The seatbelt sign came on moments after the plane started shaking, but, for some, it was too late.

“Whoever wasn’t buckled down, they were just launched into the air within the cabin,” said Dzafran Azmir, who was among the 211 passengers on board the London-to-Singapore flight that encountered deadly turbulence on Tuesday. “Within an instant, they hit the ceiling of the cabin and dropped right back onto the floor.”

The plane, a Singapore Airlines Boeing 777-300 ER, had taken off from London’s Heathrow Airport on Monday night, about 10 hours earlier. It was about three-quarters full. Many of the travelers were Singaporeans returning home. Some were students studying in England. Others were families and some who had planned a “holiday of a lifetime” to far-flung destinations like Australia.

The bulk of the 13-hour journey of flight SQ321 was over, and many passengers had finished their last meal onboard, a breakfast that these days has been a choice between an omelet with cream cheese or stir-fried Asian noodles, both served with a side of fresh fruit.

By this time, the plane had reached the Bay of Bengal, which sits between the Indian subcontinent and the Malay Peninsula in Southeast Asia. Some pilots consider the region “notorious” this time of the year because its monsoon rains can cause turbulence.

But commercial pilots know how to prepare for such scenarios. They rely on weather radar and carry extra fuel so they can fly around and wait for the weather to ease, if needed. Or they follow the course charted by other planes that recently have passed through the area and have warned air traffic controllers about weather upheavals.

One scenario that is impossible to prepare for is when the skies are clear and the plane’s radar does not detect anything amiss. This phenomenon is known as clear air turbulence.

“It could be the plane just starts shaking, we turn on the seatbelt sign, but, unknowingly, we fall into the clear air turbulence zone,” said Captain Teerawat Angkasakulkiat, president of the Thai Pilots Association. “It’s totally unpredictable.”

It’s unclear what happened next with SQ321, but there had been thunderstorms near its flight path. As it was flying over Myanmar, cruising at 37,000 feet above the southern section of the country’s biggest river, the Irrawaddy, it hit what the airline later described as “sudden extreme turbulence.”

For the next three to five minutes, the plane shook violently, said Mr. Dzafran, 28, a university student heading home to Kuala Lumpur, Malaysia, who was buckled into a window seat in row 52.

“Then it built up, like a feeling of going up a roller coaster, up the crest, and suddenly dropping very dramatically,” he recalled.

His bag, stored under the seat in front of him, flew across the plane, and his phone was flung across the aisle. The woman sitting in the row in front of Mr. Dzafran hit her head so hard on the plastic seatbelt indicator sign above her that it broke. Oxygen masks dropped down from the overhead panel. The person behind him hit a seat. Mr. Dzafran was unharmed, but the other two passengers had bloody gashes on their heads.

At least one passenger, it appeared, was able to react quickly enough and buckle her seatbelt. It was a woman sitting behind Mr. Dzafran.

“That was miraculous luck on her side to respond so quickly,” he said.

Another passenger, Teandra Tukhunen, who was sleeping, was not able to react as fast. She was awakened by the turbulence and saw the seatbelt sign come on, but she had no time to fasten it and was thrown to the ceiling, then to the floor, Ms. Tukhunen, 30, a native of Australia, told Sky News from a hospital in Bangkok, her arm in a sling.

Elsewhere on the plane, people started crying and screaming out in pain. The whiplash was so furious that one passenger said that it appeared as if those who had been walking around on the plane were doing somersaults. Dozens of people, including some crew members, were injured.

As things settled down, it was clear that one of the worst affected passengers was a male traveler, Geoff Kitchen. A grandfather of two who ran a local theater group in the town of Thornbury in southwest England, Mr. Kitchen, 73, had planned a six-week “holiday of a lifetime” to Australia and Southeast Asia with his wife of 50 years, Linda.

Andrew Davies, who was sitting in front of Mr. Kitchen, helped carry him out of his seat and laid him on the floor, where he was given C.P.R. for at least 20 minutes.

With the plane now in Thailand’s airspace, the pilot placed a distress call to Suvarnabhumi Airport in Bangkok, asking to make an emergency landing. Then he made an in-flight announcement about the new destination, and asked for anyone who was medically trained to help passengers with injuries.

Forty-five minutes after the ordeal began, Mr. Dzafran said, the plane landed. It was 3.45 p.m. local time.

Several ambulances with flashing lights were standing by. The passengers waited patiently while nurses, emergency medical workers and doctors rushed in to treat the critically injured first. A total of 83 people were injured. Twenty of them were sent to the intensive care unit of a local hospital.

Drew Kessler, the New York-based treasurer of Rotary International who was en route to Singapore for the annual Rotary International Convention, said he had broken his neck while his wife, Vicki, had broken her back.

As Mr. Dzafran prepared to disembark, the crew told passengers to avoid one of the aisles. Mr. Dzafran said he thinks he saw someone lying on the floor. Flight attendants near the business and first class zones were bleeding. Food was strewn across the galleys.

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The passengers, dazed and confused, boarded a bus from the tarmac and arrived to a holding area inside the Bangkok airport. Conversations were struck up. A fellow passenger told Mr. Dzafran that someone had died on the plane and showed Mr. Dzafran an online news article. It was Mr. Kitchen. He was the only fatality — one of the few ever blamed on turbulence — and the cause of death hasn’t been released yet.

Singapore Airlines has apologized for the episode, and its investigators have arrived in Bangkok to try to understand what happened.

Mr. Dzafran was among the 143 passengers who emerged unscathed. So was his seatmate, who he said also had her seatbelt on.

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