Definition
A high-intensity approach lighting system installed at the approach end of a runway served by a Category II or Category III instrument approach. It consists of steady-burning white lights extending 2,400 feet from the runway threshold along the extended runway centerline, a sequenced flashing light system (a 'rabbit') that pulses toward the runway, a green threshold light bar, a 1,000-foot crossbar, and red side row barrettes in the inner 1,000 feet to provide visual guidance in low-visibility conditions.
Plain English
A specific pattern of bright lights laid out in front of the runway to help a pilot find and line up with the runway when the weather is very bad. It includes a line of lights running away from the runway, a chasing flash that points the way in, and red and green markers showing the threshold and the final stretch.
Context Anchor
Seen in airport lighting descriptions, runway information, and instrument approach material for runways equipped for low-visibility approaches.
Derivation
ALSF stands for Approach Lighting System with Sequenced Flashing Lights. The '-2' designates the second standard configuration, which is the version built for Category II/III approaches. A simpler version, ALSF-1, is used for Category I approaches.
Why Pilots Care
Provides essential visual reference in low visibility, allowing pilots to confirm runway alignment and continue the approach safely to decision height.
Grounding Statement
Picture a runway in fog: extending out from the threshold is a long bright path of white lights with a flashing light running along it toward the runway, ending at a green line of lights at the runway edge.
Intuition Check
The “2” in ALSF-2 does not mean there are two lighting systems. It identifies Configuration 2, a specific FAA-approved layout of approach lights.
Example Sentence 1
The Category II approach to Runway 27 is served by an ALSF-2, so we briefed the lighting cues we'd need to see at decision height.
Example Sentence 2
Runways equipped with ALSF-2 support Category II operations when runway visual range drops below 2,400 feet.