The designated glideslope is midway between the second and third light unit settings. The lowest angle is used for the unit furthest from the runway, the highest for the unit nearest to the runway. To form the PAPI guidance signal, the color transition boundaries of the four units are fixed at different angles. This characteristic makes the color change very conspicuous, a key feature of the PAPI signal. The transition between the two colours must take place over an angle not greater than three minutes of arc. The lower segment of the beam is red, and the upper part is white. Each light unit emits a high-intensity beam. At night the light bars can be seen at ranges of at least 20 miles (32 km).Įach light unit consists of one or more light sources, red filters and lenses. In good visibility conditions the guidance information can be used at ranges up to 5 miles (8.0 km) by day and night. The light characteristics of all light units are identical. At some locations PAPIs are installed on both sides of the runway but this level of provision is beyond the requirements of ICAO. If the PAPI is on the right-hand side of the runway (non-standard), the red lights will be on the left. The red lights are always on the side closest to the runway. A PAPI can, if required, be located on the right-hand side of the runway. The units are spaced 9 meters apart with the nearest unit 15 meters from the runway edge. The PAPI is usually located on the left-hand side of the runway at right angles to the runway center line. The VASI only provided guidance down to heights of 60 metres (200 ft) whereas PAPI provides guidance down to flare initiation (typically 15 metres, or 50 ft). An earlier glideslope indicator system, the visual approach slope indicator (VASI) is now obsolete and was deleted from Annex 14 in 1995. National regulations generally adopt the standards and recommended practices published by ICAO. 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. An abbreviated system (APAPI) consisting of two light units can be used for some categories of aircraft operations. 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. The PAPI is a light array positioned beside the runway. Individual precision approach path indicator Until they are used to the lights' meaning.