Plain English Meaning
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.
Full 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.
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.