Definition
A configuration of approach lighting installed on the extended centerline of a runway, providing medium-intensity steady white lights along the approach path together with a series of sequenced flashing lights that fire one after another toward the runway threshold. It is used at runways equipped for non-precision instrument approaches, particularly where additional visual guidance is needed to identify the approach path under reduced visibility or in surroundings with distracting background lighting.
Plain English
A line of medium-brightness white lights leading up to the runway, with extra lights that flash one after another like a chasing arrow, helping the pilot find and line up with the runway when visibility is poor.
Context Anchor
Seen on airport lighting descriptions, instrument approach charts, and runway information for airports equipped with approach lighting.
Derivation
The name describes the system directly. 'Sequenced flashing lights' refers to lights that fire in order rather than all at once — the term 'sequence' comes from the Latin 'sequi,' meaning 'to follow.' That ordered, following pattern is what creates the visual 'arrow' pointing toward the runway.
Why Pilots Care
Provides positive visual identification of the runway and alignment cues in low-visibility conditions, reducing the risk of runway incursions or unstabilized approaches.
Analogy
It works a little like a row of guide lights leading to a driveway at night, with some lights flashing in order to draw your eyes toward the entrance.
Intuition Check
“Medium intensity” does not mean “somewhat useful” or “average quality.” It refers to the brightness category of the approach lights. “Sequenced flashing” means the lights flash in order toward the runway, not all at once or randomly.
Example Sentence 1
Cleared for the approach in light rain, the pilot picked up the MALSF first and followed the sequenced flashers down to the runway threshold.
Example Sentence 2
During the night ILS approach the sequenced flashing lights of the system gave clear distance and path cues through the haze.