A precision approach path indicator (PAPI) uses lights similar to the VASI system, except they are installed in a single row, normally on the left side of the runway. PAPI consists of four light units typically situated on the left side of the runway in the form of a wing bar. When the pilot is:
- well above the approach slope, all four units show white.
- slightly above the approach slope, the one unit nearest the runway edge shows red and the other three show white.
- on or close to the approach slope, the two units nearest the runway edge show red and the two units farthest from the runway edge show white.
- slightly below the approach slope, the three units nearest the runway edge show red and the other shows white.
- well below the approach slope, all four units show red.
The APAPI system consists of two light units constructed and arranged in such a manner that a pilot making an approach will:
- when on or close to the approach slope, see the light unit nearer the runway edge as red and the light unit farther from the runway edge as white;
- when above the approach slope, see both light units as white; and
- when below the approach slope, see both light units as red.