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6/17/21 

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

3000 feet for 

precision instrument runways and 1400

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. 

Two

bar VASI installations provide one 

visual glide path which is normally set at 3 degrees. 
Three

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

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 U.S. and/or applicable notices to 
airmen (NOTAM). 

Airport Lighting Aids 

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