Definition
A high-intensity approach lighting system installed at the approach end of a Category I precision instrument runway. It consists of a 2,400-foot centerline of steady white lights extending outward from the runway threshold, a crossbar of steady white lights at 1,000 feet from the threshold, a green threshold bar, red terminating bars near the threshold, and a series of high-intensity sequenced flashing white lights (a 'rabbit') that fire in rapid succession from the far end of the system toward the runway, leading the pilot's eye to the threshold during a low-visibility approach.
Plain English
A long line of bright white lights stretching out from the runway, with one set of lights flashing in sequence toward the runway like a chasing arrow. It's designed to help a pilot find the runway when breaking out of clouds on an instrument approach.
Context Anchor
Seen on instrument approach charts and during approaches to runways equipped with high-intensity approach lighting.
Derivation
The 'sequenced flashing' part refers to the lights firing one after another in a chasing pattern, not all at once. Pilots often nickname it 'the rabbit' because the flashing point appears to run down the line toward the runway. 'Configuration 1' (the '-1') distinguishes this Category I version from ALSF-2, which serves Category II/III runways and has additional red side-row lights.
Why Pilots Care
Supports lower landing minimums and safer alignment with the runway in reduced visibility.
Grounding Statement
As you near the runway in poor visibility, the sequenced flashes appear to move toward the runway and help draw your eyes to the correct landing path.
Intuition Check
Do not think of the flashing lights as warning lights. In ALSF-1, the flashes are arranged in sequence to point the pilot toward the runway threshold.
Example Sentence 1
Breaking out just above minimums, the captain called the ALSF-1 in sight and continued the descent to the runway.
Example Sentence 2
The pilot used the sequenced flashes of the ALSF-1 system to confirm runway alignment in fog.