Lights for the Slot

Bringing any sort of big airliner down to a gentle--or safe--landing is a considerable stunt. But heavy, fast, steeply sinking jet planes have made the proposition even trickier. Their pilots cannot make a so-so approach and depend on last-minute power adjustments to keep them from overshooting or undershooting the runway. They must fly "by the numbers"--at precise letdown speeds, with their wing flaps set precisely right and their noses at the correct angle. Once a 150-ton jet is committed to land, it must follow a very narrow "slot of forgiveness," never deviating appreciably as it approaches the runway.

This is not easy to do, even with good visibility. Since regular jet flights started two years ago, no U.S. commercial jet has cracked up seriously on landing, but there have been perilously close undershoots and overshoots because of pilot misjudgment. To prevent such unpleasant incidents, the Federal Aviation Agency thoroughly tested five visual-approach aids, and it has finally recommended one that was developed by Britain's Royal Aircraft Establishment. Last week New York's International Airport offered runways to be equipped with the RAE system so that pilots can try it out for themselves.

The new system, which will soon be familiar to air travelers, is remarkably simple. Twelve powerful lights are arranged in groups of three around the threshold of the runway. In front of each lamp is a filter with a red upper half and a clear lower half. In front of the filter is a two-inch horizontal slit. When an observer is above the center of the beam, the lamp looks white; in the middle it changes to pink, and in the lower half it is red.

The lamps are set pointing upward at angles that mark out the glide path. When the pilot makes the proper approach, he sees on each side of the runway two bars of lights. The near group is white, the far group red (see diagram). As long as they stay that way, he is doing all right. But if the white lights turn pink or red, he is approaching too low. If the red lights turn pink or white, he is too high. He has plenty of time to get in the slot. Even with brilliant sunlight competing with the lights, they can be seen more than four miles away.

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