Aviation scientists are testing a new type of radar system which may have prevented the sort of incident which killed a passenger and injured dozens more on a flight over the Pacific.

The system, which works on completely different principles from traditional radar, give pilots early warning of the kind of turbulence not visible to the naked eye.

This type of "clear air" turbulence is believed to be responsible for the incident over the Pacific on Sunday when a United Airlines Boeing 747 suddenly dropped 300m (1,000ft).

When blue skies mean danger

Clear-air turbulence develops when the weather is clear, but there is no build-up of the rain clouds which ordinarily provide pilots with a vital clue that they are approaching strong thermal current flows.

"You can avoid turbulence caused by cumulonimbus clouds by spotting it on the plane's radar," said aviation expert Yoshitomo Aoki. "But in this case, where you have something like a violent decrease in pressure beneath the plane, prediction is extremely difficult."

David Learmount, the Operations and Safety Editor of Flight International magazine, said although it could not be picked up by conventional radar, a new system was in development.

"The new system operates on a completely different waveband and is currently being tested in California. It seems as if the United jumbo hit clear air turbulence caused by eddies in jet streams. These cannot be seen by pilots but would be detectable by the new system being developed," he said.

But Mr Learmount added: "The seatbelt sign on the United plane was on and passengers should have ensured they were safely strapped into their seats."

A common problem but few deaths

While turbulence is a common phenomenon on flights, it rarely leads to fatalities unless the plane crashes. Most incidents are limited to injuries to those not wearing seatbelts who bang against the ceiling.

However, this year, three other major incidents were reported.

• On June 9, a Japan Airlines jet hit turbulence 25 minutes before its scheduled arrival in Nagoya from Hong Kong. Eleven out of 180 passengers and crew were injured, one suffering a fractured pelvis.

• On July 6, a Qantas Airways plane flew into turbulence en route from Brisbane to Tokyo - 23 out of 260 passengers and crew were injured, three seriously.

• On September 13, an Alitalia jet hit turbulence shortly before landing in Caracas with 19 of 393 passengers injured, some with broken bones.

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