A precision approach path indicator (PAPI) uses lights similar to the VASI system, except they are installed in a single row, normally on the left side of the runway. PAPI consists of four light units typically situated on the left side of the runway in the form of a wing bar. When the pilot is:

well above the approach slope, all four units show white.
slightly above the approach slope, the one unit nearest the runway edge shows red and the other three show white.
on or close to the approach slope, the two units nearest the runway edge show red and the two units farthest from the runway edge show white.
slightly below the approach slope, the three units nearest the runway edge show red and the other shows white.
well below the approach slope, all four units show red.

The APAPI system consists of two light units constructed and arranged in such a manner that a pilot making an approach will:

when on or close to the approach slope, see the light unit nearer the runway edge as red and the light unit farther from the runway edge as white;
when above the approach slope, see both light units as white; and
when below the approach slope, see both light units as red.

**Precision Approach Path Indicator (PAPI)**

Definitions

Abbreviated VASI (AVASI): Consists of two light units situated on the left side of the runway in the form of a pair of wing bars (one light unit per wing bar). The display is similar to that of a VASI and depends on the position of the pilot’s eyes.
Abbreviated PAPI (APAPI): A precision approach path indicator system consisting of only two light units situated on the left side of the runway. The aircraft is on a slope if the unit nearest the runway shows red and the unit farthest from the runway shows white, too high if both units show white, and too low if both units show red. The APAPI is usually set to an eye-to-wheel height (EWH) of 10 ft (3 m).
Eye-to-Wheel Height (EWH): The vertical distance from a pilot’s eyes to the lowest portion of the aircraft in the landing configuration is called the eye-to-wheel height (EWH), and this distance varies from less than 10 ft (3 m) up to 45 ft (14 m) for some wide-bodied aircraft, such as the B-747.
Minimum Eye Height Over Threshold (MEHT): The sum of minimum wheel clearance and eye-to-wheel height of an airplane in the landing configuration.
Precision Approach Path Indicator (PAPI): A system of lights arranged to provide visual descent guidance information during the approach to a runway. The system is similar to the VASI (visual approach slope indicator). However, the light units are installed in a single row of four light units on the left side of the runway. These systems have an effective visual range of about 5 miles during the day and up to 20 miles at night.
Visual Approach Slope Indicator (VASI): A system of lights arranged to provide visual descent guidance information during the approach to a runway. Most VASI installations consist of two bars – near and far. Some VASIs consist of three bars – near, middle, and far- which provide an additional visual glide path to accommodate with a high flight deck.

References

AIM AGA – 7.6.3 Precision Approach Path Indicator (PAPI) and Abbreviated PAPI (APAPI)
AIM AGA – 7.6.4 Categories According to Eye-To-Wheel Height (EWH) in the Approach Configuration

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