Definition
A high-intensity approach lighting system installed at the approach end of a precision instrument runway, consisting of steady-burning white lights arranged in a configuration extending outward from the runway threshold, plus a series of high-intensity sequenced flashing lights that fire one after another toward the runway. Two standard versions exist: ALSF-1 (with a tall pole-mounted system and red side row bars) and ALSF-2 (with red side row bars usable as a Simplified Short Approach Lighting System with Runway Alignment Indicator Lights when visibility is reduced). Both are used to support Category I or II precision approaches.
Plain English
A long line of bright lights leading up to the runway, with a row of flashing lights that fire one after another like a chasing arrow pointing the pilot toward where to land. It is used at runways that support precision instrument approaches in poor visibility.
Context Anchor
Seen in instrument approach system diagrams, runway lighting descriptions, and approach chart notes for runways equipped with this lighting.
Derivation
The flashing lights are called 'sequenced' because they fire in sequence — one after another, in a fixed order — rather than all at once. The visual effect is a bright pulse running toward the runway, which is why pilots sometimes refer to them as 'the rabbit.'
Why Pilots Care
Enables pilots to positively identify the runway environment earlier during low-visibility approaches, supporting safer landing decisions.
Intuition Check
The flashing lights are not random warning strobes. They flash in an ordered sequence that visually leads the pilot toward the runway.
Example Sentence 1
Breaking out of the clouds at minimums, the captain called the ALSF in sight and continued the approach to landing.
Example Sentence 2
During the precision approach, the sequenced flashing lights of the ALSF confirmed runway alignment.