Precision approach path indicator: Difference between revisions

Revision as of 20:28, 28 February 2023

Revision as of 20:48, 28 February 2023

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==Meaning==

{{comparison_visual_landing_systems.svg}}

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, for most aircraft, the exceptions being the [[Boeing 747]] and now retired [[Concorde]]. With the 747, because the cockpit is approximately 20 feet behind the nose and much higher than other aircraft, the flight crew in a 747 will typically see one red and three white lights when they are on the glide slope. The aircrew of Concorde would see four white lights as the Concorde's approach angle was higher than traditional aircraft.{{cn|date=February 2023}}

==Background==

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[[Image:PAPI M39.jpg|thumb|Individual precision approach path indicator]]

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 }}).{{cn|date=February 2023}}

PAPIs are calibrated relative to the Minimum Eye Height over Threshold (MEHT). Pilots in some aircraft may see a "slightly low" indication even though they are on the ILS glideslope.<ref>https://publicapps.caa.co.uk/docs/33/CAP637%20Visual%20Aids%20Handbook.pdf</ref>