CAT'S claws

Among those still discussing and analyzing the cause of the Lockheed Electra crash (63 dead) near Tell City, Ind. last month, there appears an ominous possibility: that the aircraft was torn apart in mid-air by a phenomenon which airmen and meteorologists have taken to calling CAT--for "clear air turbulence." If the theory proves to be true, pilots will have to find ways to keep their ships out of CAT's claws.

Every pilot is familiar with ordinary turbulence, which is generally caused by thunderstorms or some other violent weather disturbance in the lower atmosphere. Pilots avoid the worst bumps by dodging the thick clouds in which vertical air currents hide. Radar helps by spotting the veils of rain or hail that mark the violent heart of a storm. But clear air turbulence is invisible both to human eyes and to any known kind of radar.

The unhappy airliner that flies into it is shaken from nose to tail without any warning whatever.

Snaking Around Earth. Dr. Harry Wexler, chief of research for the U.S. Weather Bureau, explains that CAT is generally caused by wind shear, the conflict of air masses moving at different speeds or in different directions. When such masses meet, a belt of swirls and waves appears in the boundary between them. A slow airplane can fly through moderate CAT with hardly any unpleasantness, but for a fast-flying jet the sensation is like driving a car over a cobblestone pavement with some of the stones missing.

The favorite habitat of CAT is close to the jet stream, the narrow belt of high-speed wind that snakes around the earth from west to east in mid-latitudes, often reaching 250 m.p.h. The turbulent region below the jet stream may begin as low as 15,000 ft., increasing in roughness as it nears the stream (which is not itself normally turbulent) at an average of about 30,000 ft. Another layer of CAT rides on top of the stream, reaching to 40,000 ft. There is normally some turbulence on both sides of the jet stream, but the north side is almost always the worst. No one knows why.

Waiting for Bumps. Saying that CAT surrounds the jet stream does not help detect it. The stream is capricious, whipping up and down and from side to side like a shaken rope. The only way at present to find belts of CAT is to fly an airplane through a region where it may be --and wait for the bumps to begin. The Weather Bureau intends to do this if it can get the money to fly its elaborately instrumented hurricane-hunter planes during hurricane-free seasons. Such a course of flying may suggest ways to warn pilots of CAT ahead.

Of all U.S. air disasters, the Tell City crash seems the most likely to have been caused by CAT. Otherwise, so far as is known, the worst that has happened is injuries to passengers, who got severely shaken. But jets are flying faster and higher, and when the jet stream is going their way, they make the best time by flying in it. So they meet more CAT and hit it harder.

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