Singapore’s Transport Safety Investigation Bureau Releases Preliminary Findings on SQ321 Incident

Our Bureau - : Jun 12, 2024 - : 8:39 pm

The Transport Safety Investigation Bureau of Singapore (TSIB) has issued its Preliminary Investigation findings of the SQ321 incident, which took place on 21 May and resulted in the death of one passenger with dozens injured inside the cabin on the...

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

The TSIB has extracted the data stored in the flight data recorder (FDR) and cockpit voice recorder (CVR) of the Boeing 777-300ER aircraft.

The investigation team has compiled a chronology of events based on preliminary analysis of the data from FDR and CVR and the preliminary findings for flight SQ321 which departed London on 20 May indicate that the flight was normal prior to the turbulence event. As per the preliminary findings at 07:49:21 hr (UTC) on 21 May 24, as the aircraft was passing over the south of Myanmar at 37,000 ft it was likely flying over an area of developing convective activity and begin to experience slight vibrations due to vertical accelerations that fluctuated between +ve 0.44G and +ve 1.57G for a period of about 19 sec.

The TSIB findings go on to state that at 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 findings further state that the pilots also observed an uncommanded increase in airspeed which they arrested by extending the speed brakes. While managing the airspeed, at 07:49:32 hr, pilots called out that the fasten seat belt sign had been switched on.

As per the TSIB findings, the reason for this uncommanded increase in aircraft altitude and airspeed were most likely due to the aircraft being acted upon by an updraft (the upward movement of air). 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 -ve 1.5G, within 0.6 sec. This likely resulted in the occupants who were not belted up to become airborne and 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,” the findings state.

In this period the recorded data indicated that the pilots initiated control inputs to stabilise the aircraft, disengaging the autopilot in this process and then manually controlled the aircraft for 21 sec before reengaging the autopilot at 07:50:05 hr.

Over the next 24 seconds the recorded vertical acceleration showed more gradual fluctuations, ranging from +ve 0.9G to +ve 1.1G, while the aircraft returned to 37,000 ft at 07:50:23 hr.

The aircraft diverted to Suvarnabhumi Airport, Bangkok, Thailand for emergency treatment of the injured passengers and touched down in Suvarnabhumi Airport at 08:45:12 hr.

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