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Development of PAPI
The Precision Approach Path Indicator, commonly known as PAPI, was devised by Tony Smith and David Johnson at the Royal Aircraft Establishment Bedford in 1974, and developed by them over the ensuing two years. Research Engineers (RE) were also pioneers in the development of PAPI, having produced and supplied PAPI units for the first trials conducted. The same design is still in use today.

Function of PAPI
PAPI is a visual aid that assists pilots landing aircraft. PAPI enables a pilot making an approach to a runway to acquire and maintain the correct glide path from the time that the PAPI system becomes visible until the aircraft has crossed the runway threshold. PAPI systems utilize a set of two colour high intensity light projectors. The upper half of the light beam from each projector is white and the lower half red. The transition between the two colours occurs over a very small angle. This sharp transition is an integral feature of the PAPI system. PAPI projectors are inclined at the nominal glide path angle. Typically, the elevation setting angles of each PAPI unit vary by 20 minutes of arc, the nominal 3 degree glideslope being midway between the centre pair. The approaching pilot sees either a red or white light signal from each PAPI light projector. The signal combinations provided to the approaching pilot are illustrated in Pilot's View.

Advantages of PAPI

PAPI is a digital system and has the well proven advantage that it can be interpreted as soon as the white signal becomes visible. It therefore has a greater usable range than the older VASI system which relied on the red signal being seen.