Scanning the CAT

As it soared close to Japan's Mount Fuji on a cloudless day in March 1966, a BOAC 707 jet was suddenly battered by tremendous gusts of wind that broke it apart. All 124 persons aboard were killed. High over Wyoming in equally clear skies in March 1967, a United Air Lines 720 jet was wrenched into an 8,000-ft. plunge. Inside the cabin, a passenger was flung against the ceiling and fatally injured.

Both planes were victims of a violent and invisible phenomenon called CAT (Clear Air Turbulence), a menace that has grown proportionately with increasing aircraft speeds. Clawing without warning at fast-flying jets, CAT wreaks death and injury and causes millions of dollars of damage annually to commercial and military aircraft. But CAT may soon be stripped of its coat of invisibility. Preliminary tests of a new infra-red detector indicate that areas of turbulence can be spotted as far away as 48 miles, giving jet pilots as long as four minutes to take evasive action.

Leaving a Track. Scientists generally agree that CAT occurs when a stream of fast-flowing air passes through an air mass moving at a lower velocity or in the opposite direction. The resulting shearing action produces turbulence--often severe--at the boundaries of the stream. CAT is usually encountered near the constantly shifting west-to-east jet stream and near mountain ranges, where cold air frequently spills at great speed down the leeward slopes. Although the turbulence is obvious to any pilot caught in it, it cannot be seen by the human eye. Attempts to detect CAT with devices that bounce radar or laser beams against it have so far proved either impractical or inconclusive.

Yet CAT apparently does leave a track. Turbulence tests conducted by NASA and the Federal Aviation Administration indicate that there may be a significant temperature difference between the air masses on either side of a wind shear. To scientists at North American Aviation's autonetics division, these findings seemed to provide an answer. Wherever there are sudden temperature variations in clear air, they surmised, there must also be CAT.

Kittenlike Proportions. After months of experimentation, a group led by North American Electronics Engineer Edward Flint devised a compact infrared sensor that can be mounted atop a plane. While scanning 45DEG to either side of the aircraft's flight path, the sensor can detect temperature variations as small as a fraction of a degree Fahrenheit in atmospheric carbon dioxide at a range of from 24 to 48 miles. These variations register on three side-by-side cockpit gauges that show the pilot whether a temperature gradient lies directly ahead or 45DEG to the left or right of his flight path.

Once identified, CAT can be avoided --or-its threat reduced to kittenlike dimensions. If only the left gauge shows traces of CAT, for example, the pilot can probably evade it completely by turning to the right. If the gauges indicate that the wind shear lies in a broad front across his flight path, he can change altitude or follow his gauges along the shortest possible path through the turbulence. The infra-red detector was first tested last summer aboard an Air Force version of the Boeing 707 and successfully spotted CAT before each of the dozen times that the jet plunged into turbulent air. A more sophisticated model is currently going through a three-month test aboard a Pan American 707 in regular service. So far, Pan Am officials say, it seems highly promising as a CAT detector.

North American freely admits that the detector has its limitations. Its gauges show the direction but not the precise distance or depth of the wind shear. On planes flying eastwest, it is ineffective when pointed directly into the rising or setting sun; solar radiation simply overwhelms the infra-red sensor. The device also occasionally mistakes the hot jets of a plane many miles ahead for an area of air turbulence. And with its present limited range, it is obviously impractical for use in the slow-to-react, 2,000-m.p.h. supersonic transport planes, which could plow into CAT less than a minute after it was detected.

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