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AIM 

Chapter 2.  Aeronautical Lighting and Other Airport 

Visual Aids 

Section 1.  Airport Lighting Aids 

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1.  Approach Light Systems (ALS) 

a. 

ALS provide the basic means to transition from instrument flight to visual flight for landing. Operational 

requirements dictate the sophistication and configuration of the approach light system for a particular runway. 

b. 

ALS are a configuration of signal lights starting at the landing threshold and extending into the approach 

area a distance of 2400

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3000 feet for precision instrument runways and 1400

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1500 feet for nonprecision 

instrument runways. Some systems include sequenced flashing lights which appear to the pilot as a ball of light 

traveling towards the runway at high speed (twice a second). (See FIG 2

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2.  Visual Glideslope Indicators 

a.  Visual Approach Slope Indicator (VASI) 

1. 

VASI installations may consist of either 2, 4, 6, 12, or 16 light units arranged in bars referred to as near, 

middle, and far bars. Most VASI installations consist of 2 bars, near and far, and may consist of 2, 4, or 12 light 

units. Some VASIs consist of three bars, near, middle, and far, which provide an additional visual glide path to 

accommodate high cockpit aircraft. This installation may consist of either 6 or 16 light units. VASI installations 

consisting of 2, 4, or 6 light units are located on one side of the runway, usually the left. Where the installation 

consists of 12 or 16 light units, the units are located on both sides of the runway. 

2. 

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bar VASI installations provide one visual glide path which is normally set at 3 degrees. Three

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bar 

VASI installations provide two visual glide paths. The lower glide path is provided by the near and middle bars 

and is normally set at 3 degrees while the upper glide path, provided by the middle and far bars, is normally 

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degree higher. This higher glide path is intended for use only by high cockpit aircraft to provide a sufficient 

threshold crossing height. Although normal glide path angles are three degrees, angles at some locations may 

be as high as 4.5 degrees to give proper obstacle clearance. Pilots of high performance aircraft are cautioned that 

use of VASI angles in excess of 3.5 degrees may cause an increase in runway length required for landing and 

rollout. 

3. 

The basic principle of the VASI is that of color differentiation between red and white. Each light unit 

projects a beam of light having a white segment in the upper part of the beam and red segment in the lower part 

of the beam. The light units are arranged so that the pilot using the VASIs during an approach will see the 

combination of lights shown below. 

4. 

The VASI is a system of lights so arranged to provide visual descent guidance information during the 

approach to a runway. These lights are visible from 3

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5 miles during the day and up to 20 miles or more at night. 

The visual glide path of the VASI provides safe obstruction clearance within plus or minus 10 degrees of the 

extended runway centerline and to 4 NM from the runway threshold. Descent, using the VASI, should not be 

initiated until the aircraft is visually aligned with the runway. Lateral course guidance is provided by the runway 

or runway lights. In certain circumstances, the safe obstruction clearance area may be reduced by narrowing the 

beam width or shortening the usable distance due to local limitations, or the VASI may be offset from the 

extended runway centerline. This will be noted in the Chart Supplement and/or applicable Notices to Air 

Missions (NOTAMs). 

Airport Lighting Aids 

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