What does a VASI (Visual Approach Slope Indicator) showing red over white indicate?

2-1-2. Visual Glideslope Indicators
a. Visual Approach Slope Indicator (V ASI)
1. V ASI 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 V ASI installations consist of 2 bars, near and far, and may consist of 2, 4, or 12 light
units. Some V ASIs 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. V ASI 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 V ASI installations provide one visual glide path which is normally set at 3 degrees. Three-bar
V ASI 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 V ASI 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 V ASI 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 V ASIs during an approach will see the
combination of lights shown below.
4. The V ASI 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 V ASI 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 V ASI, 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 V ASI may be offset from the
extended runway centerline. This will be noted in the Chart Supplement and/or applicable Notices to Airmen
(NOTAMs).
Airport Lighting Aids 2-1-1

 AIM 2/20/25
FIG 2-1-1
Precision & Nonprecision Configurations
NOTE-
Civil ALSF-2 may be operated as SSALR during favorable weather conditions.
2-1-2 Airport Lighting Aids

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5. For 2-bar V ASI (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
6. For 3-bar V ASI (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 V ASI configurations see FIG 2-1-4.
FIG 2-1-4
V ASI Variations
2 Bar 2 Bar 3 Bar
2 Light Units 12 Light Units 16 Light Units
On Glide Path On Glide Path on Lower Glide Path
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AIM 2/20/25
b. Precision Approach Path Indicator (PAPI). The precision approach path indicator (PAPI) uses light units
similar to the V ASI but are installed in a single row of either two or four light units. These lights are visible from
about 5 miles during the day and up to 20 miles at night. The visual glide path of the PAPI typically provides
safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 3.4 NM from
the runway threshold. Descent, using the PAPI, should not be initiated until the aircraft is visually aligned with
the runway. The row of light units is normally installed on the left side of the runway and the glide path indications
are as depicted. 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 PAPI may be offset from the extended runway centerline. This will be
noted in the Chart Supplement and/or applicable NOTAMs. (See FIG 2-1-5.)
FIG 2-1-5
Precision Approach Path Indicator (PAPI)
High Slightly High On Glide Path Slightly Low Low
(More Than (3.2 Degrees) (3 Degrees) (2.8 Degrees) (Less Than
3,5 Degrees) 2.5 Degrees)
White
Red
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 five miles 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 PathOn 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.
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FIG 2-1-7
Pulsating Visual Approach Slope Indicator
NOTE-
Since the PVASI 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.
FIG 2-1-8
Alignment of Elements
Above Glide Path On Glide Path Below Glide Path
d. Pulsating Systems. Pulsating visual approach slope indicators normally consist of a single light unit
projecting a two-color visual approach path into the final approach area of the runway upon which the indicator
is installed. The on glide path indication may be a steady white light or alternating RED and WHITE light. The
slightly below glide path indication is a steady red light. If the aircraft descends further below the glide path, the
red light starts to pulsate. The above glide path indication is a pulsating white light. The pulsating rate increases
as the aircraft gets further above or below the desired glide slope. The useful range of the system is about four
miles during the day and up to ten miles at night. (See FIG 2-1-7.)
e. Alignment of Elements Systems. Alignment of elements systems are installed on some small general
aviation airports and are a low-cost system consisting of painted plywood panels, normally black and white or
fluorescent orange. Some of these systems are lighted for night use. The useful range of these systems is
approximately three-quarter miles. To use the system the pilot positions the aircraft so the elements are in
alignment. The glide path indications are shown in FIG 2-1-8.
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