Definition
A high-intensity approach lighting system installed at the approach end of a runway to guide aircraft visually during the final stage of an instrument approach. It includes a series of steady white lights arranged along the extended centerline, plus a row of rapidly sequenced flashing white lights that appear to race toward the runway threshold. Two versions exist: ALSF-1 (used with Category I precision approaches) and ALSF-2 (used with Category II and III approaches, and which can also serve as a simplified system in Category I conditions).
Plain English
A bright, high-end runway approach lighting system that includes a line of flashing lights that visually 'pull' the pilot's eye toward the runway in low visibility.
Context Anchor
Seen on instrument approach charts, runway lighting information, and in notes about lighting equipment that is not working.
Derivation
The flashing portion is often nicknamed the 'rabbit' because the sequenced flashes appear to hop along the ground toward the runway. 'Sequenced flashing' literally means the lights flash one after another in a fixed order, creating that running visual effect.
Why Pilots Care
When part of the ALSF is inoperative, instrument approach minimums change — visibility requirements typically increase. Pilots must check the inoperative components table to know what minimums actually apply for the approach they are flying.
Grounding Statement
In poor visibility, the ALSF can be the first clear visual cue that points the pilot toward the runway.
Intuition Check
Do not think of the flashing lights as random warning lights. In an ALSF, the flashes are sequenced in order toward the runway to guide the pilot’s eyes and alignment.
Example Sentence 1
Breaking out of the clouds at minimums, the pilot saw the ALSF lights flashing toward the threshold and continued the approach to landing.
Example Sentence 2
Sequenced flashing lights from the ALSF gave the crew a clear visual path to the runway threshold.