What happened
On July 5, 2012, a Korean Air Lines Boeing 747-40 and registered HL7473 was operating a scheduled flight from Gimpo International Airport to Tokyo International Airport. The flight, carrying 194 people, had been experiencing light shaking for some time, leading the crew to turn off the fasten seat-belt sign after reaching cruising altitude and beginning a descent in visual meteorological conditions.
At approximately 20:30 local time, while the aircraft was descending through 22,000 feet about 160 km north-northeast of Tokyo, it encountered a sudden and violent bout of turbulence lasting two to three seconds. During this event, a passenger who was standing in the aisle to walk to the lavatory was thrown off balance. The individual struck the L4 door with their back before falling to the floor. Flight attendants assisted the passenger back to their seat, but the individual later required a wheelchair upon arrival in Tokyo and was subsequently transported by ambulance to a hospital, where they were diagnosed with a bone fracture.
The investigation
The Japan Transport Safety Board (JTSB) examined data from the digital flight data recorder (DFDR), air traffic control communications, and statements from the crew and the injured passenger. The investigation focused on the atmospheric conditions at the time of the incident and the crew's decision-making regarding the seat-belt sign.
Investigators analyzed weather radar imagery from the Japan Meteorological Agency (JMA) and vertical wind shear forecasts. While the aircraft's onboard weather radar did not show any echoes at the time of the event, JMA imagery revealed that convective clouds were developing and expanding in the area. Furthermore, the DFDR recorded a significant spike in vertical acceleration—increasing to +1.54G and then dropping to +0.34G—coinciding with a rapid fluctuation in wind direction and velocity.
Findings
- The sudden, violent shaking was the direct cause of the passenger being thrown against the cabin structure.
- Unstable atmospheric conditions, likely involving vertical wind shear or developing convective clouds, generated the turbulence.
- The crew likely turned off the fasten seat-belt sign because the onboard radar showed no immediate threats, and the expected period of significant turbulence had passed.
- The aircraft's weather radar settings may have been unable to detect the developing cloud activity occurring below the aircraft's flight level.