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AIM

10/12/17

2−1−1

Airport Lighting Aids

Chapter 2. Aeronautical Lighting and 

Other Airport Visual Aids

Section 1. Airport Lighting Aids

2−1−1. 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 2−1−1.)

2−1−2. 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 

1

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4

 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).