Definition
A standard approach lighting system installed at the approach end of a runway, consisting of a 1,400-foot configuration of steady-burning white lights combined with a sequence of five flashing strobe lights (the runway alignment indicator lights, or RAIL) on the outer portion. The MALSR provides visual guidance to pilots transitioning from instrument flight to visual references during the final stage of an instrument approach, and it is the most common approach lighting system serving Category I precision approaches in the United States.
Plain English
A line of lights stretching out from the runway end to help a pilot see and line up with the runway when breaking out of cloud or low visibility. The flashing strobes on the far end act like a 'follow me' arrow pointing to the runway.
Context Anchor
You will see MALSR listed on approach charts and may see it ahead of the runway when breaking out of clouds or reduced visibility near the end of an instrument approach.
Derivation
The name is descriptive: 'medium-intensity' refers to the brightness level (between low- and high-intensity systems), and 'runway alignment indicator lights' refers specifically to the flashing strobe portion that points the pilot toward the runway centerline.
Why Pilots Care
Provides essential visual cues that allow safe continuation of an instrument approach below decision altitude or minimum descent altitude when the runway environment becomes visible.
Grounding Statement
Picture a line of lights reaching out from the runway toward the arriving airplane, guiding your eyes to the correct runway end.
Intuition Check
Do not read “medium-intensity” as meaning “not very useful.” It describes the brightness category of the lights, not their operational importance.
Example Sentence 1
Approaching minimums on the ILS, the pilot called the MALSR in sight and continued the descent toward the runway.
Example Sentence 2
During the missed approach briefing, the crew noted that the MALSR was out of service and planned accordingly.