Definition
A medium-intensity approach lighting system installed at the approach end of a runway, consisting of steady-burning white light bars extending outward from the runway threshold along the extended centerline, plus a series of sequenced flashing lights in the outer portion that fire one after another toward the runway. The sequenced flashers create a visible 'ball of light' that appears to travel down the lighting array toward the threshold, helping the pilot align with and identify the runway during an instrument approach.
Plain English
A row of approach lights leading up to the runway, set at medium brightness, with flashing lights at the far end that pulse one after another in a wave pointing toward the runway. The wave of flashes shows the pilot which way the runway is and helps them line up with it as they break out of the clouds.
Context Anchor
Seen on instrument approach charts and during final approach to a runway equipped with this lighting system.
Derivation
The name is built from its parts. 'Medium Intensity' describes the brightness setting (between low and high). 'Approach Lighting System' is the family of light arrays leading up to the runway threshold. 'Sequenced Flashers' refers to lights that flash in order, one after another, rather than all at once. Together the term tells you exactly what the system is and what makes it different from other approach lighting systems.
Why Pilots Care
Helps pilots acquire the runway visually and maintain alignment in marginal weather or at night, improving safety during the critical transition from instrument to visual flight.
Intuition Check
“Medium intensity” does not mean the system is only moderately important; it describes the brightness level of the lights. “Sequenced flashers” means the flashing lights turn on in order, creating a visual lead-in toward the runway.
Example Sentence 1
The approach chart showed a MALSF installed on Runway 27, so the crew knew to look for the sequenced flashers leading them to the threshold.
Example Sentence 2
MALSF provides sequenced flashers that lead the eye to the runway when visibility is limited.