The Precision Approach Path Indicator (PAPI) is the commonly used glidepath system. It provides the glidepath information required during approaches. It is usually found on the left side of the runway and consists of a single row of lights. The glidepath is normally set at 3 degrees.
In a typical system (see the figure shown above) a single row concicting of 4 light units projects the following colours based on the visual approach path:
- If the aircraft is below the glidepath, all 4 lights are red.
- If the aircraft is above the glidepath, all 4 lights are white.
- If the aircraft is on the glidepath, the PAPI projects 2 red and 2 white lights.
