Visual Approach slope indicator
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Visual approach slope indicators are special lights on the side of a runway to assist pilots with staying on the glide slope for the runway. there are four different types of VASIs
VASI Visual approach slope indicators
Two sets of lights one near the beginning of the runway and one about twenty feet down the runway. Each set of lights is designed so that viewing it form above a specific angle it shows two white lights and below that angle two red lights. If the pilot can see red on the set farther down the runway and white on the closest set then he is on the glide slope. White over white means he is too high and red over red too low. This is the most common type of VASI system.
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Airport_VASI_example.jpg
Image:Airport VASI example.jpg
PASI Precision approach slope indicator
These are four lights in a line perpendicular to the runway. These work in about the same way as VASIs but allow more accuracy in that they can show how far off the glide slope the aircraft is. Two white lights next to two red lights shows that the plane is right on the glide slope. One white next to three red shows that the plane is slightly below the glide slope. Three white lights and one red: slightly above. Four red far below. Four white far above. This system is most common in large airports.
PVASI Pulsating visual approach slope indicator
This is a single light next to the runway. It shows steady white on the glide slope Pulsating white above the glide slope. steady red below the glide slope that starts pulsing faster and faster the farther off the glide slope the plane is. This is a vary rare slope indicator because it is easy to mistake it for other runway lights.
Tri-colored VASI
This is a single light that shows amber above the glide slope, white on the glide slope and red below it. This is also a rarity because slightly below the glides slope it indicates amber causing pilots inexperienced with it to correct in the wrong direction.
Mnemonic
Red over white, you're all right. White over white, you're out of sight (alt. you'll fly all night or you're high as a kite). Red over red, you're dead.