Early investigations into severe turbulence that hit a Singapore Airlines flight last Tuesday reveal it dropped around 178ft (54m) over 4.6 seconds, BBC reported.

One British passenger died and dozens more were injured when the London-Singapore flight encountered the turbulence over Myanmar and had to make an emergency landing in Thailand. 

Singapore Airlines said it was co-operating with investigators and supporting affected passengers and crew, including with medical and hospital expenses.

The Transport Safety Investigation Bureau's (TSIB) preliminary findings found rapid changes in gravitational force (G) caused the altitude drop - which is just shy of the height of Italy's Leaning Tower of Pisa - and likely injured those who were not wearing seatbelts.

It said the flight operated as normal until it probably flew over an area of "developing convective activity" while passing over the south of Myanmar at 37,000ft (11,300m).

"The rapid changes in G over the 4.6 seconds duration resulted in an altitude drop of 178ft (54m), from 37,362ft to 37,184ft," the TSIB report said.

"This sequence of events likely caused the injuries to the crew and passengers.

"After the pilots were informed by the cabin crew that there were injured passengers in the cabin, the decision was made to divert to Suvarnabhumi Airport, Bangkok, Thailand."

Approximately 17 minutes after the turbulence event, the pilots were able to make a "controlled descent from 37,000ft", the report said, adding the aircraft did not encounter any more severe turbulence during its diversion to Bangkok.

The pilots also requested for medical services to meet the aircraft on arrival.

There were 211 passengers and 18 crew onboard the Boeing 777-300ER aircraft.

Geoff Kitchen, 73, died from a suspected heart attack onboard, while 104 other passengers were taken to Bangkok's Samitivej Hospital for treatment.

Ali Bukhari, 27, who was seated with his wife Ramiza, told the BBC the aircraft went into "straight freefall".

"It was terrifying. It's just like going down a vertical roller coaster," he said. 

bd-pratidin/GR