Precision approach path indicator

To form the PAPI guidance signal, the color transition boundaries of the four units are fixed at different angles.

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).

In good visibility conditions the guidance information can be used at ranges up to 5 miles (8.0 km) by day and night.

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 controlled lighting (PCL) frequency.

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.

[5] 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] 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] An earlier glideslope indicator system, the visual approach slope indicator (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 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.

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.