Incident Overview
On April 11, 2011, at 6:58:38 coordinated universal time (UTC), American Airlines Flight 170, a Boeing 777-223 (registration N766AN), experienced severe turbulence during climb over international waters approximately 194 nautical miles east of Narita, Japan. The aircraft was not damaged. Four flight attendants were injured: two sustained serious injuries and two sustained minor injuries. The other 227 passengers and crew were not injured.
The scheduled international passenger flight originated from Narita International Airport, Japan, and was en route to Los Angeles International Airport, California, operating under Title 14 CFR Part 121.
Sequence of Events
According to the flight crew, as the airplane was climbing, they observed a small weather buildup ahead. They requested a deviation around the weather, but air traffic control instructed them to standby. The first officer reported that when clearance to turn was received, the airplane was already in the weather. At approximately 24,000 feet, the airplane encountered moderate then severe turbulence for about 11 seconds. Although the seatbelt signs were illuminated from takeoff, the flight attendants were working in the cabin at the time.
The number 2 flight attendant was working in the aft galley when she was thrown to the floor, 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 lost balance and fell, striking her right foot. Both sustained fractured ankles.
Flight Data Recorder Analysis
The Digital Flight Data Recorder (DFDR) was sent to the National Transportation Safety Board (NTSB) for analysis. Data indicated that at the time of the event (6:58:38), the airplane was at a pressure altitude of about 24,000 feet with a speed of 336 knots indicated airspeed (KIAS). Vertical acceleration changed from 2.2 G to 0.6913 G within one second, reaching a minimum of 0.496 G approximately five seconds after the maximum.
Weather Conditions
A weather study by an NTSB meteorologist revealed a frontal wave over the Japanese coast with a warm front extending eastward near the turbulence location; developing gale-force winds were expected. An upper-level chart for 18,000 feet showed west-southwesterly winds of 40-45 knots in the region. Low to mid-level clouds with tops near 14,000 feet were present, and cumulonimbus cloud development with tops up to 26,000 feet occurred slightly upstream from the encounter. At 0732, an aircraft on a similar route reported winds at 24,000 feet from 245 degrees at 65 knots.
The weather package provided to the flight crew by American Airlines dispatch contained no turbulence forecasts. No SIGMETs were in effect for the route. At the time, there were no reports of turbulence from preceding aircraft, and the flight crew observed no returns on their weather radar.