How do they know that? It’s a common question about PAPI (precision approach path indicator) lights, implying that the lights themselves are judging the height of your approach to landing. But, no, the lights don’t know. You’re the one making the system appear to change color.
PAPI lights usually are placed to the left of the runway. They are visible within four miles in daylight and up to 20 miles at night. PAPI lights provide safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline, and most PAPIs are set to a 3-degree glideslope.
A 150-watt bulb inside a standard four-light PAPI illuminates the system, and a filter sets the angle for the colors you see. “White/white, you’re high as a kite. Red/white, you’re alright. Red/red, you’re dead.”
If you’re high on approach, you see the white light because the red light is blocked by the filter. The four lights in the system judge you independently—each light is a judge. If all the lights are white, you are extremely high on approach. If the far right bulb appears red, you’re slightly high. If you see two white and two red, you’re right on the glideslope. If you see three reds, you’re slightly low. And if all four lights appear red, well, see above. Time to climb.
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