishes this configuration from ALSF-2, a similar system used for Category II approaches.
Why Pilots Care
Enables safe visual acquisition of the runway environment in low visibility, directly affecting approach minimums and reducing the chance of misalignment or controlled flight into terrain.
Grounding Statement
Picture short bursts of bright light stepping toward the runway end through haze or darkness, helping you pick out where the runway begins.
Intuition Check
Do not think of the flashers as general warning lights. In an ALSF-1, the flashers are guidance lights that flash in a planned order toward the runway.
Example Sentence 1
After breaking out of the overcast at minimums, the pilot saw the ALSF-1 sequenced flashers leading her straight to the runway threshold.
Example Sentence 2
With the ALSF-1 operating, the approach minimums were lower than they would have been with only basic runway edge lights.