Flight UA826 was cruising at 33,000 feet, two hours out of Narita airport in Tokyo, when it lurched, began to rattle then suddenly dropped like a stone.
Passengers who had just finished their evening meal were hurled into the ceiling of the aircraft. Some people suffered broken limbs, others had bloodied faces and neck injuries.
Amid the screaming, the pilot pleaded for calm. "We have just hit air turbulence and the aircraft descended 300 metres. There is no danger of a crash," he called over the intercom.
But among the 374 passengers flying to Honolulu, Hawaii, there was panic, captured by one of them on video and shown yesterday on television.
"Suddenly the plane dropped and people were jumping and falling, and things came flying at me - like juice cans, food," said Chieko Ejiri, 28, who was on her way to a holiday with her boyfriend. "I saw people with bleeding bandages on their heads and someone with his arm in a sling. And one person was crying out in pain," she added.
"I thought I was dying," said Kiyotaka Eto, a 16-year-old student from Osaka who was going on a surfing holiday. He said his seat belt saved him from injury, although he felt his body floating upwards during the drop.
His friend, Yuji Takahashi, 17, who was not wearing a belt, said, "I hit my head on the ceiling. It was like something straight out of the movies."
The plane was dropping towards the northern Pacific until the pilot brought it back under control and flew back 1,100 miles to Tokyo to be met by the emergency services. Some passengers emerged from the plane on Sunday night in neck braces, others on stretchers or wrapped in blankets.
United Airlines blamed the incident, which was over in seconds, on "severe clear-air turbulence". A spokesman said 10 passengers required hospital treatment.
"The seat belt sign was on when the plane was hit by severe turbulence." said spokesman Tony Molinari. "There had to be folks who weren't belted."
The Federal Aviation Authority said it would now investigate "because we're always interested in turbulence issues".
Clear air turbulence is the atmosphere's equivalent to the eddies that form in rivers where fast-moving waters and slower waters come together. Because it occurs in areas without cloud it is invisible and difficult for pilots to detect.
However, meteorologists issue charts which warn airlines of likely areas of clear-air turbulence, as well as thunderstorms and dense areas of cloud.
The turbulence typically occurs at a height of between 30-35,000ft. The plane experiences "wind shear" as it moves from a jet stream of, say, 300 knots into one of 40 knots, causing rapid loss of lift as it meets the eddy.
Such turbulence is fairly common and there have been a succession of such incidents this year. In June, a Japan Airlines jet hit turbulence near Hong Kong and 11 passengers and crew were injured, one suffering a fractured pelvis. A month later, a Qantas aircraft flew into turbulence between Brisbane and Tokyo leaving 23 injured, three seriously. Then an Alitalia jet hit turbulence shortly before landing in Caracas in September injuring19, some with broken bones.
Air passengers more frequently experience turbulence when flying through cumulonimbus clouds but the affects are far milder.
Researchers in California are close to developing a radar system which they hope will enable pilots to identify areas of clear-air turbulence on their instrument panel.Reuse content