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- to 3,000-foot centerline bar of steady white lights extending outward from the runway threshold, a 100-foot crossbar of steady lights located 1,000 feet from the threshold, a green threshold light bar, red terminating bars near the threshold, and a series of sequenced flashing white lights (often called the 'rabbit') that pulse rapidly toward the runway to give the pilot a strong visual lead-in during the final stages of an instrument approach.
Plain English
ALSF-1 is a specific layout of bright runway approach lights, including a row of fast-flashing lights that appear to race toward the runway, used to help pilots see and line up with a Category I precision runway as they break out of the clouds.
Context Anchor
Seen on instrument approach charts, airport lighting descriptions, and in inoperative-components guidance when the approach lighting system is not working.
Derivation
The name is built from its function: 'Approach Lighting System' (ALS) describes what it is, 'F' adds 'with sequenced Flashing lights,' and '-1' marks it as the first of two standard configurations (the other being ALSF-2, used for Category II/III runways). Knowing the letters helps the pilot decode similar acronyms (MALSR, SSALR, ALSF-2) on charts.
Why Pilots Care
Pilots depend on the system to confirm runway alignment and position; partial outages can raise landing minimums or prevent an approach from continuing.
Intuition Check
ALSF-1 is not the instrument approach itself. It is the lighting system that helps the pilot visually find and align with the runway near the end of the approach.
Example Sentence 1
Because the ALSF-1 was out of service, the crew had to use the higher inoperative-components minimums published for the ILS approach.
Example Sentence 2
When segments of the ALSF-1 are inoperative, the pilot checks the approach chart to see if higher visibility requirements apply.