On April 11, 2011, at 6:58:38 coordinated universal time (UTC), American Airlines Flight 170, a Boeing 777-223, N766AN, experienced severe turbulence during climb over international waters about 194 nautical miles east of Narita, Japan. Four flight attendants were injured; two flight attendants sustained minor injuries, while two flight attendants sustained serious injuries. There were no injuries to the other 227 passengers and crew, and the airplane was not damaged. The scheduled international passenger flight originated from Narita International Airport, Narita Japan and was en route to Los Angeles International Airport, Los Angeles, California under the provisions of Title 14 Code of Federal Regulations Part 121. The flight crew reported that as the airplane was climbing, they observed a small weather build up in front of them. They requested a deviation around the weather, but ATC informed them to standby. The first officer reported that when the flight crew received the clearance to turn the airplane was already in the weather. At about 24,000 feet, the airplane encountered moderate then severe turbulence for about 11 seconds. Although the seatbelt signs were still illuminated from takeoff, the flight attendants were working in the cabin at the time of the occurrence. During the turbulence encounter, both the number 2 and number 3 flight attendants received fractured ankles. The number 2 flight attendant was working in the aft galley when she was thrown to the floor of the airplane, landing hard on her right hand and the outside of her right ankle. The number 3 flight attendant was washing her hands in the 4R lavatory when she could not maintain balance and fell, striking her right foot. The Digital Flight Data Recorder (DFDR) was sent to the National Transportation Safety Board (NTSB) recorders division for data analysis. According to the DFDR data, the event occurred at 6:58:38; at this time, the airplane was at a pressure altitude of about 24,000 feet, with a speed of 336 KIAS. During the event, the vertical acceleration went from 2.2G’s to 0.6913G’s within 1 second, to a minimum G of 0.496 about 5 seconds after the 2.2G maximum. A weather study conducted by a NTSB meteorologist revealed a frontal wave over the Japanese coast with a warm front extending eastward in the vicinity of the turbulence encountered; developing gale force winds were expected. An upper level chart for 18,000 feet depicted west-southwesterly winds of 40-45 knots in the region. The region 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. At 0732, an airplane along a similar route reported wind at 24,000 feet from 245 degrees at 65 knots. Review of the weather package given to the flight crew by American Airlines dispatch found no forecasts for turbulence. No SIGMETS were in effect for the route of flight. At the time of the occurrence, there were no reports of turbulence from airplanes ahead of the accident flight, and the flight crew saw no indications on their weather radar of any returns.