May 30, 2024
SINGAPORE – Over 4.6 seconds on May 21, Singapore Airlines flight SQ321 dropped 178 feet, or about 54m, which wreaked havoc in the plane cabin and likely caused the injuries to cabin crew and passengers.
The Boeing 777-300ER had encountered rapid changes in gravitational forces (G-forces) in those few seconds, which resulted in the drop in altitude, according to the findings of a preliminary investigation issued on May 29.
Investigators from the Transport Safety Investigation Bureau (TSIB) set out a timeline of the incident, based on a preliminary analysis using the data stored in the flight data and cockpit voice recorders.
Their initial findings showed that passengers had only eight seconds to react after pilots turned on the “fasten seat belt” sign before the aircraft was hit by severe turbulence, which injured dozens of people and claimed the life of a 73-year-old British man.
The TSIB investigates air, marine and rail accidents, and is part of the Ministry of Transport. The SQ321 investigation team comprised TSIB investigators and representatives from the United States’ National Transportation Safety Board, Federal Aviation Administration and aircraft manufacturer Boeing.
Investigators found that the aircraft was flying at its cruise altitude of 37,000ft south of Myanmar and likely “over an area of developing convective activity” at 3.49.21pm Singapore time in the afternoon, when turbulence started to cause fluctuations between plus 0.44G and plus 1.57G for 19 seconds. Convective activity refers to weather phenomena like thunderstorms.
G-forces measure rapid acceleration or deceleration by comparing it with the normal pull of gravity on earth, which is considered plus 1G. Hence, at plus 1.57G, a person would feel like they were 1.57 times their body weight.
On board SQ321, which was bound for Singapore from London, the 211 passengers and 18 crew members would also start to feel the plane beginning to vibrate slightly. The aircraft started to climb, reaching 37,362ft in altitude.
It was at this point that the plane’s autopilot tried to bring the plane down to its set cruise altitude, with the pilots turning on the seat belt sign at 3.49.32pm.
Eight seconds later, the plane experienced a rapid change in G-forces from plus 1.35G to minus 1.5G within 0.6 seconds. Negative G-forces move opposite to the direction of gravity, causing people to experience a sensation of “lightness” or “floating”.
In SQ321’s case, it flung passengers and crew who did not manage to fasten their seat belts off their seats and towards the cabin ceiling, possibly causing injuries.
At 3.49.41pm, the G-forces went from minus 1.5G back again to plus 1.5G in four seconds. This resulted in the aircraft dropping from 37,362ft to 37,184ft and all passengers who were flung off their seats would have fallen back down, also likely causing injuries, which included head and spinal injuries.
During this time, the pilots tried to retain control of the aircraft in a bid to stabilise it, disengaging the autopilot in the process, before re-engaging it after manually controlling the plane for 21 seconds.
About 17 minutes after the turbulence incident, the pilots started a “normal, controlled” descent to divert into Bangkok’s Suvarnabhumi Airport, landing at 4.45pm that afternoon. The data showed that the Singapore-bound aircraft did not encounter further severe turbulence during the diversion, investigators said.
Mr Michael Daniel, managing director of consultancy Aviation Insight and a member of the International Society of Air Safety Investigators, said the rapid transition from positive to negative G-forces would have flung unsecured passengers against the roof of the aircraft cabin.
“This is probably why there was so much damage seen to the cabin roof in the photos of the aftermath,” he said. He added that the report issued by the TSIB contained good, useful information, and that the investigators appeared to be on the right track to find the probable cause of the incident.
In a statement on May 29, SIA acknowledged the preliminary findings from TSIB and said it is fully cooperating with the authorities in the ongoing investigations.
The airline said it is committed to supporting the passengers and crew members on board SQ321, including covering their medical and hospital expenses, as well as any additional help they may need.
“We deeply appreciate the invaluable assistance provided by the governments of Singapore and Thailand, as well as our many partners and the medical teams in both countries and around the world,” SIA said. The airline said 26 passengers from SQ321 were still hospitalised in Bangkok as at 5.30pm on May 29.