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{{Short description|Visual aid in aviation}}

[[Image:PAPI M39.jpg|thumb|250px|Individual Precision Approach Path Indicator]]

{{More citations needed|date=December 2009}}

The '''Precision Approach Path Indicator (PAPI)''' is a light system positioned beside the [[runway]] that consists of two, three, or four boxes of lights that provide a visual indication of a [[fixed-wing aircraft]]'s position on the [[glidepath]] for the associated runway. The [[Federal Aviation Administration|FAA]] standard for the PAPI is the same as the [[ICAO]]'s standard [[VASI|Visual Approach Slope Indicator]].

[[Image:PAPI Jersey Airport.JPG|thumb|The PAPI can be seen to the right (non-standard) side of the runway. The aircraft is slightly below the glideslope.]]

The PAPI is usually located on the left side of the runway and can be seen up to five miles during the day and twenty miles at night. It has two or four lights installed in a single row instead of far and near bars that would be characteristic of Visual Approach Slope Indicator (VASI).

A '''precision approach path indicator''' ('''PAPI''') is a system of [[Electric light|lights]] on the side of an airport [[runway]] threshold that provides visual [[descent (aircraft)|descent]] guidance information during [[final approach]]. It is generally located on the left-hand side of the runway approximately {{convert|300|m|ft}} beyond the landing threshold of the runway.

Each box of lights is equipped with an optical apparatus that splits light output into two segments, red and white. Depending on the angle of approach, the lights will appear either red or white to the [[aviator|pilot]]. Ideally the total of lights will change from white to half red, moving in succession from right to left side. The pilot will have reached the normal glidepath (usually 3 degrees) when there is an even split in red and white lights. If an aircraft is beneath the glidepath, red lights will outnumber white; if an aircraft is above the glidepath, more white lights are visible.

==Design and installation==

During aircraft [[descent (aircraft)|descent]], this system, along with other airport lights, may be activated by the pilot by keying the aircraft microphone with the aircraft's communication radio tuned to the [[CTAF]].

A typical engineering design specification for a PAPI light unit is shown below:

[[File:PAPI-Section.svg|thumb|350px|Schematic diagram of longitudinal section<br />

1 = Axis datum<br />2 = Light source<br />3 = Red filter<br />4 = Lenses<br />5 / 6 = Light beam- white/red]]

Optical construction:

* Two lamps for redundancy;

* Anodized aluminium reflectors;

* Red color filter;

* One or two lenses;

* Lamps and reflectors replaceable without recalibration.

Each light unit consists of one or more light sources, red filters and lenses. A color filter may not be necessary with colored LED lights. Each light unit emits a high-intensity beam. The lower segment of the beam is red, and the upper part is white. The transition between the two colours must take place over an angle not greater than three minutes of arc. This characteristic makes the color change very conspicuous, a key feature of the PAPI signal. To form the PAPI guidance signal, the color transition boundaries of the four units are fixed at different angles. The lowest angle is used for the unit furthest from the runway, the highest for the unit nearest to the runway. The designated glideslope is midway between the second and third light unit settings.

The PAPI operates on the principle of the [[Fresnel lens]].

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"/>

==See also==

* [[Pilot Controlled Lighting]] (PCL)

* [[Visual Approach Slope Indicator]] (VASI)

* [[Runway End Identification Lights]] (REIL)

* [[Runway Edge Lights]] (HIRL, MIRL, LIRL)

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 counted starting from the side of the PAPI array closest to the runway. If the PAPI lights are on the right-hand side of the runway (non-standard), then the red lights will counted up 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).

== Sources ==

* [http://www.faa.gov/ATPubs/AIM/Chap2/aim0201.html#2-1-2 FAA Aeronautical Information Manual: Visual Glideslope Indicators]

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}}

* [http://av-info.faa.gov/terps/Directives_files/8260.49.pdf FAA TERPS directive]

For a typical 3 degree [[approach slope]], PAPI lights should be angled as follows: 3°30', 3°10', 2°50', 2°30' (3.50°, 3.17°, 2.83°, 2.50°).<ref name="ICAO"/>{{rp|8–36}}

==Interpretation==

[[File:comparison_visual_landing_systems.svg|thumb|250px|Comparison of PAPI, [[Visual approach slope indicator|VASI]], and [[Optical landing system|OLS]] meatball and datum lights {{nowrap|(not to scale)}}]]

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 see more white lights than red; below the ideal angle more red lights than white will be seen. At the optimum approach angle the ratio of white to red lights will be equal, for most aircraft.

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)

until they are used to the lights' meaning.

[[Image:PAPI M39.jpg|thumb|250px|Individual Precisionprecision Approachapproach Pathpath Indicatorindicator]]

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://www.caa.co.uk/our-work/publications/documents/content/cap-637/ |title=CAP 637: Visual Aids Handbook |date=31 May 2007 |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 |url-status=live |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 |archive-date=2024-01-20 |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}}.

PAPI systems are readily available from airfield lighting manufacturers worldwide. PAPI is normally operated by [[air traffic control]] (ATC). If ATC services are not normally provided at an aerodrome, PAPI along with other airport lights may be [[pilot-controlled lighting|activated by the pilot]] by keying the aircraft microphone with the aircraft's communication radio tuned to the [[CTAF]] or dedicated pilot controlled lighting (PCL) frequency.

==History==

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. 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.<ref>{{cite web |url=http://www.research-engineers.com/about_papi.html |title=Research Engineers PAPI |access-date=2017-05-14 |url-status=dead |archive-url=https://web.archive.org/web/20140125163859/http://research-engineers.com/about_papi.html |archive-date=2014-01-25 }}</ref>

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.{{citation needed|date=March 2023}}

PAPIs were used by NASA's Space Shuttle for its safe landing, for which Johnson was interviewed by UK local news media and TV.{{citation needed|date=March 2023}}

An earlier glideslope indicator system, the [[visual approach slope indicator]] (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 }}).{{citation needed|date=February 2023}}

2008 saw the advent of new PAPI devices manufactured using solid state [[LED lamp]]s instead of [[incandescent lamp]]s. The LEDs produce sufficient brightness to satisfy ICAO light intensity and beamspread standards, and average lifetime with the LED based systems is 50,000 hours or more. By using LEDs, the device's power consumption is lowered considerably. The LED systems run internally on DC voltage, so the DC voltage requirements, along with the LEDs' inherently low power consumption, now allow for solar-powered PAPIs, enabling them to function completely independently of a power grid.<ref>{{cite web |url=http://www.flightlight.com/airportlighting/4.0.1/4.0.1.html |title=Solar PAPI & APAPI (Precision Approach Path Indicator) |access-date=2012-04-10 |url-status=dead |archive-url=https://web.archive.org/web/20111230132434/http://www.flightlight.com/airportlighting/4.0.1/4.0.1.html |archive-date=2011-12-30 }}</ref>

The PAPI system is co-opted for use by the [[final approach (aviation)|Final Approach]] Runway Occupancy Signal (FAROS) system being introduced<ref>{{Cite web|url=http://www.faraim.org/aim/aim-4-03-14-87.html|title=Aeronautical Information Manual (AIM) - Page 87|website=www.faraim.org|access-date=2019-12-24}}</ref> by several major airports in the United States for the purpose of allowing pilots to resolve a [[runway incursion]] without requiring ''a priori'' notice of an occupied runway from the [[control tower]]. In FAROS, automated line-of-sight runway sensors detect if a vehicle has committed a runway incursion, and if so, will flash the PAPI lights to alert the pilot of an aircraft on final approach that the runway is currently occupied. The pilot then becomes responsible for resolving the conflict by notifying the [[air traffic controller]] and executing a [[go-around]]. Once the tower has ascertained that the runway has been cleared, the ground controller resets the PAPI so that landing operations may resume normally.<ref>{{cite web |url=https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/operations/td/projects/faros/solution/animation.cfm |title=Final Approach Runway Occupancy Signal (FAROS) - Animation |access-date=2010-06-07 |url-status=dead |archive-url=https://web.archive.org/web/20100209024258/https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/operations/td/projects/faros/solution/animation.cfm |archive-date=2010-02-09 }}</ref><ref>{{cite web |url=https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap2_section_1.html |title=Section 1. Airport Lighting Aids |access-date=2019-09-19 }}</ref>

==See also==

* [[Approach lighting system]] (ALS)

* [[Pilot Controlledcontrolled Lightinglighting]] (PCL)

* [[Visual Approachapproach Slopeslope Indicatorindicator]] (VASI)

* [[Runway Endend Identificationidentifier Lightslights]] (REIL)

* [[Runway Edgeedge Lightslights]] (HIRL, MIRL, LIRL)

* [[Optical landing system]]

* [[Leading lights]]

==References==

[[Category:Aviation terminology]]

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