What is the PAPI beam?
Each PAPI unit sends out one beam, white above and red below. Four units set at slightly different angles turn that beam into a glide-path signal.
A PAPI (Precision Approach Path Indicator) is a wing bar of four light units beside the runway. Each unit projects a single beam that is split vertically: white light above its setting angle and red light below it, with a very sharp transition between the two. The pilot does not need to judge brightness or distance, only the colour of the lights.
The four units are set at different angles. The unit nearest the runway has the highest setting and the unit farthest from the runway the lowest, spaced around the nominal approach slope θ. As an aircraft descends it drops below each unit’s angle in turn, and that unit changes from white to red. The result is five readable indications: four white (too high), three white and one red (slightly high), two white and two red (on slope), one white and three red (slightly low), and four red (too low).
The signal depends on the elevation angle of the aircraft, so the same indication is seen at any distance along that angle, within the beam coverage. This is why the setting angle of every unit is checked at installation and again after any maintenance that could disturb the fixture.
Frequently Asked Questions
Q1What is the PAPI beam?
Each PAPI unit projects a beam that is split vertically: white light above and red light below, with a sharp transition between them. Four units in a wing bar are set at slightly different elevation angles, so the pilot sees a different mix of red and white lights depending on whether the aircraft is above, on or below the approach slope.
Q2Why is the PAPI beam half white and half red?
The colour split turns an elevation angle into a signal the pilot can read at a glance. Above a unit's setting angle the pilot sees white, below it the pilot sees red. Because the colour depends only on the angle, the indication does not depend on how far the aircraft is from the runway.
Q3What do the four PAPI lights show when the aircraft is on the approach slope?
Two red and two white. The two units nearest the runway show red and the two farthest from the runway show white. One more red means the aircraft is slightly low, one more white means it is slightly high. Four reds is too low and four whites is too high.
Q4How sharp is the red-to-white transition?
Very sharp. ICAO Annex 14 requires the change from red to white to occur within a vertical angle of about three minutes of arc, so the pilot sees a decisive change rather than a gradual colour drift. Confirm the exact wording and values against the current edition.
Q5What are the unit setting angles for a 3 degree approach slope?
For a nominal slope of 3°, the four unit transitions are typically set at 3°30′, 3°10′, 2°50′ and 2°30′, with the unit nearest the runway at the highest angle. Always use the setting angles specified for the actual approach slope, in the standard and the manufacturer's installation data.
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