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The visual approach slope indicator (VASI) is a system of lights on the side of an airport runway threshold that provides visual descent guidance information during approach. 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.
Basic visual approach slope indicators consist of one set of lights set up some 7 metres (23 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 indicators (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
- Red over White, you're alright.
- Red over Red, you're dead.
- White over Red, unsaid / you're under head / you land on your head.
("White over red" isn't actually possible unless the VASI installation is badly off, the white and red filtered glasses on the lights were fitted upside down by accident, or else if the pilot is accidentally flying upside-down on final.)
- Leading lights, similar aid to navigation
- Pilot controlled lighting (PCL)
- Precision approach path indicator (PAPI)
- Runway end identifier lights (REIL)
- Runway edge lights (HIRL, MIRL, LIRL)
- Approach lighting system (ALS)