On May 20, Singapore Airlines Flight SQ321 took off from London’s Heathrow Airport at 10.15pm local time with 211 passengers and 18 crew members on board.
The flight had no incidents until it reached the Irrawaddy Basin in southern Myanmar.
At 3.49.21pm (Singapore time), while cruising at 37,000 ft, the plane started to vibrate, likely after entering an area of convective activity —upward and downward currents of air often associated with thunderstorms.
At the same time, and while still on autopilot, the plane started to rise rapidly.
In response, the autopilot pitched the aircraft downward to return to 37,000 ft.
At 3.49.32pm, one of the pilots announced that the seat-belt sign had been turned on.
Less than 10 seconds later, the aircraft experienced a sudden drop in altitude, causing unbelted passengers and crew to be flung into the air.
According to the investigation, within four seconds, the plane’s vertical acceleration went back to positive 1.5G, causing those who were flung off their seats to fall back down. The plane dropped 178 ft, from 37,362 ft to 37,184 ft. As a result, some passengers and crew members were injured.
During this time and for 21 seconds, the pilots took manual control of the aircraft to stabilise it. They re-engaged the autopilot at 3.50.05pm.
The plane continued to fluctuate, although more gradually, until it reached its original cruising altitude of 37,000 ft at 3.50.23pm.
After being informed by the cabin crew about injured passengers, the pilots diverted to Suvarnabhumi Airport in Bangkok, Thailand. En route, they requested medical services to meet the aircraft upon arrival.
Approximately 17 minutes after the turbulence, at 4.45.12pm, the aircraft landed at Suvarnabhumi Airport.
The preliminary findings released by Singapore’s Transport Safety Investigation Bureau revealed some technical details that explain what occurred during the three minutes of turbulence that affected the Boeing 777-300 aircraft.
The investigation aligns with flight-tracking site Flightradar24’s data, which is obtained from a global network of receivers, satellites and radars that receive information from aircraft transponders.
This data shows that the aircraft rapidly climbed and descended twice within 62 seconds, between 3.49pm and 3.51pm.
The flight's altitude changed by more than 362 ft in less than a minute
The rapid change in G-force caused passengers who were not wearing their seat belts to be lifted out of their seats. G-forces measure how quickly something speeds up or slows down compared with the normal pull of gravity on earth, which is called 1G.
In the 4.6-second event, these rapid changes in G-force caused the plane to drop 178 ft in altitude, from 37,362 ft to 37,184 ft.
This indicates that the turbulence caused rapid transitions between climbing and descending, rather than the minor changes in altitude, and caused the disturbance in the cabin.
The plane’s vertical speed surged to 1,664 feet per minute (fpm) before dropping to -1,536 fpm in a few seconds