Editing Visual approach slope indicator (section)

==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]]
[[File:comparison_visual_landing_systems.svg|thumb|250px|Comparison of [[Precision approach path indicator|PAPI]], VASI, and [[Optical landing system|OLS]] meatball and datum lights {{nowrap|(not to scale)}}]]

Basic visual approach slope indicators consist of one set of lights set up {{convert|7|m|ft}} from the start of the runway. Each light is designed so that it appears as either white or red, depending on the angle at which it is viewed. When the pilot is approaching the lights at the proper angle, meaning the pilot is on the [[glide slope]], the first set of lights appears white and the second set appears red. When both sets appear white, the aircraft is too high, and when both appear red it is too low. This used to be the most common type of visual approach slope indicator system; however, it is being phased out and replaced by [[precision approach path indicator]]s (PAPIs), which are closer together and therefore more efficient to sight and maintain.

A mnemonic to remember the colors and their meaning is:

: White over White, you're high as a kite / you'll fly all night / check your height / you're out of sight / too much height.
: Red over White, you're alright / height's alright.
: Red over Red, you're dead / pilot's dead.<ref>[https://www.skybrary.aero/index.php/Visual_Approach_Slope_Indicator_Systems_(VASIS) Skybrary]</ref>

(White over Red is not possible.)

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>