The Singapore Airlines flight that suffered severe turbulence, leaving one passenger dead and dozens more injured, dropped 178 feet (54 metres) in less than five seconds, an early investigation has found.
Singapore's Transport Safety Investigation Bureau today blamed the drop and an associated "rapid" change in gravitational force for the injuries to crew and staff who were unbuckled at the time.
UK grandfather Geoffrey Kitchen, 73, died of a suspected heart attack and more than 50 were injured when the Boeing 777, which was flying from London's Heathrow airport to Singapore on May 21, ran into turbulence that hurled people and items around the cabin.
Fifty-six Australians, including at least 12 who were hospitalised, were among the 211 passengers and 18 crew onboard.
After examining the plane's flight data and cockpit voice recorders, investigators from Singapore, Boeing and two American aviation agencies said SQ321 was a normal flight until G-force changes started to cause a "slight vibration" about 7.50am London time (9.50am AEST), while flying over the south of Myanmar at 37,000 feet.
Roughly 19 seconds later, the plane suffered a "rapid change in G" as it fell for 0.6 seconds, which "likely resulted in the occupants who were not belted up to become airborne".
A four-second rise that immediately followed likely caused them to fall back down.
"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 feet," the investigators wrote.
"This sequence of events likely caused the injuries to the crew and passengers."
About the same time as the vibration began, the plane's instruments recorded an "uncommanded" jump up to 37,362 feet, causing the autopilot to pitch the plane down to return to cruising altitude.
"In addition, the pilots observed an uncommanded increase in airspeed which they arrested by extending the speed brakes," the investigators said.
"While managing the airspeed, at 07:49:32 (GMT), it was heard that a pilot called out that the fasten seat belt sign had been switched on."
Judging by the timing, unbelted passengers would've had only seconds to react to the seatbelt sign being turned on.
The pilots disengaged the autopilot and took manual control of the plane for about 21 seconds, before deciding to divert to Bangkok when they received word about the injured passengers.
A normal, controlled descent from 37,000 feet to 31,000 feet roughly 15 minutes after the severe turbulence was initiated by the pilots, not caused by turbulence, according to the preliminary report.
Passengers have described the "sheer terror" of the aircraft shuddering, loose items flying and injured people lying paralysed on the floor of the plane.
Twenty-six people remained in hospital in Bangkok on Wednesday. Hospital authorities earlier said injuries included spinal or spinal cord damage, skull or brain injuries and damage to bones or internal organs.
It was unclear what caused the turbulence. 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.
- Reported with AAP