What happened
On December 25, 2019, an Airbus A320-232, registered B50001, operated by Tigerair Taiwan, encountered extreme turbulence while cruising at FL300. The flight, traveling from Hakodate Airport to Taiwan Taoyuan International Airport, was approximately 100 km north-northeast of Miyazaki Airport when the aircraft began to shake violently.
As the aircraft entered a zone of intense vertical acceleration, the vertical speed surged to a maximum of 9,300 ft/min, and the aircraft climbed to 33,200 ft. During this period of instability, the First Officer applied a sidestick input that exceeded the threshold for autopilot disengagement, forcing the crew to manage the aircraft's attitude manually.
The violent motion caused significant disruption in the cabin. A cabin crew member (the CIC) was walking through the aisle to return to a jump seat when they fell, resulting in a right ankle bone fracture. Additionally, one passenger sustained a minor lumbar injury, and two other cabin crew members and one passenger suffered minor injuries. The turbulence also caused damage to the cabin ceiling and door panels at seven different points.
The investigation
The Japan Transport Safety Board (JTSB) examined flight data recorder (FDR) logs, meteorological charts, and satellite imagery. The investigation focused on the atmospheric conditions near the jet stream and the communication of weather warnings. Investigators analyzed the aircraft's flight path, noting that while the crew had been briefed on potential turbulence, the specific severity of the encounter was not fully anticipated.
Technical analysis of the FDR confirmed that vertical acceleration fluctuated significantly between 2.17 G and -0.57 G. The investigation also looked into the company's Operation Control Center (OCC) procedures regarding the receipt and distribution of significant meteorological information (SIGMET).
Findings
- The primary cause of the injuries was the aircraft fiercely shaking due to an encounter with wind shear near the jet stream.
- Severe wind shear was present in the area, evidenced by the presence of a transverse line of high clouds and vertical wind shear.
- The flight crew likely did not anticipate the extreme intensity of the turbulence, despite having predicted some turbulence earlier in the flight.
- The crew's ability to prepare for the event was hindered because the company's OCC had not obtained the specific SIGMET released by the Japan Meteorological Agency earlier that day.
- The autopilot disengaged because the aircraft's motion caused the First Officer to unintentionally move the sidestick forward beyond the 5-degree threshold.