Definition
A high-intensity approach lighting system installed at the approach end of a runway served by a Category I precision instrument approach. It consists of a 2,400-foot centerline bar of steady-burning white lights extending outward from the runway threshold, a 1,000-foot crossbar, threshold lights, a red terminating bar near the threshold, and a sequence of high-intensity flashing strobe lights that fire in rapid order from the far end of the system toward the runway, producing a visible 'rabbit' of light leading the pilot to the touchdown zone.
Plain English
A long line of bright lights laid out on the ground before the runway, with a row of strobe lights that flash one after another in a fast moving stream pointing toward the runway. It tells a pilot flying an instrument approach exactly where the runway begins and which way to align the aircraft.
Context Anchor
Seen on instrument approach charts and during the final part of an instrument approach, especially at airports equipped for low-visibility landings.
Derivation
ALSF stands for 'Approach Lighting System with Sequenced Flashing Lights.' The '-1' designates the first of two standard configurations (ALSF-1 and ALSF-2); ALSF-2 supports Category II/III approaches and adds extra side-row barrettes and a different terminating bar arrangement. Knowing the '-1' means 'Category I version' helps pilots recognize what minima the system supports.
Why Pilots Care
Provides reliable visual cues that allow safe transition from instrument references to the runway environment, reducing the risk of misalignment or controlled flight into terrain during the final stages of an approach.
Grounding Statement
On final approach, ALSF-1 looks like a bright, organized light path leading your eyes toward the runway.
Intuition Check
Do not think of ALSF-1 as ordinary runway lighting. It is approach lighting placed before the runway to help you find and align with the runway, not lights along the runway edges.
Example Sentence 1
Breaking out at minimums, the captain called the ALSF-1 lights in sight and continued the approach to landing.
Example Sentence 2
Runway 27 is equipped with an ALSF-1 system that activates automatically when the approach lights are turned on.