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The PAPI is a light array positioned beside the [[runway]]. It normally consists of four equi-spaced light units color-coded to provide a visual indication of an aircraft's position relative to the designated [[glideslope]] for the runway. An abbreviated system (APAPI) consisting of two light units can be used for some categories of aircraft operations. The international standard for PAPI is published by the [[International Civil Aviation Organization]] (ICAO) in ''Aerodromes, Annex 14 to the Convention on International Civil Aviation, Volume 1, Chapter 5''. National regulations generally adopt the standards and recommended practices published by ICAO. An earlier glideslope indicator system, the [[visual approach slope indicator]] (VASI) is now obsolete and was deleted from ''Annex 14'' in 1995. The VASI only provided guidance down to heights of {{convert|60|m|ft|sigfig=1}} whereas PAPI provides guidance down to flare initiation (typically {{convert|15|m|ft|sigfig=1|disp=x|, or }}).
The PAPI is usually located on the left-hand side of the runway at right angles to the runway center line. The units are spaced 9 meters apart with the nearest unit 15 meters from the runway edge. A PAPI can, if required, be located on the right-hand side of the runway. The red lights are always on the side closest to the runway. If the PAPI is on the right-hand side of the runway (non-standard), the red lights will be on the left. At some locations PAPIs are installed on both sides of the runway but this level of provision is beyond the requirements of ICAO.
Each light unit consists of one or more light sources, red filters and lenses. 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. Depending on the position of the aircraft relative to the specified angle of approach, the lights will appear either red or white to the [[aviator|pilot]]. The pilot will have reached the normal glidepath (usually 3 degrees) when there is an equal number of red and white lights. If an aircraft is beneath the glidepath, red lights will outnumber white; if an aircraft is above the glidepath, more white lights are visible.
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