Precision approach path indicator

A precision approach path indicator (PAPI) is a visual aid that provides guidance information to help a pilot acquire and maintain the correct approach (in the vertical plane) to an airport or an aerodrome. It is generally located beside the runway approximately 300 meters beyond the landing threshold of the runway.

The precision approach path indicator system was first devised in 1974 by Tony Smith and David Johnson at the Royal Aircraft Establishment in Bedford, England. It took them a further two years to fully develop the technology. Smith and Johnson's work was honoured by a commendation from the RAE, a Fellowship from the Aeronautical Society, an award from the American Flight Safety Foundation, and a Gold Medal from the British Guild of Air Pilots. Engineering firm Research Engineers (RE) were also heavily involved in the project, having produced and supplied PAPI units for the first trials that were conducted. The same design is still in use today, and in fact was used by NASA's Space Shuttle for its safe landing, for which Johnson was interviewed by UK local news media and TV.

The ratio of white to red lights seen is dependent on the angle of approach to the runway. Above the designated glide slope a pilot will observe more white lights than red, at approaches below the ideal angle more red lights than white will be seen. For the optimum approach angle the ratio of white to red lights will remain equal throughout.

The greater number of red lights visible compared with the number of white lights visible in the picture means that the aircraft is flying below the glideslope. To use the guidance information provided by the aid to follow the correct glide slope a pilot would manoeuvre the aircraft to obtain an equal number of red and white lights.

Student pilots in initial training may use the mnemonic,

until they are used to the lights' meaning.

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 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 Organisation (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 60 metres (200 ft) whereas PAPI provides guidance down to flare initiation (typically 15 metres, or 50 ft).

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