Definition
A high-intensity approach lighting system installed at the approach end of a runway served by a Category I instrument landing system (ILS). It consists of a 2,400-foot configuration of steady-burning white lights arranged along the extended runway centerline, a green threshold bar, a red terminating bar, and a series of sequenced flashing white lights (often called the rabbit) that fire in rapid succession from the outermost light toward the runway threshold to lead the pilot visually to the landing surface.
Plain English
A long, bright lighting pattern leading up to the runway, with a chain of fast-flashing white lights that appear to race toward the runway. It helps a pilot flying an instrument approach see and line up with the runway when breaking out of cloud or low visibility.
Context Anchor
Seen on instrument approach charts and at airports with precision instrument approaches, especially when landing in low visibility.
Derivation
The 'sequenced flashing lights' part of the name describes how the lights flash one after another in order, rather than all at once. This sequencing creates the visual effect of motion toward the runway, which is why pilots nicknamed it the rabbit.
Why Pilots Care
Provides positive visual identification of the runway and glide path alignment, allowing safe continuation of an approach when the runway is not yet in sight.
Intuition Check
Do not read “sequenced flashing” as lights blinking randomly. In an ALSF-1, the flashing lights fire in order toward the runway to help guide the pilot’s eyes in the correct direction.
Example Sentence 1
Breaking out just above minimums, the captain called the ALSF-1 in sight and continued the approach to landing.
Example Sentence 2
Airports equipped with the ALSF-1 system give pilots an earlier visual cue to line up with the runway.