What happened
On June 24, 2018, a Boeing 777-300, registered JA8944, was operating a scheduled flight from New Chitose Airport to Tokyo International Airport. While cruising at FL300 over Kurihara City, Miyagi Prefecture, the aircraft encountered sudden and violent turbulence.
During the period of instability, the aircraft experienced rapid fluctuations in vertical acceleration, with loads reaching as high as 1.71G. At the time of the event, the seat belt sign was turned off as the flight had been stable since takeoff. A cabin attendant, who was moving through the aft aisle with a service tray, attempted to steady herself by grasping a passenger seat armrest. However, the intensity of the second shock caused her to lose her footing and feel as though she were floating, leading to a fall that resulted in a bone fracture to her ankle.
Following the disturbance, the flight crew immediately illuminated the seat belt sign and initiated a descent to FL260 to avoid the turbulent air. The aircraft landed safely at Tokyo International Airport later that afternoon.
The investigation
The Japan Transport Safety Board (JTSB) examined flight data recorder (FDR) information, weather analysis charts, and crew statements. The investigation focused on the meteorological conditions, specifically the presence of a jet stream and vertical wind shear (VWS) in the area. Investigators also reviewed the pre-flight briefing procedures and the communication between the flight deck and cabin crew regarding weather precautions.
Findings
- The aircraft encountered clear air turbulence (CAT) while passing through the edge of a jet stream. This type of turbulence is notably difficult to detect as it does not produce echoes on airborne weather radar.
- The vertical wind shear in the flight path was significantly stronger than what had been predicted during the pre-flight briefing.
- While the flight crew had warned the cabin crew of potential shaking during the initial briefing, they did not provide a specific updated warning as the aircraft approached the area of predicted wind shear.
Safety action
Following the incident, the operator implemented several safety measures, including:
- Instructing all crew members to ensure that any manual changes to flight altitude for weather avoidance are explicitly noted in briefing memos.
- Enhancing cabin crew training to emphasize preparedness for sudden turbulence and the importance of adapting service plans based on real-time flight conditions.
- Updating guidelines to allow cabin attendants to formally request the illumination of the seat belt sign when they perceive changing conditions.