Definition
A medium-intensity approach lighting system installed on the extended centerline of a runway, used to help pilots transition from instrument flight to visual flight during an approach. It consists of a series of steady-burning white light bars extending 1,400 feet from the runway threshold, combined with a sequence of flashing white lights (the Runway Alignment Indicator Lights, or RAIL) on the outer portion. MALSR is a standard approach lighting configuration for runways served by precision and certain non-precision instrument approach procedures, and its presence or absence affects the visibility minimums published on an approach chart.
Plain English
A line of bright white lights leading up to the runway, with flashing lights on the far end, that helps a pilot find and line up with the runway when breaking out of cloud or low visibility on an instrument approach.
Context Anchor
Seen on instrument approach charts, airport lighting information, and in inoperative-component notes that tell you whether landing minimums change if the MALSR is not working.
Derivation
The name is descriptive: 'medium intensity' refers to the brightness setting of the lights (between low and high intensity systems); 'approach lighting system' is the lead-in lighting toward the runway; and 'runway alignment indicator lights' are the flashing lights that show the pilot where the centerline points. Knowing each piece tells you exactly what the system is and does.
Why Pilots Care
Knowing the status of a MALSR helps determine whether approach minimums must be raised or the approach becomes unusable when lights are inoperative.
Grounding Statement
Picture a row of lights leading your eyes from the dark or hazy approach path toward the runway, with flashing lights pointing in the landing direction.
Intuition Check
Medium intensity does not mean medium importance; it describes the brightness category of the lights. Runway alignment indicator lights are not runway edge lights; they are flashing approach lights that help show whether you are lined up before you reach the runway.
Example Sentence 1
Because the MALSR was reported out of service, we added a quarter mile to the required visibility before flying the ILS.
Example Sentence 2
With the MALSR reported inoperative, the crew increased the decision height by 50 feet per the inoperative-components table.