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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

/

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).