Many of the more seriously injured people who were on the Singapore Airlines flight that hit severe turbulence need operations on their spines, the Bangkok hospital where they are being treated says.

Twenty people remain in intensive care and a 73-year-old British man died after the Boeing 777, which was flying from London’s Heathrow airport to Singapore, suddenly descended sharply after hitting the turbulence over the Andaman Sea on Tuesday.

"The number of patients in ICU remain the same," Adinun Kittiratanapaibool, Director of Bangkok's Samitivej Srinakarin Hospital told reporters, referring to the medical facility's intensive care unit.

"The meaning of those in ICU are those that need close attention," he said, and added that currently there were no life-threatening cases.

Of the 40 people on the flight still under treatment, 22 patients have spinal cord injuries and six have brain and skull injuries, he said.

Adinun had said 41 people were still under treatment, but later said one person had been discharged.

Ten British, nine Australian, seven Malaysian and four Philippine citizens were among the 41 people, according to a presentation made by Adinun. The oldest patient at the hospital is 83 years old and the youngest a two-year-old child who suffered a concussion, he added.

British citizen Andrew Davies, 54, speaks about the turbulence on the Singapore Airlines flight. – Reuters

Passengers described the “sheer terror” of the aircraft shuddering, loose items flying and injured people lying paralysed on the floor of the plane.

An Australian passenger injured in the turbulence said the airline had not been forthcoming with information following the deadly incident.

Speaking to media at Bangkok’s Samitivej Srinakarin Hospital from a wheelchair and with his head bandaged, Australian citizen Keith Davis said:

"No information from Singapore at all, not a single word."

Davis said he has not received information on whether he will be able to make an insurance claim. His wife is in intensive care and needs a medical evacuation, he added.

It remains unclear what exactly caused the turbulence that sent the plane, which was carrying 211 passengers and 18 crew members, on a 6000-foot (1800m) descent in about three minutes. The flight from London to Singapore was diverted to Thailand.

In one of the latest accounts of the chaos on board, 43-year-old Malaysian Amelia Lim described finding herself face down on the floor.

“I was so afraid. I could see so many individuals on the floor, they were all bleeding. There was blood on the floor as well as on the people,” she said.

The woman who had been seated next to her was “motionless in the aisle and unable to move, likely suffering from a hip or spinal injury”, she added.

Adinun Kittiratanapaibool, Director of Samitivej Srinakarin Hospital, speaks to the media. – Reuters

Another passenger onboard the flight spoke about the moment he helped medics with the man who died.

Fifty four-year-old Andrew Davies, an event project manager from London, was on his way to New Zealand via Singapore for a business trip.

Davies said the cabin was left in "disbelief" after the incident, adding that he quickly helped bring the 73-year-old British passenger, who later died, to the front of the plane for medics to perform CPR, which he said lasted for roughly 20 minutes.

"My heart goes out to the to the wife of the man who lost his life. You know, she was... I spent a little bit of time talking to her and held her hand briefly," said Davies.

He also commended the Singapore Airlines cabin crew, who had seemingly kept their composure and continued to help passengers despite some getting injured themselves.

"Every single cabin crew I saw was injured. I didn't see anybody who worked for Singapore Airlines that was not injured," he said.

When asked how the ordeal affected his outlook on air travel, Davies said he would continue flying, but mentioned that he had felt every bump and made sure his seatbelt was firmly fastened when flying to Singapore from Bangkok, where flight SQ321 had been diverted to.

A passenger who was on board the flight SQ321 arrives at Changi Airport in Singapore. – Reuters

Australian officials were helping 12 Australian citizens and one permanent resident in hospital in Bangkok who were injured during the incident, a Department of Foreign Affairs spokesperson said on Thursday.

Another Australian had been hospitalised but has since been discharged, the spokesperson said.

"The Australian government offers its sympathies to those affected by the Singapore Airlines flight SQ321 incident," they said in a statement.

Some of the 56 Australians on board the flight have begun making their way home as relieved passengers touched down in Sydney on Wednesday night.

Beverley Mayers recalled non-stop tears after getting off the plane.

"My son has come to pick me up and I said to him, 'I have to stay with you tonight and see my other children, and just hold them close'," she told Nine News.

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.