N794UA
BOEING 777
The flight encountered clear air turbulence near the southern tip of Greenland. A review of the weather briefing provided to the accident flight crew by company Flight Dispatch contained relevant SIGMETs along the proposed route of flight. The package was not found to be missing any pertinent published information that could have alerted the flight crew to an enroute encounter with turbulence over Greenland. Analysis of the calculated winds indicated the airplane encountered a downdraft on the order of 13 feet/sec followed sharply by an updraft of approximately 24 feet/sec. Simulation analysis of the upset confirmed that the airplane reacted as expected to the calculated wind profile and the recorded control inputs. Based on the simulation study that was performed, the recorded elevator input from the autopilot during the disturbance reduced the magnitude of the upset. Analysis of the surrounding atmosphere and GOES-3 satellite photographs indicated the presence of icecap wave instability along the airplane's route of flight that was capable of producing rapidly changing vertical winds on the order of 20 feet per second, which resulted in conditions conducive to severe turbulence.
HISTORY OF FLIGHT On May 25, 2010 at 1310 UTC, a Boeing 777-200B, registered in the United States as N794UA, encountered icecap wave turbulence near the southern tip of Greenland. One flight attendant and one passenger were seriously injured. None of the 3 flight crewmembers, other 11 cabin crewmembers or other 195 passengers were injured. The flight was registered to and operated by United Airlines under the provisions of 14 Code of Federal Regulations Part 121 as a passenger-carrying flight. Visual flight rules (VFR) conditions prevailed for the flight, which operated on an instrument flight rules (IFR) flight plan. The flight originated from Heathrow International Airport (LHR), London, England and its destination was to be Los Angeles International Airport (LAX),Los Angeles, California. Following the encounter, a medical emergency was declared and the flight diverted to Pierre Elliott Trudeau International Airport (YUL), Montreal, Quebec, Canada, and landed uneventfully. According to the captain, who was the nonflying pilot at the time, the seat belt sign had been off for approximately 30 minutes prior to the turbulence encounter, which he described as "unexpected and extremely violent". The airspeed quickly increased into the overspeed range and the first officer attempted to control the airspeed by retarding the throttles. The airspeed decayed rapidly then increased immediately back into the overspeed range with a maximum speed of about .875 mach. He stated that the altitude deviations appeared to be plus or minus 80 to 100 feet and the autopilot remained engaged during the entire encounter. INJURIES One flight attendant suffered a fractured leg and one passenger suffered a fractured ankle. DAMAGE TO AIRPLANE The airplane suffered no damage during the event. METEOROLOGICAL INFORMATION A review of the weather briefing provided to the accident flight crew by company Flight Dispatch contained relevant SIGMETs along the proposed route of flight. The package was not found to be missing any pertinent published information that could have alerted the flight crew to an enroute severe turbulence encounter over Greenland. Analysis of the calculated winds indicated that the airplane encountered a downdraft on the order of 13 feet/sec followed sharply by an updraft of approximately 24 feet/sec. Analysis of the surrounding atmosphere indicated the presence of an icecap wave instability that was capable of producing rapidly changing vertical winds on the order of 20 feet per second, which could result in conditions conducive to severe turbulence. FLIGHT RECORDERS The FDR data show a significant decrease in normal load factor (minimum of 0.33 g’s) immediately followed by a significant increase (maximum of 2.08 g’s). The disturbance spanned approximately 2 seconds while the airplane was cruising with the autopilot engaged at approximately FL330. TESTS AND RESEARCH Simulation analysis of the upset confirmed the airplane reacted as expected to the calculated wind profile and the recorded control inputs. Based on the simulation study that was performed, the recorded elevator input from the autopilot during the disturbance reduced the magnitude of the upset.
An inadvertent encounter with unforecast turbulence in cruise flight.
Source: NTSB Aviation Accident Database
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