NTSB Identification: DCA10FA064
Scheduled 14 CFR Part 121: Air Carrier operation of United Airlines, Inc.
Accident occurred Tuesday, May 25, 2010 in Southern Tip of Greenland, Denmark
Probable Cause Approval Date: 06/28/2011
Aircraft: BOEING 777, registration: N794UA
Injuries: 2 Serious,209 Uninjured.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

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.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

An inadvertent encounter with unforecast turbulence in cruise flight.

Full narrative available

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