Precision approach path indicator: Difference between revisions

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A PAPI installation consists of a bar of four units. Units should be [[Frangibility|frangible]] but not susceptible to [[jet blast]]. The inner edge of the PAPI installation should be situated {{convert|15|m|ft}} from the runway edge, and not closer than {{convert|14|m|ft}} to any runway or taxiway. The units should be spaced {{convert|9|m|ft}} apart. An abbreviated system, A-PAPI, can be used for some categories of aircraft operations. It consists of two units with the inner unit located {{convert|10|m|ft}} from the runway edge.<ref name="ICAO"/>

The PAPI should be located on the left-hand side of the runway at right angles to the runway center line, although can be located on the right-hand side of the runway if required. The red lights are always oncounted starting from the side of the PAPI array closest to the runway. If the PAPI islights are on the right-hand side of the runway (non-standard), then the red lights will becounted onup starting from the left of the array. At some locations, PAPIs are installed on both sides of the runway but this level of provision is beyond the requirements of the [[International Civil Aviation Organization]] (ICAO).

The optimum distance from the runway threshold depends on the wheel clearance over the threshold of the types of aircraft expected to land on the runway; compatibility with non-visual glide paths such as [[instrument landing system]] (ILS) down to the minimum possible range and height; and any difference in elevation between the PAPI installation and the runway threshold. This optimum distance may be adjusted depending on runway length and obstacle clearance. Harmonisation between PAPIs and an ILS system must take into account the distance between eye height and ILS receiver height for various aircraft.<ref name="ICAO"/>{{rp|8–33}}

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Student pilots in initial training may use the mnemonic

* WHITE on WHITE -– "Check your height" (or "You're gonna fly all night") (too high)

* RED on WHITE – "You're all right"

* RED on RED – "You're dead" (too low)

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

PAPIs are calibrated relative to the Minimum Eye Height over Threshold (MEHT).<ref name="ICAO">{{cite book |title=ICAO Doc 9157, Aerodrome Design Manual. Fifth edition, 2021. Part 4, Visual Aids. Chapter 8 |publisher=ICAO |url=http://icscc.org.cn/upload/file/20220712/20220712090914_13813.pdf |access-date=1 March 2023 |archive-date=1 March 2023 |archive-url=https://web.archive.org/web/20230301152804/http://icscc.org.cn/upload/file/20220712/20220712090914_13813.pdf |url-status=dead }}</ref> For certain aircraft with a low pilot eye height, the pilot will see a "slightly low" indication even though they are on the ILS glideslope.<ref>{{Cite book |url=https://publicappswww.caa.co.uk/docsour-work/33publications/documents/content/cap-637/CAP637%20Visual%20Aids%20Handbook.pdf {{bare|title=CAP URL637: Visual Aids Handbook PDF|date=31 May 20232007 |publisher=[[Civil Aviation Authority (United Kingdom)]] |isbn=978 0 11790 844 4 |access-date=5 November 2024 |orig-date=First issued January 1997 |archive-url=https://web.archive.org/web/20240625184548/https://www.caa.co.uk/publication/download/12192 |archive-date=25 June 2024 |url-status=live |issue=2}}</ref> Pilot eye height is usually above the ILS receiver antenna.<ref>{{Cite web |date=2019-04-19 |title=Subject: Airbus Aircraft Data for Visual Aids Calibration |url=https://www.airbus.com/sites/g/files/jlcbta136/files/2021-11/Airbus-Aircraft-Data-for-Visual-Aids-Calibration-v5.0.pdf {{bare|url-status=live URL|archive-url=https://web.archive.org/web/20240120122413/https://www.airbus.com/sites/g/files/jlcbta136/files/2021-11/Airbus-Aircraft-Data-for-Visual-Aids-Calibration-v5.0.pdf PDF|archive-date=May2024-01-20 2023|access-date=2024-11-05 |website=Airbus |series=Issue 1.0 |id=REF:ME1911191}}</ref> [[Concorde]] had a particularly high eye height because the main undercarriage was so far behind the cockpit, so the pilots needed to land with a "slightly high" indication.<ref name="Bannister">{{cite book |last1=Bannister |first1=Mike |title=Concorde: The thrilling account of history's most extraordinary airliner |date=29 September 2022 |publisher=Random House |isbn=978-0-241-55702-0 |url=https://books.google.com/books?id=QSthEAAAQBAJ |access-date=1 March 2023 |language=en}}</ref>

The light characteristics of all light units are identical. In good visibility conditions the guidance information can be used at ranges up to {{convert|5|mi|km}} by day and night. At night the light bars can be seen at ranges of at least {{convert|20|mi|km}}.

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[[Category:Airport lighting]]

[[Category:British inventions]]

[[Category:Types of final approach (aviation)]]