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Passengers on Singapore Airlines flight SQ321 endured 19 seconds of extreme turbulence that included a drop of 178 feet in just 4.6 seconds – representing a vertical speed of 26mph. They also suffered violent changes in vertical acceleration.

“This sequence of events likely caused the injuries to the crew and passengers,” say investigators.

The Boeing 777 had departed from London Heathrow the previous day on a routine flight to Singapore. As it was flying south of Myanmar at 37,000 feet on 21 May, it hit violent turbulence. A 73-year-old British man, Geoff Kitchen, died of a heart attack during the incident. Other passengers and crew were injured, some of them seriously.

Singapore’s Transport Safety Investigation Bureau (TSIB) has now issued its Preliminary Investigation Findings of Incident Involving SQ321.

The investigation team comprises TSIB investigators and US representatives from the National Transportation Safety Board (NTSB), Federal Aviation Administration (FAA) and Boeing.

They extracted the information stored in the flight data recorder (FDR) and cockpit voice recorder (CVR) of flight SQ321.

The inflight drama began at 7.49am GMT as the plane flew through “an area of developing convective activity”. The aircraft was on autopilot.

The report says: “An uncommanded increase in aircraft altitude, reaching a peak of 37,362 feet, was recorded. In response to this uncommanded altitude increase, the autopilot pitched the aircraft downwards to descend back to the selected altitude of 37,000 feet.

“In addition, the pilots observed an uncommanded increase in airspeed which they arrested by extending the speed brakes. While managing the airspeed, at 07:49:32 hr, it was heard that a pilot called out that the fasten seat belt sign had been switched on.”

The investigation appears to show that it took 11 seconds for the seatbelt sign to be switched on after the turbulence began. The pilots took over control from the autopilot.

The ordeal for passengers and crew continued.

“At 07:49:40 hr, the aircraft experienced a rapid change in G as recorded vertical acceleration decreased from positive 1.35G to negative 1.5G, within 0.6 sec.

“This likely resulted in the occupants who were not belted up to become airborne.”

The forces reversed almost instantaneously. “This likely resulted in the occupants who were airborne to fall back down,” investigators say.

“In the midst of the sequence of rapid changes in G, recorded data indicated that the pilots initiated control inputs to stabilise the aircraft, disengaging the autopilot in this process.”

The pilots manually controlled the aircraft for 21 seconds before reengaging the autopilot. When cabin crew told pilots of the injuries, they decided to divert to Bangkok. The “normal, controlled descent” began 17 minutes after the plane first hit turbulence.

Flight SQ321 touched down safely at the Thai capital at 8.45am GMT, almost an hour after the extreme turbulence began.

A spokesperson for Singapore Airlines said: “We are committed to supporting our passengers and crew members who were on board SQ321 on that day, as well as their families and loved ones.

“This includes covering their medical and hospital expenses, as well as any additional assistance they may need.

“We deeply appreciate the invaluable assistance provided by the governments of Singapore and Thailand, as well as our many partners and the medical teams in both countries and around the world.

“We are also humbled by and grateful for the encouragement from our passengers and members of the public.”

The airline has changed its in-flight procedures and no longer allows cabin crew to serve meals while the seatbelt sign is on.

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