Definition
A category of airport approach lighting installed at the approach end of a runway, consisting of a 1,400-foot configuration of medium-intensity steady-burning white lights aligned with the runway centerline, augmented by sequenced flashing lights in the outer portion of the system. The sequenced flashers fire in rapid succession toward the runway threshold, producing a visual 'rabbit' effect that helps pilots identify and align with the runway during instrument approaches in reduced visibility.
Plain English
A row of approach lights at the runway end that includes flashing lights firing one after another toward the runway, helping pilots see and line up with the runway when visibility is poor.
Context Anchor
Seen on instrument approach charts, airport lighting legends, and discussions of approach lighting that may be working or not working.
Derivation
The name is built from its function: 'Medium-intensity' describes the brightness class, 'Approach Lighting System' describes its purpose, and 'Sequenced Flashers' describes the chasing strobes that distinguish it from the plain MALS. Knowing the parts of the acronym makes the system easy to recognize on charts.
Why Pilots Care
Allows pilots to confirm runway alignment and glide path visually during the final segment of an instrument approach, directly affecting minimums and go/no-go decisions.
Grounding Statement
Picture a row of lights leading toward the runway, with a few flashing in sequence to draw your eyes in the landing direction.
Example Sentence 1
Runway 27 is served by a MALSF, so the sequenced flashers should help us pick up the runway environment as we break out of the clouds.
Example Sentence 2
With the MALSF in view, the crew could confirm the runway centerline and continue the descent without hesitation.