Precision approach path indicator
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A precision approach path indicator (PAPI) is a visual aid that provides guidance information to help a pilot acquire and maintain the correct approach (in the vertical plane) to an airport or an aerodrome. It is generally located beside the runway approximately 300 meters beyond the landing threshold of the runway.
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. 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. 
What the lights mean
The ratio of white to red lights seen is dependent on the angle of approach to the runway. Above the designated glideslope 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.
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 glideslope. 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.
Student pilots in initial training may use the mnemonic, "RED on WHITE you're alright, RED on RED and you're dead" until they are used to the lights' meaning.
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 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 Organisation (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 60 metres (200 ft) whereas PAPI provides guidance down to flare initiation (typically 15 metres, or 50 ft).
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. 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 five miles by day and night. At night the light bars can be seen at ranges of at least twenty miles.
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 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 activated by the pilot by keying the aircraft microphone with the aircraft's communication radio tuned to the CTAF or dedicated pilot activated lighting (PAL) frequency.
A typical engineering design specification for a PAPI light unit is shown below:
- Preadjusted 2-lamp optical assembly.
- Anodized aluminium reflectors.
- 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 lamps instead of incandescent lamps. 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 power consumption is lowered considerably. The LED systems run internally on DC voltage so the DC voltage requirements, along with the LED's inherent low power consumption, now allow for solar-powered PAPIs, completely removing them from the power grid.
The PAPI system is co-opted for use by the final approach runway occupancy signal (FAROS) system being introduced at 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.
- Approach lighting system (ALS)
- Pilot controlled lighting (PCL)
- Visual approach slope indicator (VASI)
- Instrument landing system (ILS)
- Runway end identifier lights (REIL)
- Runway edge lights (HIRL, MIRL, LIRL)