(Bloomberg) -- Passengers aboard the Singapore Airlines Ltd. flight that hit extreme turbulence last week had seconds to react between when the seatbelt sign came on to when the aircraft plunged, causing many to be flung up to the ceiling and back. 

In a preliminary report on the accident, investigators found the aircraft appeared to have been caught in an updraft, Singapore’s Transport Safety Investigation Bureau said Wednesday.

Inside the Boeing 777 jetliner, passengers went from encountering 1.35G, or 1.35 times the normal force of gravity, to negative 1.5G in less than one second, the report said. The entire incident lasted around 62 seconds, but the most violent part of the turbulence was 4.6 seconds long, according to the report.

Read More: One Week After Singapore Air Turbulence, 34 Remain in Hospital

A British man died and several dozens required critical care treatment for spinal, brain and other injuries, when SQ321 from London to Singapore encountered severe turbulence flying over Myanmar. Many passengers were not wearing their seatbelt at the time of the incident, causing them to fly out of their seats and hit the ceiling of the aircraft. 

In its findings, the TSIB said that the aircraft was “likely flying over an area of developing convective activity” when it encountered severe turbulence. Convective activity is defined when hot air rises and cold air sinks due to temperature differences.

The jet, which was cruising at 37,000 feet, suddenly climbed to 37,362 feet uncommanded. Pilots tried to return the plane to its previous altitude when it dropped 178 feet.

“This sequence of events likely caused the injuries to the crew and passengers,” the report said, based on an analysis of the flight data and cockpit voice recorder.

Singapore Airlines said it was fully cooperating with the investigation, and will continue to support passengers and crew members, including covering medical and hospital expenses, and any other assistance they require. 

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