Precision Approach Path Indicator (PAPI) was devised at the Royal Aircraft Establishment Bedford in 1974. Research Engineers (RE) were pioneers in the development of PAPI, having produced and supplied PAPI units for the first PAPI trials. The same design is still in use today.
Research Engineers PAPI's compact modular construction and three point mounting system reduces field maintenance to the extent that only the elevation angle and cross leveling need to be checked. Projectors are secured to the base frame by three, quarter turn DZUS fasteners. Research Engineers PAPI projectors are completely interchangeable. The re-lamping of projectors is also a very simple task.
Research Engineers PAPI is fitted with a purpose designed parabolic reflector that produces a parallel light beam. This makes it possible to use a single lens system. Advantages of single lens system are: Optics demist automatically in a few minutes without the need for separate heating circuits. Modern high transmission dichroic filters with brighter, more accurate red colour are used. Less surface reflections means better signal contrast. Less glass components in the light beam means lower absorption losses. Reduced brightness difference between red and white sectors of light beam means less pilot dazzle.