Definition
A high-intensity approach lighting system installed at the approach end of a runway used for Category II and Category III precision instrument approaches. It consists of steady-burning white lights arranged along the extended runway centerline for 2,400 feet from the runway threshold, a row of red side bars near the threshold, and a sequenced flashing white strobe light in each centerline position from 1,000 feet outward, which fire in rapid succession toward the runway to create a visible 'ball of light' that travels toward the threshold.
Plain English
It is the most capable runway approach lighting system. It uses long rows of bright white lights leading to the runway, plus a chain of fast-flashing strobes that appear to race toward the runway end, helping the pilot find and line up with the runway in very poor visibility.
Context Anchor
Seen on airport diagrams, approach charts, runway lighting descriptions, and NOTAMs for runways equipped with this approach lighting system.
Derivation
ALS stands for Approach Lighting System. The 'F' refers to the sequenced flashers (the racing strobes), and 'II' distinguishes this version from the earlier ALSF1 design. The two systems look similar but differ in the threshold and decision-bar lighting.
Why Pilots Care
Determines the available visual guidance and may affect published approach minimums and landing safety.
Grounding Statement
On final approach in haze, rain, or darkness, ALSF2 gives the pilot a bright visual lead-in to the runway.
Intuition Check
Do not treat the “II” as meaning simply “the second set of lights.” In this term, it identifies a specific FAA approach-lighting configuration used for certain precision runways.
Example Sentence 1
Runway 4R is equipped with ALSF2, which supports the Category II ILS approach in low visibility.
Example Sentence 2
During the briefing the pilot noted the ALSF2 lights would provide sequenced flashing guidance to the threshold.