Aviation investigators arrived in Bangkok on Wednesday to learn how and why severe turbulence sent a Singapore Airlines plane into a sudden dive that tossed passengers and crew around the cabin, leaving a British man dead and dozens others injured.

Twenty people remained in intensive care in hospital after Flight SQ321, which was flying from London's Heathrow airport to Singapore, hit the turbulence Tuesday over the Andaman Sea.

The Boeing 777, which carried 211 passengers and 18 crew members, descended 6,000 feet (around 1,800 metres) in about three minutes, the carrier said.

The captain diverted the plane to Bangkok's Suvarnabhumi Airport, where medical teams evaluated those aboard and sent over 80 to hospital.

Singapore Airlines sent a special flight to Bangkok on Tuesday night to pick up those well enough to travel. The airline said that 131 passengers and 12 crew members arrived shortly after 5 a.m. at Singapore's Changi Airport.

An additional 79 passengers and six crew members stayed in Bangkok, where the majority remained in hospital, said Singapore Airlines CEO Goh Choon Phong.

Samitivej Srinakarin Hospital, where most of the injured were taken, said 20 people were being treated in intensive care while 27 others have been discharged.

The ICU patients include six Britons, six Malaysians, three Australians, two Singaporeans and one person each from Hong Kong, New Zealand, and the Philippines, it said.

The hospital said nine people underwent surgery Tuesday and five more operations were expected to be completed Wednesday. It said it had provided 104 people with medical care, including 19 at its clinic at the airport.

Officers from Singapore's Transport Safety Investigation Bureau arrived in Bangkok late Tuesday, Singapore Transport Minister Chee Hong Tat said Wednesday.

He added that the US National Transportation Safety Board is also sending an accredited representative and four technical advisors to support the investigation because the incident involved a Boeing plane.

One of 56 Australians listed as passengers on board the flight told Sky News that the seatbelt sign had come on just ahead of the turbulence, but she could not act in time because she had been asleep.

Thirty-year-old Teandra Tukhunen, speaking from a hospital with her left arm in a sling, said she had been “thrown to the roof and then to the floor.”

“It was just so quick, over in a couple of seconds and then you're just shocked. Everyone's pretty freaked out," she said.

Recounting the accident, British passenger Andrew Davies also told Sky News that the seatbelt sign was illuminated but crew members didn't have time to take their seats.

“Every single cabin crew person I saw was injured in some way or another, maybe with a gash on their head,” Davies said. “One had a bad back, who was in obvious pain.”

Thai officials had withheld the name of the dead man, but British media identified him as Geoffrey Kitchen, 73, who was going on a six-week holiday with his wife. She was among the passengers taken to hospital in Bangkok.

Kitchen was described as formerly working in the insurance industry, and in retirement was continuing his decades-long involvement with amateur theatre.

A Thai airport official said Kitchen might have had a heart attack, though that hadn't been confirmed.

Tracking data captured by FlightRadar24 and analyzed by The Associated Press showed Tuesday's flight cruising at an altitude of 37,000 feet (11,300 metres).

At one point, the Boeing 777-300ER suddenly and sharply descended to 31,000 feet (9,400 metres) over about three minutes, according to the data.

The aircraft then stayed at 31,000 feet for under 10 minutes before diverting and landing in Bangkok less than a half-hour later.

Most people associate turbulence with heavy storms, but the most dangerous type is so-called clear air turbulence. Wind shear can occur in wispy cirrus clouds or even in clear air near thunderstorms, as differences in temperature and pressure create powerful currents of fast-moving air.

According to a 2021 report by the US National Transportation Safety Board, turbulence accounted for 37.6 per cent of all accidents on larger commercial airlines between 2009 and 2018. The Federal Aviation Administration, another US government agency, has said there were 146 serious injuries from turbulence from 2009 to 2021.

“For flight attendants and passengers alike, the dangerous, shaky feeling in midair called turbulence comes from air currents shifting,” said a statement from the US-based Association of Flight Attendants.

“While details of Singapore Flight 321 are still developing, initial reports seem to indicate clear air turbulence, which is the most dangerous type of turbulence. It cannot be seen and is virtually undetectable with current technology. One second, you're cruising smoothly; the next, passengers, crew and unsecured carts or other items are being thrown around the cabin.”

“Always follow crew instructions and wear your seatbelt whenever seated,” it advised. “It is a matter of life and death.”

Officials from the British and Malaysian embassies on Wednesday visited the Bangkok hospital where most of the injured were admitted to check on their condition.

Singapore Airlines said the passengers, in addition to the 56 Australians, included two Canadians, one German, three Indians, two Indonesians, one Icelander, four Irish, one Israeli, 16 Malaysians, two from Myanmar, 23 New Zealanders, five Filipinos, 41 from Singapore, one South Korean, two Spaniards, 47 from the United Kingdom and four from the United States.