Definition
A visual landing aid installed beside the runway that shows pilots whether they are on, above, or below the correct glide path to the runway threshold. A PAPI consists of a single row of four light units (usually on the left side of the runway) that appear either white or red depending on the pilot's vertical position relative to the published approach angle, typically 3 degrees. Two white and two red lights indicate the aircraft is on the correct path; more white lights mean too high, more red lights mean too low.
Plain English
A row of four lights next to the runway that tells you, at a glance, whether you are coming in too high, too low, or just right. The mix of red and white lights is the cue: balanced means you're on the correct descent angle to the runway.
Context Anchor
Seen during final approach to a runway, especially when judging whether visual illusions are making the runway look higher, lower, closer, or farther away than it really is.
Derivation
Precision' here means accurate to a defined glide path angle. 'Approach' refers to the descent toward landing. 'Path Indicator' simply means it shows you the path. Together, the name describes exactly what the system does: it shows the precise approach path with light signals.
Why Pilots Care
Correct use prevents landing short or long due to visual illusions, directly improving landing safety.
Analogy
A PAPI is like a simple traffic-light guide for your descent: the color pattern tells you whether to adjust up, down, or keep coming down as planned.
Intuition Check
Do not assume “precision” means the runway must have an instrument precision approach. Here, PAPI means a visual light system that gives precise guidance for your descent path to the runway.
Example Sentence 1
On final approach, the pilot saw three red lights and one white on the PAPI, recognized the aircraft was below glide path, and reduced the descent rate.
Example Sentence 2
At night, the PAPI helped confirm the correct glide path when runway lights alone created an illusion of being too high.