What happened
On July 1, 2017, a United Airlines, Inc. Boeing 787-9, registered N29968, was operating as flight 875 from San Francisco International Airport to Tokyo International Airport. The aircraft was carrying 246 passengers and 15 crew members. During the descent phase of the flight, at an altitude of approximately 15,600 ft, the aircraft encountered a sudden jolt. The incident occurred roughly 64 km southwest of Fukushima Airport.
At the time of the jolt, the aircraft's vertical acceleration fluctuated significantly, reaching a peak of +1.68 G. This sudden movement caused a flight attendant working in the aft galley to lose her balance, resulting in her knee striking the floor. The impact caused a serious injury to the crew member. Following the event, the flight crew utilized speed brakes to descend to 14,000 ft to exit the cloud layers. The aircraft subsequently landed safely at Tokyo International Airport.
The investigation
The Japan Transport Safety Board (JTSB) examined several factors to determine the cause of the injury. Investigators reviewed the flight history, including statements from the flight crew and the injured flight attendant. They also analyzed Continuous Parameter Logging (CPL) data, which recorded the aircraft's vertical acceleration and outside air temperature.
Meteorological data was scrutinized, including the Japan Meteorological Agency's analysis charts and onboard weather radar records. The investigation also looked into the atmospheric stability at the time of the event, specifically comparing the temperature lapse rate to the dry adiabatic lapse rate. While the aircraft was equipped with an Enhanced Airborne Flight Recorder, the data from the specific moment of the accident had been overwritten due to the continued operation of the aircraft following the event.