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The '''visual approach slope indicator''' ('''VASI''') is a system of [[Electric light|lights]] on the side of an airport [[runway]] threshold that provides visual [[descent (aircraft)|descent]] guidance information during [[final approach]]. These lights may be visible from up to {{convert|8|km|mi}} during the day and up to {{convert|32|km|mi}} or more at night.
==Standard VASI
[[Image:Vasi-1.JPG|thumb|Standard visual approach slope indicator (circled in red)]]
[[Image:Visual Landing Lighting System 1.jpg|thumb|Working VASI]]
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[[File:Pulsating visual approach slope indicator.png|thumb|left|Pulsating VASI]]
The pulsating visual approach slope indicator (PVASI) is a system similar in purpose to a VASI, but only consists of one piece of equipment. It indicates the pilot's height on approach with a mix of red and white, steady and pulsating light. Pulsating white light indicates that the aircraft is too high, whereas steady white indicates being on the glide path. Steady red light is meant to show a position only slightly below glide path, with pulsating red light being indicative of a position well below glide path.<ref>{{Cite book |url=https://www.columbus.af.mil/Portals/39/documents/New%20Students/FIH_1MAR18.pdf#page=90 |title=DoD Flight Information Publication (Enroute) - Flight Information Handbook |publisher=[[National Geospatial-Intelligence Agency]] |year=2018 |___location=St. Louis, Missouri |pages=B-34 |archive-url=https://web.archive.org/web/20210829190252/https://www.columbus.af.mil/Portals/39/documents/New%20Students/FIH_1MAR18.pdf |archive-date=2021-08-29 |url-status=live}}</ref> This type of system is less prevalent than other visual glide slope indicators like regular VASIs or PAPIs: as of May 2022 there were only 84 PVASIs installed at 51 airports and heliports in the United States and its territories according to [[Federal Aviation Administration]] data, compared to 6730 PAPIs and 623 VASIs.<ref>{{Cite web |title=Airport Data and Information Portal |url=https://adip.faa.gov/agis/public/#/public |access-date=2022-05-09 |website=adip.faa.gov |publisher=[[Federal Aviation Administration]]}}</ref>
== Dual VASI ==
Some airports serving long-bodied aircraft have three-bar VASIs that provide two visual glide paths to the same runway. The first glide path encountered is the same as provided by the standard VASI. The second glide path is about 25 percent higher than the first and is designed for the use of pilots of long-bodied aircraft.<ref name="IFH">{{cite book |title=Instrument Flying Handbook |date=2012 |publisher=[[Federal Aviation Administration]] Flight Standards Service |edition=FAA-H-8083-15B |url=https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/FAA-H-8083-15B.pdf |chapter=Chapter 9. Navigation Systems |pages=38}} {{PD-notice}}</ref>
==See also==
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[[Category:Airport lighting]]
[[Category:British inventions]]
[[Category:Optical communications]]
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