Editing Precision approach path indicator (section)

==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:
* Two lamps for redundancy;
* Anodized aluminium reflectors;
* Red color filter;
* One or two lenses;
* Lamps and reflectors replaceable without recalibration.

Each light unit consists of one or more light sources, red filters and lenses. A color filter may not be necessary with colored LED lights. Each light unit emits a high-intensity beam. The lower segment of the beam is red, and the upper part is white. The transition between the two colours must take place over an angle not greater than three minutes of arc. This characteristic makes the color change very conspicuous, a key feature of the PAPI signal. To form the PAPI guidance signal, the color transition boundaries of the four units are fixed at different angles. The lowest angle is used for the unit furthest from the runway, the highest for the unit nearest to the runway. The designated glideslope is midway between the second and third light unit settings.

A PAPI installation consists of a bar of four units. Units should be [[Frangibility|frangible]] but not susceptible to [[jet blast]]. The inner edge of the PAPI installation should be situated {{convert|15|m|ft}} from the runway edge, and not closer than {{convert|14|m|ft}} to any runway or taxiway. The units should be spaced {{convert|9|m|ft}} apart. An abbreviated system, A-PAPI, can be used for some categories of aircraft operations. It consists of two units with the inner unit located {{convert|10|m|ft}} from the runway edge.<ref name="ICAO"/>

The PAPI should be located on the left-hand side of the runway at right angles to the runway center line, although can be located on the right-hand side of the runway if required. The red lights are always counted starting from the side of the PAPI array closest to the runway. If the PAPI lights are on the right-hand side of the runway (non-standard), then the red lights will counted up starting from the left of the array. At some locations, PAPIs are installed on both sides of the runway but this level of provision is beyond the requirements of the [[International Civil Aviation Organization]] (ICAO).

The optimum distance from the runway threshold depends on the wheel clearance over the threshold of the types of aircraft expected to land on the runway; compatibility with non-visual glide paths such as [[instrument landing system]] (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. Harmonisation between PAPIs and an ILS system must take into account the distance between eye height and ILS receiver height for various aircraft.<ref name="ICAO"/>{{rp|8–33}}

For a typical 3 degree [[approach slope]], PAPI lights should be angled as follows: 3°30', 3°10', 2°50', 2°30' (3.50°, 3.17°, 2.83°, 2.50°).<ref name="ICAO"/>{{rp|8–36}}