'Rapid changes' in G-force, altitude drop likely caused injuries to unbelted passengers and crew on SQ321
Rapid changes in gravitational forces (G-forces) along with an altitude drop likely cause the injuries to unbelted crew and passengers on Flight SQ321, according to the findings of a preliminary investigation.
In a statement on Wednesday, investigators from the Transport Safety Investigation Bureau (TSIB) said that there was a "rapid change" in G-force over 4.6 seconds, resulting in the injuries. This was based on findings from data stored in the flight data recorder and cockpit voice recorder.
The TSIB is the air, marine and rail accidents and incidents investigation authority in Singapore and is part of the Ministry of Transport (MOT).
Investigators found that the flight, which departed from London on May 20, was normal prior to the turbulence incident.
At around 3.49.21pm (Singapore time) on May 21, the aircraft flew over the south of Myanmar at an altitude of 37,000 ft (11,300m) and was "likely flying over an area of developing convective activity," MOT stated.
Turbulence started to cause fluctuations between +0.44G and +1.57G for a period of 19 seconds.
G or gravitational force, refers to changes in speed and are expressed in multiples of gravitational acceleration.
"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," MOT said.
The investigation team had comprised of TSIB investigators and representatives from the US’ National Transportation Safety Board, Federal Aviation Administration and aircraft manufacturer Boeing.
Rapid change in G-forces
It was at this point that the autopilot attempted to pitch the aircraft downwards to descend back to the selected altitude of 37,000 ft. Pilots also observed an uncommanded increase in airspeed which prompted the extension of speed breaks.
About 32 seconds at 3.49.32pm, the pilot called out that the seatbelt sign was switched on.
Eight seconds later, a rapid change in G-forces was recorded, going from +1.35G to -1.5G within the span of 0.6 seconds.
"This likely resulted in the occupants who were not belted up to become airborne," MOT explained.
Within the next four seconds, the vertical acceleration of the plane went from -1.5G to +1.5G, which caused airborne occupants 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," MOT wrote in the release. "This sequence of events likely caused the injuries to the crew and passengers."
Amid the rapid changes in G-force, pilots took control of the aircraft, disengaging the autopilot.
They manually controlled the plane for 21 seconds, re-engaging the autopilot five seconds past 7.50am.
Over the proceeding 24 seconds, there were more gradual fluctuations in G-force ranging from +0.9G to +1.1G before the plane returned to an altitude of 37,000 ft.
After pilots were informed of the injured passengers on board their aircraft, the decision was made to divert the plane to Suvarnabhumi Airport, Bangkok, Thailand.
On the way to Bangkok, the pilots requested for medical services to meet the aircraft on arrival.
How much G is too much?
According to the BBC, an upwards acceleration of +5G is enough to prevent your heart from pumping blood to your brain, causing oxygen starvation leading to a blackout within seconds.
Downward acceleration, or negative G-force, will cause blood to pool in the head.
This causes the face to swell up and lower eyelids forced over the eyes, resulting in a "redout" - all that is visible is light shining through eyelids, which would appear red.
At -3G, the blood cannot return to the lungs to re-oxygenate, which will result in the individual passing out.
ALSO READ: 'Nobody eff around now': Man on flight from London to Singapore experiences turbulence, notes how compliant passengers are
khooyihang@asiaone.com
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