Definition
A visual landing aid located beside the runway that uses a row of light units to show the pilot whether the aircraft is on, above, or below the correct approach path during the final descent to the runway. Each light unit projects a beam that appears either white or red depending on the aircraft's vertical position relative to the designed glide path. A standard PAPI consists of four lights in a single row; the combination of red and white lights tells the pilot the angle of approach. Two white and two red indicates on the correct path; more white means too high; more red means too low.
Plain English
A row of lights next to the runway that tells the pilot if they are coming in at the right angle. The mix of red and white lights shows whether the approach is on the correct slope, too high, or too low.
Context Anchor
Seen beside many runways during landing, especially when a pilot is lined up on final approach.
Derivation
The name describes its job directly: it indicates the path for a precision approach. 'Precision' here refers to the accuracy of the visual glide path it provides, not to the radio-based precision approach systems like ILS.
Why Pilots Care
Maintains a stable descent angle that prevents landing short of the runway or floating too far down the pavement.
Intuition Check
Do not read “precision” as meaning the PAPI is an instrument landing system. A PAPI is a visual aid: the pilot must see the lights and use them as visual guidance.
Example Sentence 1
On short final, the PAPI showed three white and one red, so the pilot reduced power to settle back onto the correct path.
Example Sentence 2
At night the PAPI gave immediate confirmation that the approach path was correct before touchdown.