Visual Approach Slope Indicator
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The Visual Approach Slope Indicator (VASI) is a system of lights on the side of a runway that provide visual descent guidance information during the approach to a runway. These lights are visible from 3-5 miles during the day and up to 20 miles or more at night.
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[edit] Four types of VASIs
[edit] 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 from 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.
[edit] PAPI (Precision Approach Path 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.
[edit] 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 very rare slope indicator because it is easy to mistake it for other runway lights.
[edit] 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 glide slope it indicates amber causing pilots inexperienced with it to correct in the wrong direction.
[edit] Mnemonics
- Red over white, you're all right.
- White over white, you're out of sight.
- Alternative: You'll fly all night or you're high as a kite.
- Red over red, you're dead.
- Alternative: Watch your head.
[edit] Reference
[edit] See also
- Pilot Controlled Lighting (PCL)
- Precision Approach Path Indicator (PAPI)
- Runway End Identification Lights (REIL)
- Runway Edge Lights (HIRL, MIRL, LIRL)
- Approach Lighting System (ALS)