Dzafran Azmir was among many passengers on a Singapore Airlines SIAL.SI flight hit by heavy turbulence which left dozens injured and one dead, who finally reached Singapore on Wednesday morning.

The scheduled flight on a Boeing 777-300ER plane diverted to Bangkok after the plane was buffeted by turbulence that flung passengers and crew around the cabin, slamming some hard into the ceiling.

What is in-flight turbulence, and when does it become dangerous for passengers and crews?

A 73-year-old British passenger died of a suspected heart attack and at least 30 people were injured.

British man may have suffered fatal heart attack in turbulence-hit Singapore Airlines flight

Photographs from the interior of the plane showed gashes in the overhead cabin panels, oxygen masks and panels hanging from the ceiling and luggage strewn around. A passenger said some people's heads had slammed into the lights above the seats and punctured the panels.

"Suddenly the aircraft starts tilting up and there was shaking so I started bracing for what was happening, and very suddenly there was a very dramatic drop so everyone seated and not wearing a seatbelt was launched immediately into the ceiling," Azmir, a 28-year-old student on board the flight told Reuters.

Singapore Airlines took those who could continue on a flight from Bangkok that reached Singapore just before 5 a.m. (2100 GMT).

Singapore's Transport Safety Investigation Bureau (TSIB) is looking into the incident, and the U.S. National Transportation Safety Board is also sending representatives for support.

The sequence of events is still unclear, as the plane declared a medical emergency - but Reuters has not been able to confirm whether that happened before the turbulence erupted and the plane's altitude dropped from 37,000 feet to about 31,000 feet in a short period of time.

The sudden turbulence occurred over the Irrawaddy Basin in Myanmar about 10 hours into the flight, the airline said. Turbulence has many causes, most obviously the unstable weather patterns that trigger storms, but this flight could have been affected by clear air turbulence, which is very difficult to detect.

Turbulence-related airline accidents are the most common type of accident, according to a 2021 NTSB study.

While the airline said 30 people were injured, Samitivej Hospital in Thailand said it was treating 71 passengers. There were 211 passengers and 18 crew on board.

From 2009 through 2018, the U.S. agency found that turbulence accounted for more than a third of reported airline accidents and most resulted in one or more serious injuries, but no aircraft damage.

Singapore Airlines, which is widely recognized as one of the world's leading airlines and is a benchmark for much of the industry, has not had any major incidents in recent years.

Its last accident resulting in casualties was a flight from Singapore to Los Angeles via Taipei, where it crashed on Oct. 31, 2000 at the Taiwan Taoyuan International Airport, killing 83 of the 179 people on board.

(Production: Joseph Cambell, Ng Kokai)