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The '''Precision Approach Path Indicator (PAPI)''' is a light system positioned beside the [[runway]] that consists of two, three, or four boxes of lights that provide a visual indication of an airplane's position on the [[glidepath]] for the associated runway. The [[Federal Aviation Administration|FAA]] standard for the PAPI is the same as the [[ICAO]]'s standard [[VASI|Visual Glide Slope Indicator]].
The PAPI is usually located on the left side of the runway and can be seen up to five miles during the day and twenty miles at night.
Each box of lights is equipped with an optical apparatus that splits light output into two segments: red (the lower segment) and white. Depending on the viewing, and thus the descent, angle, a [[aviator|pilot]] will see either the red or the white portion.▼
▲Each box of lights is equipped with an optical apparatus that splits light output into two segments: red (the
A normal glidepath (usually 3%) is indicated by an even split in red and white lights. If an airplane is beneath the glidepath, more red lights are visible to the pilots; if an airplane is above the glidepath, more white lights are visible.
During aircraft [[descent_(aircraft)|descent]], this system, along with other airport lights, may be activated by the pilot by keying the airplane microphone with the airplane's communication radio tuned to the [[CTAF]].
The PAPI operates on the principle of the Fresnel lens.
==See also==
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