Visual Approach Slope Indicator Lights
VASI is an acronym for Visual Approach Slope Indicator that is located approximately twenty-five feet in from the beginning or start of the runway. The system consists of two separate rows of lights on a box usually located on the left side of the runway. It provides a proper glide path for a plane to follow to the runway.
For the two row VASI system both rows, for clarity of explanation, consists of one row of white lights and one row of red lights. The normal glide slope is three degrees with variations for sloped runways and high obstructions nearby with different banks of lighting.
When the pilot approaches the runway he is faced with three choices. He is too high. (Both rows of double lights show only the white lights.) He is right on the glide path. (Top row of double lights shows only the white lights and the bottom row of double lights show only the red lights.) He is below the glide path. Top row of double lights show only red lights and the bottom row of double lights show only red lights.
To the Pilot:
Two White Bars - you are above the glide slope.
Upper Bar White - Lower Bar Red - you are on the glide path
Two Red Bars - you are below the glide slope
The lighting system I describe is very simplistic. It is just my way of illustrating a very complex system of banks of lights.
Visibility of the lighting system is between two and four miles in daylight and upwards of twenty miles at night.
VASI allows a pilot to follow a normal glide slope of three degrees down to the runway.
The depth of explanation is too much for a brief post . You know the basics now click on VASI. It will allow you to read the details of the system and the variations that exist at different airports.
