PAPI (Precision Approach Path Indicator) is one of the main Visual Aids established right and left sides of runway (sometimes PAPI can be just in the left or just in the right side of runway but usually on the left because the captain pilot sits also on the left side in the cockpit). PAPI lights generally consist of four equivalent round shaped light units to indicate the vertical position of the approaching aircraft.
The international standard of PAPI is published by ICAO (International Civil Aviation Organisation) Annex 14 Volume 1, Chapter 5.
The earlier glideslope indication system VASI (Visual Approach Slope Indicator) is now inadequate and was deleted by ICAO in 1995. The main difference between VASI and PAPI is VASI only provides guidance down to heights of 60 metres (200 ft) whereas PAPI provides guidance down to flare initiation (typically 15 metres, or 50 ft).
PAPI lights indicate the vertical position of the aircraft according to the optimum approach slope. In detail there are four lights on a PAPI and if the approaching traffic is on the optimum position PAPI lights are two red and two right.
(Red lights on the module always burn near to the runway)
Let’s continue. If the approaching aircraft is flying above the optimum height than white lights increase and there are three white lights and one red light burn on PAPI. If the aircraft is too high than the optimum then it turns to four white lights.
On the other side, if the traffic is lower than the optimum height, red lights start to increase. As expected if the traffic lower than the optimum then three red light burn on PAPI (Red lights are always near to the runway) and if the traffic is too low than the optimum then PAPI turns to four red lights.
(Relatively being lower than the optimum level (height) is more dangerous for an aircraft. So the colour red symbolizes danger as we all know).
Who Operates PAPI Lights?
PAPI is normally operated by Air Traffic Controllers (ATCs). If ATC services are not normally provided at an aerodrome, along with other airport lights they may be 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.
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