Investigation Summary
The flight, operating as United Airlines Flight 826, departed Narita with 374 passengers and 19 crew members. The captain had selected a specific route (Track 12) to avoid known areas of turbulence, but approximately two hours into the flight, the aircraft encountered 'wave action' followed by two severe turbulence events. Although the seat belt sign was illuminated at the time of the encounter, many passengers were not buckled. A Japanese-speaking flight attendant, who was responsible for translating the captain's announcements, admitted she did not translate the full warning about potential turbulence because she did not want to alarm the passengers. Post-accident testing of the passenger address system revealed a failed photoelectric cell in the amplifier which may have prevented the seat belt chime from sounding, though investigators could not confirm if this failure occurred before or after the accident. The aircraft sustained no structural damage but the interior cabin was heavily damaged by the impact of unrestrained occupants and objects.
Final Conclusions
The National Transportation Safety Board determined the probable cause was the pilot-in-command's inadvertent flight into adverse weather conditions and the inherent difficulty of obtaining adequate weather forecasts for over-ocean turbulence. Contributing factors included the presence of clear air turbulence and the failure of a flight attendant to issue a complete safety advisory in the passengers' native language.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
First Officer
Second Officer
Systems & Failure Modes
Passenger Address (PA) Amplifier
A teardown of the Collins PA Amplifier revealed a failed photoelectric cell which prevented the low chime from operating when the seat belt sign was toggled.
Inertial Navigation System (INS)
Warning lights illuminated on INS units 1 and 3 during the turbulence, but the units functioned normally after error codes were cleared.
Weather Radar
The crew utilized the radar's turbulence detection mode (Doppler), but no Doppler returns or red echoes were observed along the flight path prior to the encounter.
Interesting Facts
- 01The aircraft was a Boeing 747-122 with 374 passengers and 19 crew members on board.
- 02One passenger, seated in 46F with an unfastened seat belt, suffered a fatal cervical spinal injury after hitting the ceiling.
- 03In addition to the fatality, 18 people sustained serious injuries and 161 to 171 others sustained minor injuries.
- 04None of the passengers who sustained fatal or serious injuries were wearing seat belts at the time of the encounter.
- 05The NTSB officially recorded the aircraft damage as 'Minor' as there was no external or internal structural damage.
- 06Weather forecasts and SIGMETs for the area did not indicate the presence of severe turbulence at the time of the accident.
- 07The aircraft was declared an economic total loss and written off because it was scheduled for retirement in early 1998 and the cost of cabin repairs exceeded its remaining value.
Safety Actions & Advisories
United Airlines Safety Bulletin 98-1
Issued to reiterate the importance of effective communication between pilots and flight attendants and to reinforce the policy of encouraging passengers to wear seat belts at all times when seated.