Rapid changes in g-force, altitude caused injuries in SQ321 flight

The extreme changes in flight conditions happened in around five seconds.

Initial findings on the turbulence-hit Singapore Airlines (SIA) flight SQ321 showed that sudden changes in gravitational force and altitude possibly led to the injury of crew members and passengers who were not wearing their seat belts at the time of the incident. 

In a statement, the Ministry of Transport (MOT) said the Transport Safety Investigation Bureau of Singapore has extracted data from the flight data recorder and cockpit voice recorder of flight SQ321.

It worked with United States representatives, who were from the National Transportation Safety Board, Federal Aviation Administration, and Boeing, in the ongoing probe.

According to MOT, the SQ321 flight was normal when it left London on 20 May. At around 7:49 AM UTC (3:49 PM, Singapore time) on 21 May, it passed over the south of Myanmar at 37,000 feet (ft) and likely flew over an area of developing convective activity.

The gravitational force fluctuated between positive (+ve) 0.44G and +ve 1.57G for about 19 seconds, which would have caused slight vibration.

Around this time, the flight reached an uncommanded peak of 37,362 ft. The autopilot then pitched the aircraft downwards to return to the selected 37,000 ft altitude. 

Meanwhile, there was also an uncommanded increase in airspeed, which the pilots controlled by extending speed brakes. Whilst doing this, a pilot called out that the fasten seat belt sign had been switched on.

MOT said the sudden increase in altitude and airspeed may be due to an updraft or the upward movement of air. It noted autopilot was engaged during this period. 

The aircraft then experienced a rapid change in gravitational force, which decreased to negative (-ve) 1.5G from +ve +ve 1.35G within 0.6 sec. 

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

This was immediately followed by an increase to +ve 1.5G within 4 sec, which led occupants who were airborne to fall back down, the ministry added.

“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,” MOT said. 

During these events, MOT said the pilots manually controlled the aircraft for 21 seconds before reengaging the autopilot at 7:50 AM UTC (3:50 PM Singapore time). 

More gradual fluctuations were observed over the next 24 seconds, which ranged from +ve 0.9G to +ve 1.1G, before it returned to 37,000 ft 18 seconds after.

The pilots decided to divert the flight to Thailand after they were informed that some passengers were injured. 

MOT said 17 minutes after the incident, SQ321 normally descended from 37,000 ft to 31,000 ft, and that no further severe turbulence occurred before arriving in Bangkok.