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{{Short description|Visual aid in aviation}}

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

A '''precision approach path indicator''' ('''PAPI''') is a [[visual aid]] that provides guidance information to help a pilot to acquire and maintain the correct approach (in the vertical plane) to an airport or an aerodrome. It is generally located on the left-hand side of the runway approximately 300 meters beyond the landing threshold of the runway.

[[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.]]

A '''precision approach path indicator''' ('''PAPI''') is a system of [[visualElectric aidlight|lights]] thaton providesthe guidanceside informationof toan helpairport a[[runway]] pilotthreshold tothat acquireprovides andvisual maintain the correct approach[[descent (in the vertical planeaircraft)|descent]] toguidance aninformation airportduring or[[final an aerodromeapproach]]. It is generally located on the left-hand side of the runway approximately {{convert|300 meters|m|ft}} beyond the landing threshold of the runway.

==Development==

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. 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. 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, and in fact was used by NASA's Space Shuttle for its safe landing, for which Johnson was interviewed by UK local news media and TV.<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>

==Design and installation==

==Meaning==

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

{{comparison_visual_landing_systems.svg}}

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

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}}

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

Optical construction:

* Two lamps for redundancy;

* Anodized aluminium reflectors.;

* Red color filters.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. Depending on the position of the aircraft relative to the specified angle of approach, the lights will appear either red or white to the [[aviator|pilot]]. The pilot will have reached the normal glidepath (usually 3 degrees) when there is an equal number of 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.

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 isshould usuallybe located on the left-hand side of the runway at right angles to the runway center line., The units are spaced 9 meters apart with the nearest unit 15 meters from the runway edge. A PAPIalthough can, if required, 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}}

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}}

==BackgroundInterpretation==

[[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 greater number of red lights visible compared with the number of white lights visible in the picture means that the aircraft is flying below the glide slope. To use the guidance information provided by the aid to follow the correct glide slope a pilot would manoeuvre the aircraft to obtain an equal number of red and white lights.

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 observesee more white lights than red; at approaches below the ideal angle more red lights than white will be seen. ForAt the optimum approach angle the ratio of white to red lights will remainbe 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}}

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)

Line 21 ⟶ 38:

[[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).<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>

PAPIs are calibrated relative to the Minimum Eye Height over Threshold (MEHT). Pilots in certain 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> when pilot eye height is above the ILS receiver antenna.<ref>https://www.airbus.com/sites/g/files/jlcbta136/files/2021-11/Airbus-Aircraft-Data-for-Visual-Aids-Calibration-v5.0.pdf</ref>

The PAPI is usually located on the left-hand side of the runway at right angles to the runway center line. The units are spaced 9 meters apart with the nearest unit 15 meters from the runway edge. A PAPI can, if required, be located on the right-hand side of the runway. The red lights are always on the side closest to the runway. If the PAPI is on the right-hand side of the runway (non-standard), the red lights will be on the left. At some locations PAPIs are installed on both sides of the runway but this level of provision is beyond the requirements of ICAO.

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}}.

Each light unit consists of one or more light sources, red filters and lenses. 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. Depending on the position of the aircraft relative to the specified angle of approach, the lights will appear either red or white to the [[aviator|pilot]]. The pilot will have reached the normal glidepath (usually 3 degrees) when there is an equal number of 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.

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.

==DesignHistory==

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. 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. 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, and in fact was used by NASA's Space Shuttle for its safe landing, for which Johnson was interviewed by UK local news media and TV.<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>

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 />

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}}

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

Optical construction:

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}}

* Preadjusted 2-lamp optical assembly.

* Anodized aluminium reflectors.

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}}

* Red color filters.

* Precision-ground lenses.

* Lamps and reflectors replaceable without recalibration.

* 2 x 200 W / 6,6 A prefocused halogen lamps, Pk30d base.

* Average lifetime 1000 hours at rated current.

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>

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* [[Pilot controlled lighting]] (PCL)

* [[Visual approach slope indicator]] (VASI)

* [[Instrument landing system]] (ILS)

* [[Runway end identifier lights]] (REIL)

* [[Runway edge lights]] (HIRL, MIRL, LIRL)

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[[Category:Airport lighting]]

[[Category:British inventions]]

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