A sudden 54-metre altitude drop caused unbelted passengers on a Singapore Airlines flight hit by deadly turbulence to be thrown violently inside the cabin, causing injuries, the transport ministry said Wednesday.
One person died and several other passengers and crew onboard flight SQ321 from London suffered skull, brain and spine injuries during last week's terrifying high-altitude ordeal.
The pilots diverted the Singapore-bound Boeing 777-300ER carrying 211 passengers and 18 crew to Bangkok, where the injured were taken to hospitals.
The transport ministry said in a statement on Wednesday that the aircraft experienced a "rapid change" in gravitational force, or G-force, while the plane was passing over the south of Myanmar, citing a preliminary report by Singapore's Transport Safety Investigation Bureau (TSIB).
The ministry said that "vertical acceleration" decreased from positive 1.35G to negative 1.5G within 0.6 seconds, likely resulting "in the occupants who were not belted up to become airborne".
Vertical acceleration again changed from negative 1.5G to positive 1.5G within 4.0 seconds and "this likely resulted in the occupants who were airborne to fall back down", it added.
"The rapid changes in G over the 4.6 sec duration resulted in an altitude drop of 178 ft, from 37,362 ft to 37,184 ft," according to the statement.
"This sequence of events likely caused the injuries to the crew and passengers."
The transport ministry said the early report was based on data taken from the plane's flight data recorder and cockpit voice recorder.
The investigation team included experts from the TSIB, the US National Transportation Safety Board, the US Federal Aviation Administration and the plane's manufacturer Boeing.
The plane initially experienced slight vibration which resulted in an "uncommanded increase in aircraft altitude" from 37,000 feet to 37,362 feet before the autopilot pitched the plane downward to its original altitude.
The pilots also observed an "uncommanded increase in airspeed" which they slowed.
While managing the speed, a pilot was heard calling out that the fasten-seat-belt sign had been turned on, according to the ministry.
The "uncommanded" increases in altitude and airspeed were most likely caused by an "updraft", or the upward movement of air, it said.
The ministry statement did not mention the plane plunging 1,800 metres (6,000 feet) in just a few minutes which was shown on flight tracking data.
Singapore Airlines said in a statement on Wednesday that 42 passengers from the flight were still in Bangkok, including 26 in hospital.
The airline also said it was cooperating in the investigation.