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Singapore Boeing 777-300 Accident Investigation
Singapore B773 near Bangkok on May 21st 2024 due to severe turbulence kills one and injures 85 people on board. A Singapore Airlines Boeing 777-300, registration 9V-SWM, carrying 211 passengers and 18 crew members on flight SQ-321 from London Heathrow, England to Singapore, was flying at FL370, approximately 350 nm west-northwest of Bangkok while enroute at FL370 over Myanmar about 15 minutes earlier, when it experienced severe turbulence, injuring several passengers. The crew chose to divert to Bangkok after 17 minutes of descent and the plane touched down on runway 19R approximately half an hour later. Upon arrival, 1 passenger was declared dead and 6 crew members and 79 passengers were taken to the hospitals.
Turbulence
In commercial aviation, turbulence continues to be one of the most unpredictable and potentially dangerous meteorological phenomena. Turbulence remains a major cause of in-flight injuries and unplanned diversions, despite the fact that modern aircraft are built to withstand large aerodynamic loads and pilots are trained to react to unforeseen atmospheric conditions. Particularly in clear air close to jet streams or convective boundaries, turbulence frequently forms without visual warning, in contrast to thunderstorms, icing, or poor visibility. Even with onboard weather radar and contemporary forecasting tools, this makes it especially challenging to detect and avoid.
Accident Overview
Report
Boeing 777-300 aircraft experienced a severe turbulence and had to make emergency landing in Bangkok and not reaching flight's final point.
Aircraft
Wide-body long-range twin-engine jet with capacity for 388 passengers.
Investigation
The Transport Safety Investigation Bureau of Singapore initiated a comprehensive investigation focusing on meteorological factors.
Operational Context
Geographical location
The plane was cruising at FL370 over southeast Myanmar, close to the Bay of Bengal airspace, about 350 nautical miles northwest of Bangkok, when the incident occurred. Particularly during the pre-monsoon season, in May, when deep convective activity starts to increase and develop vertically, this area experiences climatological activity. Since vertical wind shear can be strong near the tropopause, turbulence in this area is frequently hard to detect, particularly when it is linked to developing convection or clear-air turbulence.
Previous Decisions
The aircraft experienced a strong updraft at around 07:49 UTC while cruising at FL370, which resulted in an unplanned climb to 37,362 feet and a sharp increase in airspeed. The crew applied speed brakes to regulate the airspeed after the autopilot pitched the plane down in response. At 07:49:32, the seatbelt sign went on. Shortly afterward, the aircraft's vertical acceleration dropped sharply from +1.35G to -1.5G in 0.6 seconds, causing unbelted passengers and crew to become airborne.
Weather Alerts
The plane was operating on a regular airway, in autopilot mode, in a region with little radar coverage and little infrastructure for reporting turbulence. At the time, there were no known SIGMETs for turbulence over Myanmar. Convective clouds were forming in the area, but the weather system was probably still in its early stages and did not yet have the complete cumulonimbus cloud structure that pilots or flight planning systems could see.
Surface Reports and Forecasts
METAR and TAF reports
Looking at the METAR before the event, we can that visibility was about 6000 meters and also formation of few cumulonimbus clouds - indicating local thunderstorms developing. After an accident METAR report has changed a little bit, visibility increased to 7000 meters and cumulonimbus clouds altitude has increased to 2000 feet. According to TAF report between 06:00 and 12:00 UTC, temporary conditions were expected:
Winds shifting to 180° with gusts up to 18 knots, thunderstorms with rain, reduced visibility to 4000m and low cloud layers at 1200 feet and cumulonimbus clouds at 1800 feet
This forecast indicates that during the period when the SQ321 turbulence event occurred, convective activity and storm-related turbulence were expected.
Satellite Data Analysis
Airmass
Strong vertical development typical of cumulonimbus activity was evidenced by satellite imagery over southern Myanmar on May 21, 2024, which showed a dominant warm, moist tropical airmass with deep convective clouds with bright white tops. In line with the in-flight updraft that impacted Singapore Airlines Flight SQ321, this unstable environment, which was fueled by surface heating and high humidity, produced ideal conditions for severe turbulence.
On the other hand, a more stable, still moist, airmass with limited vertical development and drier upper layers encroaching from the northwest seemed to cover the region surrounding Bangkok, Thailand, with thinner, broken cloud cover. The turbulence over Myanmar and the comparatively calm conditions over Bangkok, where the aircraft safely diverted and landed, can be explained by these opposing conditions.
WV chart
Large areas of bright white cloud cover are visible, especially over southern Myanmar and bordering Thailand, suggesting the presence of deep, cold-topped convective clouds, like cumulonimbus.
These are typical of thunderstorms or rapidly developing convection, which can be associated with severe turbulence.
This chart shows the environmental conditions behind the turbulence event encountered by SQ321 flight.
Skew-T chart analysis
Up to 600 hPa, the Skew-T diagram represents a moist and unstable lower atmosphere. The temperature and dew point are closely aligned, suggesting high humidity and saturation, which are ideal for cloud formation and convection. The profile becomes noticeably drier above 500 hPa, indicating possible instability in the event that air parcels rise from below. Convective development is supported by the lack of a strong inversion, which allows for unrestricted vertical motion.
Clear-air turbulence near cruising altitudes is known to be caused by moderate to strong vertical wind shear, which is indicated by the wind barbs, which show increasing wind speed with height. Overall, the sounding points to a convectively unstable environment that is likely to have contributed to the SQ321 turbulence event, complete with updraft potential and shear-induced turbulence.
Investigation Conclusions
Strong updrafts and atmospheric instability close to rapidly developing convective cloud cells over Myanmar caused severe clear-air turbulence for Singapore Airlines Flight SQ321. Extreme in-flight motion caused by vertical wind shear, potential microbursts, and gravity waves resulted in one fatality and numerous injuries. Although the crew reacted appropriately by diverting to Bangkok, the incident underscores the need for better forecasting, weather avoidance, and consistent seatbelt use during cruises, as well as the danger of undetectable turbulence close to convection.