Precision Approach Lighting systems

Part of the training for a Commercial Pilot's Licence includes understanding the different lighting systems in operation at airports. Pictured here is a Category 1 precision approach lighting system which features a 900m stretch of lighting before you get to the start of the runway (threshhold). It features a line of lights in the direction of the runway, and crossbars, and the lights are white, varying in intensity as you get closer to the threshhold for landing. In the closest 300m the line comprises a single line of lights, in the next 300m there is a double line of lights and in the furthest 300m there is a triple line of lights.

Green threshhold lights

The beginning of the runway or threshhold is marked by a crossbar of green lights and the end of the runway is marked by a crossbar of red lights.

In poor visibility a pilot may not land the aircraft unless he has the runway or approach lights in sight when he reaches the minimum decision height which is generally about 200 ft above the runway surface, otherwise he needs to do a missed approach, taking off without landing and going around. Many accidents have been caused by pilots landing without having clearly identified the runway in misty conditions and the lighting systems are intended to make identification clearer. 

In the photo below you can see King Shaka International Airport's approach lights at night in a slightly different configuration. The lines of lights on ether side of the centre line from the green threshhold bar (start of the runway) extending into the runway indicate the touchdown zone. The two bars of lights on either side of the runway adjacent to the touchdown zone are the Papi (Precision Approach Path Indicator) lights... when the two outer lights are white and the two inner lights are red, you are on the correct glide slope for the landing, whereas more red lights will show you are too low and more white lights will show you are too high.

King Shaka International runway lights at night

An illustration of PAPI lights