Precision Approach Path Indicator - Used in avionics to guide planes to the runway using the approach path (Glide Path).
A series of 3 or 4 lights that change colour from white to red in accordance to the height of the aircraft.
Red over Red - Too low
Red over White - On the Glide path
White over White - Too high
The PAPI lights can be seen up to 5 miles in daylight and upto 20 miles at night.
The PAPI system will help us find the runway
Red over Red - You'll soon bang your head
Red over White - You'll be allright
White over White - You'll be out of sight