Precision approach path indicator

The PAPI is an instrument helping to carry out a correct approach (in the vertical plane) on an aerodrome or an airport. It is generally located approximately 300 meters beyond the landing threshold of the runway.

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 an aircraft's position on the glidepath for the associated runway. The FAA standard for the PAPI is the same as the ICAO's standard Visual Approach Slope Indicator.

The PAPI is usually located on the left or right side of the runway(at 90˚ to the runway centre line which are typically spaced at 9 metres apart.) Units are identical 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).

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 pilot. Ideally the total of lights will change from white to half red, moving in succession from the runway side to the outer 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.

During 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.

The Precision approach path indicator is based on the Fresnel Lens principle.

Optical construction: - Preadjusted 2-lamp optical assembly. - Anodized aluminium reflectors. - Red colour 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.

As of 2008 PAPIs are starting to be made using solid state LED technology instead of incandescent lamps. Average lifetime with the LED based systems is 50,000 hours or more. By using the LED technology the power consumption can be lowered considerably. The LED systems run internally on DC voltage so the DC voltage requirements along with the low power consumption now allow for Solar powered PAPIs.

See also