Definition
A high-intensity approach lighting system installed at the runway end of a Category I precision instrument runway, consisting of a row of steady-burning white lights extending out from the runway threshold, with a sequence of brilliant strobe lights that fire one after another from the far end toward the threshold, creating a visible 'rabbit' that appears to run toward the runway. ALSF1 helps the pilot transition from instrument flight to visual landing by giving a clear, directional cue showing alignment with the runway centerline.
Plain English
A bright runway approach lighting setup with a line of white lights and a chasing strobe effect that points toward the runway, helping the pilot line up for landing when breaking out of clouds.
Context Anchor
Seen on approach charts, airport lighting descriptions, and runway information for instrument approaches.
Derivation
The 'sequenced flashers' part of the name describes the lights' behavior: they flash one after another in sequence, not all at once. The Roman numeral 'I' distinguishes this configuration from ALSF2, which is a similar but more elaborate system used for Category II approaches.
Why Pilots Care
Provides positive visual identification of the runway environment, allowing safe continuation of the approach to a landing decision point in reduced visibility.
Grounding Statement
On final approach, the sequenced flashes appear to move toward the runway, drawing the pilot’s attention to the correct landing path.
Intuition Check
Do not read the “1” as meaning there is only one flashing light. It identifies the ALSF1 lighting configuration.
Example Sentence 1
The approach plate showed ALSF1 lighting for runway 27, so we briefed that the sequenced flashers would lead us to the threshold once we broke out.
Example Sentence 2
Runways served by an ILS Category I approach are often equipped with ALSF1 lighting to improve visual cues in marginal weather.