What happened
On January 12, 2020, a Jin Air Co., Ltd. Boeing 737-800, registered HL8243, was performing a scheduled flight from Kitakyushu Airport to Incheon International Airport. The aircraft was climbing through flight level 250, approximately 30 km northwest of Fukuoka Airport, when it was suddenly struck by intense shaking.
At the time of the event, the seat belt sign had been turned off after the aircraft passed 10,000 feet, as the flight crew had not detected any immediate signs of instability. During the sudden onset of turbulence, the purser was standing in the aft galley. The violent vertical and lateral accelerations caused the crew member to lose her balance and fall, resulting in a fractured right ankle. The pilot in command immediately activated the seat belt sign and instructed the crew to take their seats. The turbulence continued intermittently until the aircraft reached flight level 270.
The investigation
The JTSB investigation examined flight data recorder (FDR) records, meteorological charts, and crew statements. Analysis of the FDR revealed significant fluctuations in acceleration, bank angle, and wind velocity during the period of instability. Specifically, vertical acceleration spiked to 1.49 G, and the headwind component dropped sharply from 68 kt to 44 kt.
Meteorological data indicated that a jet stream was present near flight level 320, with significant wind shear (increasing 20 kt per 1,000 feet) occurring above flight level 240. While the flight crew had reviewed weather forecasts that suggested moderate turbulence was possible, the investigation found that the crew had not specifically updated the cabin crew on the aircraft's approach to the high-shear airspace after the seat belt sign was deactivated.
Findings
- The primary cause of the accident was the aircraft encountering clear air turbulence caused by significant wind shear associated with the jet stream.
- The suddenness of the turbulence meant there were no detectable warning signs on cockpit instruments prior to the impact.
- Although the pilot had briefed the crew to be wary of turbulence during the pre-flight meeting, the flight crew did not provide subsequent updates to the cabin crew as the aircraft approached the specific area of predicted instability.
- The purser was unable to maintain her footing due to the simultaneous vertical, lateral, and rotational movements of the aircraft.