Visual approach slope indicator: Difference between revisions

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{{Short description|Runway light set}}

{{redirect|VASI|other uses|Vasi (disambiguation){{!}}Vasi}}

{{Refimprove|date=November 2012}}

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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Simple VASIs are obsolete, having been deleted from ICAO Annex 14 in 1995, however T-VASIS and AT-VASIS are still specified. T-VASIS is defined as twenty light units symmetrically disposed about the runway centre line in the form of two wing bars of four light units each, with bisecting longitudinal lines of six lights". AT-VASIS is an abbreviated form of T-VASIS, defined as "ten light units arranged on one side of the runway in the form of a single wing bar of four light units with a bisecting longitudinal line of six lights."<ref name="Annex14">{{cite web |title=Annex 14 to the Convention on International Civil Aviation. Aerodromes. Volume 1, Aerodrome Design and Operations |url=https://skylibrarys.files.wordpress.com/2016/07/annex-14-aerodromes.pdf |publisher=ICAO |access-date=1 March 2023 |quote=deletion of specifications on VASIS (AVASIS) and 3-BAR VASIS (3-BAR AVASIS)}}</ref>

== Tri-color VASI ==

[[File:Tri-color visual approach slope indicator.png|thumb|left|Tri-color VASI]]

A tri-color system consists of a single-light unit projecting a three-color visual approach path. Below the glide path is indicated by red, on the glide path is indicated by green, and above the glide path is indicated by amber. When descending below the glide path, there is a small area of dark amber. Pilots should not mistake this area for an “above the glide path” indication.<ref>{{Cite PHAK|year=2023|chapter=14|page=17}}</ref>

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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==

* [[Index of aviation articles]]

* [[Approach lighting system]] (ALS)

* [[Leading lights]]

* [[Optical landing system]] (OLS)

* [[Pilot controlled lighting]] (PCL)

* [[Runway edge lights]] (HIRL, MIRL, LIRL)

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==References==

{{reflist}}

<references/>

[https://web.archive.org/web/20090904034137/http://www.faa.gov/air_traffic/publications/ATpubs/AIM/Chap2/aim0201.html FAA Aeronautical Information Manual, Chapter 2 (Aeronautical Lighting and Other Airport Visual Aids), Section 1 (Airport Lighting Aids)]

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[[Category:Airport lighting]]

[[Category:British inventions]]

[[Category:Optical communications]]