What happened
On December 16, 2014, an American Airlines, Inc. Boeing 777-200, registered N751AN, was operating a scheduled flight from Incheon International Airport to Dallas Fort Worth International Airport. While cruising at approximately 27,000 feet over Northern Kanto, Japan, the aircraft encountered severe turbulence.
Prior to the event, the flight crew noticed the thrust levers moving backward, a sign of impending instability, and activated the seatbelt signs. However, no passenger address announcement was made to instruct passengers to remain seated. During the period when the seatbelt signs were illuminated, a passenger moved toward the rear of the cabin to use a lavatory.
At approximately 1-35 UTC, the aircraft entered a period of intense shaking. Despite the pilot attempting to change altitude to avoid the disturbance, a second major bout of turbulence occurred. The vertical accelerations reached between +1.8G and -0.88G. During these oscillations, a passenger in the lavatory and a cabin attendant in the aisle were struck against the ceiling, resulting in one passenger and one cabin attendant seriously injured, along with several minor injuries to other passengers and crew.
Following the turbulence, the crew assessed the cabin for damage and consulted their headquarters regarding potential airframe integrity issues. Due to the injuries and concerns regarding the aircraft's condition, the flight destination was changed, and the aircraft landed at Narita International Airport.
The investigation
The Japan Transport Safety Board (JTSB) examined the flight data recorder, cockpit voice recorder, and weather information systems. The investigation focused on the flight planning process, the accuracy of the weather forecasts provided to the crew, and the communication of turbulence warnings between the dispatcher and the pilot. Investigators also reviewed ground-based weather radar data and the Japan Meteorological Agency (JMA) forecasts to determine if the clear air turbulence (CAT) was predictable.
Findings
- The primary cause of the intense shaking was the aircraft's unexpected entry into an airspace containing clear air turbulence (CAT).
- The crew and dispatcher failed to predict the occurrence of the CAT because the company's specific method for utilizing weather information prevented the identification of critical warnings.
- While the dispatcher used a private weather service, the specific severe turbulence warnings issued by the JMA were not monitored or utilized by the company's operational control.
- The pilot did not grasp the significance of the JMA's Forecast of Significant Weather (FBJP), which indicated severe turbulence in the flight path.
- The cabin attendants were unable to prevent the passenger from moving through the cabin due to the demands of in-flight service at the time the seatbelt signs were first illuminated.