What happened on the turbulence-hit SIA flight that left one dead and 79 hurt?

SINGAPORE – On May 21, 2024, Singapore Airlines (SIA) Flight SQ321 hit severe turbulence as it flew over south-west Myanmar.

A 73-year-old Briton died of heart failure and a build-up of fluid in the lungs, while 79 were injured.

In its final report detailing its investigation findings, the Transport Safety Investigation Bureau said on May 19 that inclement weather had not been detected by the weather radar on board.

The pilots did not observe any clouds in their immediate flight path prior to the incident either.

The bureau noted that their actions were “understandable and appropriate for the situation”.

This is a timeline of events, in Singapore time:

5.38am: The Boeing 777-300ER departs from Heathrow Airport in London, bound for Changi Airport in Singapore.

The flight crew consists of a pilot-in-command, 60, a senior first officer, 40, and an augmenting pilot, 51. Both the pilot-in-command and the augmenting pilot held the rank of captain.

In his career, the pilot-in-command had accumulated more than 21,922 hours of flight time, while the augmenting pilot had more than 13,791 hours. The senior first officer had 6,320 hours of flight time.

3.30pm: The aircraft begins approaching south-west Myanmar.

At this point, the senior first officer is in the crew rest facility behind the cockpit, while the other two pilots are manning the plane.

The pilots navigate by taking reference from their flight instruments, with the plane cruising at 37,000ft.

The cabin crew, meanwhile, prepare for breakfast service as passengers start waking up.

According to the flight crew, their navigation displays did not show any precipitation based on weather radar information in the 15 minutes before they flew over a particular waypoint in south-west Myanmar.

Both pilots noted that their immediate flight path was clear, although the pilot-in-command said he saw cirrus clouds, or high-level clouds, in the distance.

3.44pm: The flight crew request that Yangon air traffic control reroute them, citing weather as the reason.

Based on interviews with the flight crew, however, the report said that “there was no concern regarding the weather”.

They merely cited it to increase the odds of being rerouted, as the requested route was more direct.

Air traffic control counters with an alternative route instead, which the flight crew accept.

3.49.23pm: The aircraft is cruising at 37,000ft at a speed of 0.84 Mach, or 1,037kmh, when it encounters turbulence.

This causes the vertical acceleration to rise to about plus 1.2G, meaning that people would feel as if they were 1.2 times their body weight.

The autopilot pitches the plane’s nose down to maintain its altitude, but the aircraft only rises higher.

3.49.25pm: The airspeed of the wide-body jet reaches about 0.88 Mach.

This particular model of plane has a maximum operating Mach of 0.89, and the augmenting pilot uses the speed brakes to prevent the aircraft from exceeding it.

The pilot-in-command also reduces the cruising speed to 0.82 Mach, the recommended speed for flying through turbulence.

He switches the seat-belt sign on and calls out “seat belt on”, but has no time to make an announcement to passengers and cabin crew.

3.49.40pm: The plane reaches an altitude of 37,228ft, with the gravitational force at plus 1.35G.

3.49.41pm: The plane reaches an altitude of 37,362ft, but in a span of 0.6 seconds, the vertical acceleration drops from plus 1.35G to minus 1.5G.

3.49.43pm: The flight crew flies the plane manually for the next 21 seconds to stabilise it. Autopilot is disengaged.

3.50.05pm: The flight crew switches on autopilot again.

3.51pm: The flight crew note that the area outside the aircraft is clear.

The pilot-in-command is heard asking: “How come suddenly? Let me remove this (the sunshade on the side window) and see.”

There is no further discussion on what he sees, but a later recording has him saying that he had set the weather radar at the highest sensitivity at the time of the incident, but saw nothing on the display.

3.55pm: The flight crew report to Yangon air traffic control that they encountered severe turbulence.

They ask if other aircraft reported turbulence as well, but air traffic control says it did not receive any other reports.

4.01pm: The flight crew declare an emergency to Yangon air traffic control. They request to divert to Suvarnabhumi Airport in Bangkok, Thailand.

4.03pm: SIA is alerted by the flight crew about the diversion.

4.06pm: Yangon air traffic control instructs the plane to descend from 37,000ft to 31,000ft.

At the lower altitude, the pilot-in-command observes altostratus, a type of greyish or bluish cloud sheet, as well as scattered cumulus clouds.

However, these were not shown on the plane’s navigation displays and appeared only when the aircraft went below 31,000ft.

4.07pm: The augmenting pilot goes to the cabin to assess the situation.

One passenger is reported unconscious, and cardiopulmonary resuscitation is administered.

4.45pm: The aircraft lands at Suvarnabhumi Airport, reaching its assigned parking bay about five minutes later.

Medical personnel arrive, and those injured are given treatment and sent to hospitals.