What happened
On December 28, 1997, United Airlines Flight 826 was en route from New Tokyo International Airport (Narita) in Japan to Honolulu International Airport in Hawaii. The flight was being operated by a Boeing 747-100 aircraft. Approximately two hours into the journey, while cruising at an altitude of 31,000 feet (9,400 m), the crew received reports indicating severe clear-air turbulence ahead. In response, the seat belt sign was illuminated.
Shortly after the warning, the aircraft experienced a sudden and significant drop of approximately 100 feet (30 m). This abrupt movement resulted in serious injuries to 15 passengers and three crew members. Following the incident, the decision was made to divert the flight back to its point of origin. The Boeing 747-100 landed safely at Narita Airport. Tragically, one passenger, a 32-year-old Japanese woman, succumbed to her injuries.
Investigation
The available source material does not provide specific details regarding the formal investigation process or official findings beyond the immediate sequence of events and the resulting casualties.