Investigators have released their preliminary findings from a data extraction of the flight data recorder (FDR AKA the ‘black box’) and the cockpit voice recorder following a fatal turbulence incident on SQ321.
https://www.travelweekly.com.au/article/heavy-turbulence-kills-one-with-dozens-injured-on-singapore-airlines-flight/
Investigators from the Transport Safety Investigation Bureau (TSIB) and US representatives from the National Transportation Safety Board (NTSB), Federal Aviation Administration (FAA) and Boeing compiled a chronology of events based on preliminary analysis of the data from FDR and CVR.
The flight departed London on 20 May and flew without issue until 07:49:21 hr (UTC) on 21 May when passing over an area south of Myanmar at 37,000 feet the aircraft encountered an area of developing convective activity – this would likely have been the point where the aircraft began experiencing slight vibrations.
Around this time, an ‘uncommanded’ altitude increase to 37,362 feet was recorded as well as an increase in aircraft speed. In response, the autopilot pitched the aircraft down and pilots extended speedbreaks. It was around this time that the pilot notified the cabin that the fasten seatbelt sign had been turned on.
The uncommanded increase in aircraft altitude and airspeed is most likely due to the aircraft being acted upon by an updraft, the autopilot was engaged during this period.
In the moments that followed, a rapid change in gravitational force was recorded as vertical acceleration decreased, the cause of cabin crew and staff becoming ‘airborne’ in the cabin.
Data then shows that pilots took complete control of the aircraft for 21 seconds and stabilised the aircraft before re engaging autopilot and returning to 37,000 feet.
After the pilots were informed by the cabin crew that there were injured passengers in the cabin, the decision was made to divert to Suvarnabhumi Airport, Bangkok, Thailand. On the way to Bangkok, the pilots requested for medical services to meet the aircraft on arrival.
Approximately 17 minutes after the turbulence event, at 08:06:51 hr, pilots initiated a normal, controlled descent from 37,000 ft and the aircraft reached 31,000 ft at 08:10:00 hr.
Data shows the aircraft did not encounter further severe turbulence during this diversion, and touched down in Suvarnabhumi Airport at 08:45:12 hr.
Investigations are ongoing.
