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Reference.org
Visual approach slope indicator
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<p>The visual approach slope indicator (VASI) is a system of <a href="/facts/Electric_light/v3fb5WBp">lights</a> on the side of an airport <a href="/facts/Runway/EYy72SPJ">runway</a> threshold that provides visual <a href="/facts/Descent_(aircraft)/V28gn433">descent</a> guidance information during <a href="/facts/Final_approach/TsvNsvp2">final approach</a>. These lights may be visible from up to 8 kilometres (5.0 mi) during the day and up to 32 kilometres (20 mi) or more at night. </p>