Visual approach slope indication systems (VASIS) provide the pilot with glidepath information that can be used for day or night approaches. By maintaining the proper glidepath as provided by the system, a pilot should have adequate obstacle clearance and should touch down within a specified portion of the runway. VASIS consists of a series of lights visible from approximately 4 NM and is designed to provide visual indications of the desired approach slope to a runway (usually 3°).

Precision Approach Path Indicator (PAPI) located to the left of the runway

At a certified airport, aircraft following the on-slope signal are provided with safe obstruction clearance to a minimum of 6° on either side of the extended runway centreline out to 7.5 km (4.1 NM) from the runway threshold. Newly certified airports are commonly protected up to 8° on each side of the extended runway centreline and up to 15 km (8 NM) from the runway threshold. Exceptions will be noted in the CFS. Descent using VASIS should not be initiated until the aircraft is visually aligned with the runway centreline.

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.
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 aircraft with a high flight deck.

References

AIM AGA 7.6 – Visual Approach Slope Indicator Systems (VASIS)

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