NTSB Identification: DCA11FA091
Scheduled 14 CFR Part 121: Air Carrier operation of United Airlines
Accident occurred Friday, July 29, 2011 in Tangshan, China
Probable Cause Approval Date: 01/15/2013
Aircraft: BOEING 747-422, registration: N173UA
Injuries: 1 Serious,4 Minor,380 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 crew reported a normal takeoff and climbout with a small amount of clouds and rain in the area. The preflight weather briefing package included a forecast of convective activity including a SIGMET (significant meteorological information) for thunderstorms south of the Beijing area moving northeastward. The crew was aware of the potential for convective activity during the climbout from Beijing, and the initial enroute segment over the Bohai Sea. During flight, the Captain was actively using the airborne weather radar to search for convective returns, including using the Doppler functions. Satellite imagery showed a signature termed “transverse banding” which is indicative of thunderstorms and turbulence at altitude. Based on the lack of weather radar returns, the Captain kept the cabin seat belt sign illuminated, but advised the cabin crew that they could begin service. The airplane encountered convectively induced turbulence, resulting in a peak vertical acceleration of over 2 G’s. The activity was likely developing upwards into the flight path, and precipitation attenuation and/or radar over-scanning may have contributed to the lack of visibility of the condition, which led to the risk assessment to allow the flight attendants to begin service.

Following the turbulence encounter, the flight crew controlled the airplane appropriately to avoid an overspeed or other hazardous condition. The flight and cabin crew consulted with a physician passenger and determined continuing the flight per the injured flight attendant’s desire, was appropriate.

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

an inadvertent encounter with convectively induced turbulence likely due to radar limitations and convective weather developing upwards into the flight path.

Full narrative available

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