What is the PAPI elevation angle?
The vertical angle at which each PAPI unit’s beam changes from white to red. Four units, four angles, spread around the approach slope.
The elevation angle of a PAPI unit is the angle, measured up from the horizontal, at which its beam changes from white to red. A pilot above that angle sees white from the unit, and a pilot below it sees red. The change is sharp, so the pilot reads the angle by eye as a colour.
The four units of the wing bar are not set to the same angle. For a 3° slope they are typically set at 3°30′, 3°10′, 2°50′ and 2°30′, which is the slope plus or minus 10 and 30 arc-minutes, with the highest setting at the unit nearest the runway. On the slope the pilot is above the two lower settings and below the two higher ones, which gives two red and two white. Each step off the slope changes one more unit.
The angle is set with an inclinometer placed on the unit’s reference surface, adjusting the unit until the reading equals its setting angle. Elevation sets the height of the glide path signal, while azimuth only aims the beam sideways, so the two are set and read separately and then read again together, because moving one can disturb the other.
Frequently Asked Questions
Q1What is the PAPI elevation angle?
It is the vertical angle at which a PAPI unit's beam changes from white to red, measured up from the horizontal. Above that angle the pilot sees white from that unit, below it red. Each of the four units has its own setting angle.
Q2What are the four setting angles for a 3° slope?
Typically 3°30′, 3°10′, 2°50′ and 2°30′, which is the slope plus or minus 10 and 30 arc-minutes, with the highest setting at the unit nearest the runway. Always take the setting angles for the actual approach slope from the standard and the manufacturer's data.
Q3Why are the four units set at different angles?
So that the colour pattern changes in steps as the aircraft moves off the slope. Two red and two white is on the slope, and each unit that changes colour tells the pilot the aircraft has moved about 20 arc-minutes higher or lower. If all four were set the same, there would be no steps, only a sudden change from all white to all red.
Q4How is the elevation angle set and read?
With an inclinometer placed on the unit's reference surface, then adjusting the unit until the reading equals its setting angle. Some fixtures have a built-in scale, but the inclinometer reading is the one to trust. Read it again after tightening, because tightening can move the unit.
Q5Does the elevation angle change the azimuth?
It can. Tilting a unit on an uneven base can turn it slightly sideways, and turning it can change its tilt. Set the elevation, set the azimuth, then read both again, and use the tolerances in the manufacturer's data and the applicable standard.
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