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Four lights showing whether an approach is high or low.
Also known as: Precision approach path indicator, Glide slope lights
A precision approach path indicator is four lights beside the runway that appear red or white according to the aircraft's angle. Two red and two white is on the glide path, and the mnemonic runs red over white you're all right, all red you're dead.
Show an aircraft's position relative to the correct approach angle, appearing red or white according to the angle of view.
Contexts: Public information, Navigation
Indicate the glide path by the count: two red and two white is correct, more white means too high and more red means too low.
Contexts: Public information, Navigation
Work because each unit projects a sharp boundary between a white and a red beam, which crosses the pilot's eye at a defined angle.
Contexts: Public information, Navigation
A row of four lamps set beside the runway threshold, each showing as a single point of colour from the air.