Below Glide Path On Glide Path Above Glide Path
Airport Lighting Aids
2.1.2
6. For 3-bar VASI (6 light units) see FIG 2.1.3.
FIG 2.1.3
3-Bar VASI
7. For other VASI configurations see FIG 2.1.4.
FIG 2.1.4
VASI Variations
b. Precision Approach Path Indicator (PAPI).  5 miles during the day and up to 20 miles at night. The
The precision approach path indicator (PAPI) uses  row of light units is normally installed on the left side
light units similar to the VASI but are installed in a  of the runway and the glide path indications are as
single row of either two or four light units. These  depicted. (See FIG 2.1.5.)
systems have an effective visual range of about
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
Airport Lighting Aids
2.1.3
c.  Tri-color Systems. Tri-color visual approach  the on glide path indication is green. These types of
slope indicators normally consist of a single light unit  indicators have a useful range of approximately
projecting a three-color visual approach path into the  one-half to one mile during the day and up to
final approach area of the runway upon which the  five miles at night depending upon the visibility
indicator is installed. The below glide path indication  conditions. (See FIG 2.1.6.)
is red, the above glide path indication is amber, and
FIG 2.1.6
Tri-Color Visual Approach Slope Indicator
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
PULSATING WHITE
Threshold
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.
Airport Lighting Aids
2.1.4
FIG 2.1.8
Alignment of Elements
Above Glide Path On Glide Path Below Glide Path
d.Pulsating Systems. Pulsating visual ap-proach 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 is a steady 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.)