Aeronautical Information Manual (AIM), page 97

Index   96 -- Page 97 -- 98


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

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 2-1-1

Page 97 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   96 -- Page 97 -- 98