SQ321 turbulence: 'Rapid' G-force changes, altitude drop likely caused injuries to unbelted passengers, crew

SINGAPORE: Preliminary investigations into severe turbulence on Singapore Airlines flight SQ321 have shown that "rapid" changes in gravitational force (G) - up to three times that of Earth's - and an altitude drop of around 54m likely caused injuries to passengers and crew who were not belted up.

SQ321 had been flying from London to Singapore last Tuesday (May 21) when it encountered extreme turbulence. One passenger died and dozens were injured, resulting in an emergency diversion of the plane to Bangkok.

In a statement on Wednesday, the Ministry of Transport (MOT) said its Transport Safety Investigation Bureau (TSIB) had extracted data stored in the flight data recorder and cockpit voice recorder of the plane. 

A team comprising TSIB investigators and United States representatives - from the National Transportation Safety Board, Federal Aviation Administration and Boeing - then compiled a chronology of events based on preliminary analysis of the data. 

The investigators found that after SQ321 departed London, the flight was "normal" prior to the turbulence event.

At 7.49am UTC (3.49pm Singapore time) on May 21, the aircraft was passing over the south of Myanmar at 37,000 ft (11,300m) and “likely flying over an area of developing convective activity”, said MOT, referring to weather phenomena such as thunderstorms.

The G force then fluctuated between positive 0.44G and positive 1.57G for a period of about 19 seconds.

“This would have caused the flight to begin to experience slight vibration,” said MOT. 

This is also already slightly outside of the range of G-forces passengers experience on a typical flight, which is between 0.7G to 1.3G, Professor Rico Merkert, an aviation expert at the University of Sydney, told CNA.

Around the same time, there was an “uncommanded" increase in aircraft altitude, reaching a peak of 37,362 ft. In response, the autopilot pitched the aircraft downwards to descend back to 37,000 ft. 

The pilots also observed an uncommanded increase in airspeed, which they addressed by extending the speed brakes, said MOT.

Eleven seconds after the initial vibration, it was heard that a pilot called out that the fasten seat belt sign had been switched on.

These uncommanded increases in altitude and airspeed were most likely due to the aircraft being acted upon by an updraft, or the upward movement of air, said MOT.

The autopilot was engaged during this period, it added. 

A few seconds later, SQ321 experienced a rapid change in G as vertical acceleration decreased from positive 1.35G to negative 1.5G within 0.6 seconds. “This likely resulted in the occupants who were not belted up to become airborne,” said MOT.

One second later, vertical acceleration changed from negative 1.5G to positive 1.5G, within 4 seconds. “This likely resulted in the occupants who were airborne to fall back down,” said MOT. 

Prof Merkert said that here, the aircraft was likely accelerating upwards "faster than the passengers".

These rapid changes of up to 3G over a 4.6-second duration resulted in an altitude drop of 178 ft - the equivalent of a 19-storey building. “This sequence of events likely caused the injuries to the crew and passengers,” said MOT. 

According to Prof Merkert, a change of 3G is "quite significant force on a person's body, and anyone not wearing a seat belt would likely sustain some form of injury". 

For comparison, the fastest roller coaster in the world in Dubai lets riders experience a G-force of 4.8G. Fighter pilots can tolerate can tolerate 5 to 9Gs, but they have fastened seatbelts among other safeguards.

In summary, what happened was that the plane went up then abruptly down then abruptly up again, said Prof Merkert, adding that these moves were "uncommanded and due to nature's forces". 

MOT said that in the midst of the rapid G changes, recorded data showed that the pilots "initiated control inputs to stabilise the aircraft, disengaging the autopilot in this process”. 

The pilots manually controlled the aircraft for 21 seconds, before reengaging autopilot at 07.50am UTC, 44 seconds after the initial vibrations. 

Over the next 24 seconds, there were more gradual fluctuations ranging from positive 0.9G to positive 1.1G. The aircraft returned to 37,000 ft 18 seconds after the pilots reengaged autopilot. 

After the pilots were informed by cabin crew that there were injured passengers, the decision was made to divert to Suvarnabhumi Airport in Bangkok, Thailand. On the way there, the pilots requested medical services to meet the aircraft on arrival. 

Approximately 17 minutes after the turbulence event, at 8.06am UTC, the pilots initiated a normal, controlled descent from 37,000 ft and the aircraft reached 31,000 ft at 08.10am UTC. 

“The data showed that the aircraft did not encounter further severe turbulence during this diversion,” said MOT, adding that the flight touched down in Suvarnabhumi at 8.45am UTC. 

"AS LONG AS ONE WEARS A SEAT BELT"

Prof Merkert said that while it was scary to read what nature's forces had done to the aircraft, it was also "amazing to see what modern aircraft can sustain". 

"As long as one wears a seat belt, things should still be quite safe," said Prof Merkert of typical instances of turbulence. 

"Of course, what happens when this occurs during meal-serving hours and when one is attending the restrooms is another matter, but these are periods of time that can be kept to a minimum." 

As to why pilots took at least 19 seconds between the first vibrations before they disengaged the autopilot, experts said that the autopilot is able to handle most cases of turbulence. 

"It depends on a case-to-case basis, but usually it's preferred to engage autopilot (during turbulence), given its highly advanced capability," said aviation expert Shantanu Gangakhedkar, a senior consultant at growth advisory firm Frost and Sullivan. 

Among other things, the autopilot is designed to help an aircraft maintain the desired altitude, speed and heading despite wind sheers and other factors, said Mr Shantanu. 

Even in regular flight, the autopilot is constantly making "minor changes" to the aircraft, he added. 

That is why pilots are unlikely to disengage the autopilot, except in extreme cases. 

"Pilots need time to understand what's happening because once autopilot is disengaged, almost all the controls need to be managed manually," he said. 

Interviews with SQ321 passengers painted a harrowing scene - one where those who were unbelted flew out of their seats and onto the ceiling of the plane, before slamming back down. Others described the experience as akin to riding a roller coaster.

Many who went through the ordeal hit body parts on the cabin interior, with some suffering cuts on their heads and bleeding ears.

Photos of the plane’s interior following the incident showed broken overhead panels, oxygen masks dangling from the ceiling and food strewn all over the floor. 

Responding to the preliminary findings, Singapore Airlines (SIA) said in a statement on Wednesday that it was “fully cooperating with the relevant authorities in the ongoing investigations into this incident”.

Thailand's Samitivej Srinakarin Hospital also announced on Wednesday that 26 patients from SQ321 remain hospitalised in Bangkok.

Twenty of them are in Samitivej Srinakarin, five in Samitivej Sukhumvit Hospital and one in Bangkok Hospital. 

That would mean that one Singaporean has likely been discharged, since an earlier update indicated there were two patients - both Singaporeans - in Bangkok Hospital.

In total, 42 passengers who were on board the flight are still in Bangkok, SIA said in an update on Wednesday evening.