John Carlin on a breakthrough in the detection of a severe and unpredictable threat to aircraft
MOST air travellers may not know it, but an invisible enemy lurks in the skies, feared by all flight crews for its capacity to cause bashed heads, broken bones and sometimes death.

Clear Air Turbulence is the technical name for an unpredictable and - so far - uniquely undetectable meteorological phenomenon long identified by air safety experts as the leading cause of in-flight injuries. One man was killed and more than 80 injured on 28 December when a United Airlines flight ran unexpectedly into severe clear air turbulence over the Pacific at cruising altitude, when the seat belt lights were off. The Boeing 747 was so badly rattled that United said last week the aircraft would probably never fly passengers again.

Scores of such incidents that go unnoticed by the media are documented by America's National Transportation Safety Board. A sample of NTSB reports on accidents whose "probable cause" was clear air turbu- lence tell of broken ankles, broken ribs, of aircraft that, without warning, pitch 50 degrees to the right, 50 degrees to the left and then abruptly drop 1,500ft.

While no full-scale catastrophes have been linked to clear air turbulence, an incident over the Rocky Mountains in September 1992 suggested that there is no reason for complacency. Following an encounter with what meteorologists call a "mountain wave", a violent collision of air waves moving vertically and horizontally, a DC-8 cargo jet somehow managed to make a safe landing after shedding an engine and 19ft of metal from its left wing.

Nasa, in collaboration with the Pentagon and private sector researchers, has been working for years to come up with a system that will help avoid such accidents, and a breakthrough now appears imminent. Employing laser technology first developed by the US military, scientists expect to provide pilots with the capacity to detect these treacherous air currents early enough to allow them to turn on the seat-belt lights before they are encountered.

The system is called Lidar, which stands for Light Detection and Ranging - as opposed to radar, or Radio Detection and Ranging. While radar can detect moisture, thereby interpreting the severity of looming storm clouds, clear dry air means nothing to it. Lidar is designed to emit energy pulses from an aircraft which will instantly reflect back information on the density of the air ahead.

Rodney Bogue, a Nasa scientist who is the project manager, says the first in-flight tests would be carried out in April. The test aircraft will know where to look for clear air turbulence because there are three circumstances in which it is known typically to occur. "We see it in the boundaries of the jet stream, where high speed air transits with low speed air, mixes and produces random activity; second, over mountains, where the wind is forced to flow upwards; and third, when you're within 50 miles or less of a developing thunderstorm."

While the range of the Lidar laser beam is not yet known, Dr Bogue expects it to reach 10 miles, giving a pilot a 60-second warning - 60 seconds more than are available now to alert the crew to batten down their food carts ("they can be flying missiles in those situations") and all aboard to sit down and fasten their seat belts tight.