Definition
A medium-intensity approach lighting system installed off the approach end of a runway, consisting of a row of steady white lights extending outward from the runway threshold, combined with a sequence of flashing white lights (the Runway Alignment Indicator Lights, or RAIL) on the outer portion. It is the standard approach lighting system for Category I precision instrument runways at many U.S. airports and is one of the lighting systems referenced when determining minimum visibility on an approach chart.
Plain English
A line of lights leading up to the runway, plus a row of flashing lights farther out that point the way in. Together they help a pilot find the runway and line up with it during a low-visibility approach.
Context Anchor
Seen on instrument approach charts, runway lighting notes, and in procedures for what to do when airport lighting components are out of service.
Derivation
The name describes its parts. 'Medium Intensity' refers to the brightness level (between low and high intensity systems). 'Approach Lighting System' is the row of lights extending from the runway out into the approach area. 'Runway Alignment Indicator Lights' are the sequenced flashing lights — sometimes called the 'rabbit' because the flash appears to run toward the runway — that show the pilot where the centerline is from farther away.
Why Pilots Care
Determines whether landing minimums can be used or must be raised if the system or its RAIL component is out of service.
Analogy
Think of it like a lit path leading to a driveway at night. The steady lights show the path, and the flashing lights draw your eyes toward the correct direction of travel.
Intuition Check
Medium intensity does not mean the system is less important. It means the lights are designed for a certain brightness level, while the system still provides important runway guidance.
Example Sentence 1
The MALSR with RAIL was operational, so the crew could use the published Category I minimums for the ILS approach.
Example Sentence 2
With the runway alignment indicator lights inoperative, the crew increased their decision height by fifty feet.