NTSB Identification: WPR11LA203
Scheduled 14 CFR Part 121: Air Carrier operation of AMERICAN AIRLINES INC
Accident occurred Monday, April 11, 2011 in
Probable Cause Approval Date: 11/29/2011
Aircraft: BOEING 777-223, registration: N766AN
Injuries: 2 Serious,2 Minor,227 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

As the airplane was climbing, the flight crew observed a small weather buildup in front of them. They requested a deviation around the weather, but the air traffic controller requested that they stand by. After entering the weather, they received clearance to turn; however, about 24,000 feet, the airplane encountered about 11 seconds of moderate, and then severe, turbulence. During the encounter, two flight attendants received broken ankles while conducting work in the aft cabin area. The digital flight data recorder revealed that during the encounter vertical acceleration forces went from 2.2G to 0.6913G within 1 second, to a minimum of 0.496G about 5 seconds after the 2.2G maximum.

A weather study conducted by a National Transportation Safety Board meteorologist revealed a frontal wave over the Japanese coast with a warm front extending eastward in the vicinity of the turbulence encounter. A gale force wind was expected. The region of the turbulence encounter also contained low to mid-level clouds with cloud tops near 14,000 feet; cumulonimbus cloud development, with tops up to 26,000 feet, was slightly upstream from the turbulence encounter. Review of the weather package given to the flight crew found no forecasts for turbulence and no Significant Meteorological Information advisories (SIGMETS) were in effect for the route of flight. At the time of the occurrence, there were no reports of turbulence from pilots ahead of the accident flight and the flight crew saw no indication of turbulence on their weather radar.

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

An inadvertent encounter with convective-induced turbulence upon entering cumulonimbus clouds.

Full narrative available

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