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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
16light 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
12or 16 light units, the units are located on both sides
of the runway.
3. 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.
4. 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.
5. For 2‐bar VASI (4 light units) see FIG 2-1-2.
FIG 2-1-2
2‐Bar VASI
Far Bar
= Red
Near Bar = White
Below Glide Path On Glide Path Above Glide Path
2/14/08 AIM
Airport Lighting Aids 2-1-3
6. For 3‐bar VASI (6 light units) see FIG 2-1-3.
FIG 2-1-3
3‐Bar VASI
Far Bar
Middle Bar
Near Bar
Below Both
Glide Paths
On Lower
Glide Path
On Upper
Glide Path
Above Both
Glide Paths
7. For other VASI configurations see FIG 2-1-4.
FIG 2-1-4
VASI Variations
2 Bar
2 Light Units
On Glide Path
2 Bar
12 Light Units
On Glide Path
3 Bar
16 Light Units
on Lower Glide Path
b. Precision Approach Path Indicator (PAPI).
The precision approach path indicator (PAPI) uses
light units similar to the VASI but are installed in a
single row of either two or four light units. These
systems have an effective visual range of about
5miles during the day and up to 20 miles at night. The
row of light units is normally installed on the left side
of the runway and the glide path indications are as
depicted. (See FIG 2-1-5.)
FIG 2-1-5
Precision Approach Path Indicator (PAPI)
Slightly High
(3.2 Degrees)
White
Red
High
(More Than
3,5 Degrees)
On Glide Path
(3 Degrees)
Slightly Low
(2.8 Degrees)
Low
(Less Than
2.5 Degrees)
AIM 2/14/08
2-1-4 Airport Lighting Aids
c. Tri‐color Systems. Tri‐color visual approach
slope indicators normally consist of a single light unit
projecting a three‐color visual approach path into the
final approach area of the runway upon which the
indicator is installed. The below glide path indication
is red, the above glide path indication is amber, and
the on glide path indication is green. These types of
indicators have a useful range of approximately
one‐half to one mile during the day and up to
fivemiles at night depending upon the visibility
conditions. (See FIG 2-1-6.)
FIG 2-1-6
Tri‐Color Visual Approach Slope Indicator
Amber
Above Glide Path
On Glide Path
Below Glide Path
Amber
Green
Red
NOTE-
1. Since the tri‐color VASI consists of a single light source which could possibly be confused with other light sources, pilots
should exercise care to properly locate and identify the light signal.
2. When the aircraft descends from green to red, the pilot may see a dark amber color during the transition from green to
red.
FIG 2-1-7
Pulsating Visual Approach Slope Indicator
Above Glide Path
On Glide Path
Below Glide Path
Slightly Below Glide Path
Threshold
PULSATING WHITE
PULSATING RED
STEADY WHITE
STEADY RED
NOTESince
the PVASI consists of a single light source which could possibly be confused with other light sources, pilots should
　
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