The L-880 precision approach path indicator, PAPI, is an array of lights positioned alongside the runway. It normally consists of four equally spaced light units color-coded to provide a visual indication of an aircraft’s position relative to the designated glideslope for the runway.
The PAPI is usually located on the left-hand side of the runway and at right angles to the runway center line. At some airports PAPIs are installed on either side of the runway but this is beyond ICAO requirements. The light characteristics of all light units are identical.
Each PAPI consists of a light sources, red filters and lenses. A high-intensity beam is emitted from each unit. The lower part of the beam is red in colour, while the upper part is white. To create the PAPI guidance beams, the colour transition boundaries of the four units are fixed at different angles. Depending on the approach position and angle of the aircraft, the lights will appear either red or white to the pilot. The pilot will be on the correct glide slope, usually 3 degrees, when there are an identical number of red and white lights.
Approach lighting forms part of the complete Aviation Ground Lighting pattern. High and/or Low Intensity Approach systems may be provided. The complete Approach Lighting system is made up of several parts, each designed to meet one or more of the above objectives. The extent to which a particular airport is equipped will depend upon its licensing category and the need to continue operations during periods of poor visibility. Variations will be found between approach lighting systems for military airfields and civilian airports. (elevated ZA480 and inset ZA480)
Simple approach lighting systems consists of a row of luminaires on the extended runway centreline with a crossbar of 10 luminaires recommends that the colour of the lights shall be such that they are distinguishable from other aeronautical ground lights and from any extraneous lighting in the vicinity of the system. Where Low Intensity (LI), simple approach lighting systems are installed, they are required to have luminaires displaying Red light. (ZA450)
Supplementary Approach Lighting is only installed for CAT II or CAT III operations and has been previously referred to as SHINGALS (Supplementary High Intensity Narrow Gauge Approach Lighting System). It is the same spacing as used on Touchdown lighting, TDZ, and the aiming point surface markings. The fixtures used for Supplementary Approach applications are the same as those used for the High Intensity Approach System, but Red filters are fitted to luminaires for side row barrettes.
Threshold lights indicate the start of the usable part of the runway and dependent on the scale or category of the runway comprise two elements. Firstly a bar of inset lights at the start of the runway, and secondly two wing bars outboard of the runway edge. All threshold lights show green light into the approach. The threshold is normally uni-directional inset luminaires, however where it is co-located with the runway end bar it may consist of bi-directional luminaires showing green into the approach and red to the end of the runway. The wing bar luminaires are uni‑directional elevated or inset luminaires depending on their location.
Runway end lights signify the end of the usable runway. They show red light in the direction of aircraft landing or taking off, and denote the extremity of the usable runway for landing, manoeuvring or take off purposes.