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A preliminary report from Singapore’s Transport Safety Investigation Bureau (TSIB) has outlined the harrowing turbulence faced by passengers on Singapore Airlines flight SQ321.
Forty-three passengers and crew were injured and an elderly British passenger suffered a fatal heart attack when the flight from London to Singapore hit severe turbulence last week, forcing it to divert to Bangkok. Fifty-six Australians were on board, at least eight of whom were hospitalised.
The investigation team, comprising TSIB investigators working alongside US representatives from the National Transportation Safety Board (NTSB), Federal Aviation Administration (FAA) and Boeing, has pieced together the sequence of events based on data from the 777-300ER’s two black boxes.
According to the flight data recorder (FDR) and cockpit voice recorder (CVR), the flight began to experience a “slight vibration” at 07:49:21 (UTC) on 21 May, when it was passing over Myanmar at around 37,000ft.
“Around the same time as the onset of the slight vibration, an uncommanded increase in aircraft altitude, reaching a peak of 37,362 ft, 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 ft,” the investigators said.
“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 investigators believe that the increase in altitude was most likely due to an updraft, and that “the autopilot was engaged during this period”.
“At 07:49:40 hr, the aircraft experienced a rapid change in G as recorded vertical acceleration decreased from +ve 1.35G to negative (-ve) 1.5G, within 0.6 sec. This likely resulted in the occupants who were not belted up to become airborne,” they said.
“At 07:49:41 hr, the vertical acceleration changed from -ve 1.5G to +ve 1.5G within 4 sec. This likely resulted in the occupants who were airborne 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 ft. This sequence of events likely caused the injuries to the crew and passengers.
“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 sec and reengaged the autopilot at 07:50:05 hr.”
Investigations into the incident are ongoing.
