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A '''precision approach path indicator''' ('''PAPI''') is a system of [[Electric light|lights]] on the side of an airport [[runway]] threshold that provides visual [[descent (aircraft)|descent]] guidance information during [[final approach]]. It is generally located on the left-hand side of the runway approximately 300 meters beyond the landing threshold of the runway.
==Design and installation==
A typical engineering design specification for a PAPI light unit is shown below:
[[File:PAPI-Section.svg|thumb|350px|Schematic diagram of longitudinal section<br />
1 = Axis datum<br />2 = Light source<br />3 = Red filter<br />4 = Lenses<br />5 / 6 = Light beam- white/red]]
Optical construction:
* 2 lamps for redundancy
* Anodized aluminium reflectors.
* Red color
*
* Lamps and reflectors replaceable without recalibration.
A color filter may not be necessary with colored LED lights.
A PAPI installation consists of a bar of four units. The inner edge of the PAPI installation should be situated 15m from the runway edge, and not closer than 14m to any runway or taxiway. An abbreviated system, A-PAPI, consists of two units with the inner unit located 10m from the runway edge.<ref name="ICAO"/> Units should be frangible but not susceptible to jet blast.
The optimum distance from the runway threshold depends on the wheel clearance over the threshold of the types of aircraft landing on the runway; compatibility with non-visual glide paths such as ILS down to the minimum possible range and height; and any difference in elevation between the PAPI installation and the runway threshold. This optimum distance may be adjusted depending on runway length and obstacle clearance.<ref name="ICAO"/>{{rp|8-33}}
==Interpretation==
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