AIM 

3/21/24 

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 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

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1

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5.) 

FIG 2

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1

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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

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color Systems. 

Tri

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color visual approach slope indicators normally consist of a single light unit 

projecting a three

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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

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1

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6.) 

FIG 2

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1

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6 

Tri

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Color Visual Approach Slope Indicator 

Amber 

Above Glide Path

On Glide Path

Below Glide Path 

Amber 

Green 

Red 

NOTE

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1. 

Since the tri

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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. 

Airport Lighting Aids

2

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1

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4