Visual Approach Slope Indicator - Types - Tri-colored VASI

Tri-colored VASI

A tri-colored VASI is a single light that appears amber above the glide slope, green on the glide slope and red below it. It has fallen out of widespread use, partly because pilots who are unfamiliar with them have been known to misinterpret the lights and 'correct' in the wrong direction. These errors are increased due to a major design shortcoming of the tri-colored VASI. While on approach, the colour amber (above slope) can be seen at a very thin angle of approach between green (on slope) and red (below slope) due to the mixing of red and green which gives an amber colour. Pilots not familiar with this may see the amber light and think they are above glide slope and then descend rather than make the proper correction and ascending back to glide slope. Despite this shortcoming, it is (reportedly) in widespread use in Eastern European countries, especially Russia and Ukraine.