What is the PAPI glide path?
The descent slope the PAPI marks for the pilot. Two red and two white means on the path; more white means high, more red means low.
The glide path is the straight descent line an aircraft follows to the runway, normally about 3° from the horizontal. A PAPI does not guide the aircraft by itself: it marks the path with light. The wing bar sits beside the runway, usually about 300 m beyond the threshold, and each of its four units is aimed so that the white-to-red change occurs at a set angle above the ground at that point.
On the path, the pilot sees two red and two white lights. Rising above it turns the red lights white one at a time, and sinking below it turns the white lights red. Because the colours depend only on the angle, the pilot can hold the slope by keeping the same two-and-two pattern all the way down.
The glide path is also the reference for safety. The on-slope signal is set so that the aircraft crosses the threshold at a safe height for its wheel clearance, and the approach slope is the basis for the obstacle protection surface below it. A PAPI aimed even a fraction of a degree wrongly moves the whole indicated path, which is why the setting is verified at commissioning and after maintenance.
Frequently Asked Questions
Q1What is the PAPI glide path?
The glide path is the descent slope the aircraft follows to the runway, normally about 3°. The PAPI does not make the aircraft follow it. It shows the pilot, by the number of red and white lights, whether the aircraft is on, above or below that slope.
Q2How does the pilot know the aircraft is on the glide path?
Two lights red and two lights white. The two units nearest the runway show red and the two units farthest from the runway show white. If more lights are white the aircraft is high, and if more lights are red the aircraft is low.
Q3Where does the glide path meet the runway?
At the aiming point of the PAPI, which is set near the touchdown zone. The wing bar is normally placed about 300 metres beyond the threshold, and the signal is calibrated so that the on-slope path passes over the threshold at a safe height for the aircraft category. The exact distance comes from the standard and the aircraft's eye-to-wheel height.
Q4Is the PAPI glide path the same as the ILS glide slope?
No. They are separate systems. The PAPI is a visual aid that the pilot reads by eye from the colour of the lights, while the ILS glide slope is a radio signal read by the aircraft's receiver. At many runways the two are set to the same angle so that a pilot can cross-check one against the other.
Q5What happens if the PAPI is aimed at the wrong angle?
The on-slope indication moves to a different slope, so pilots who fly the PAPI follow a path that is too high or too low. This is why the setting angles are verified at commissioning and after any work that could move the unit. Use the setting angles for the actual approach slope in the standard and the manufacturer's data.
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