Incident
While cruising at flight level 310 (31,000 feet mean sea level), an aircraft experienced severe turbulence, leading to a serious injury to a flight attendant. The encounter occurred near 24 degrees 18 minutes south latitude and 174 degrees 20 minutes east longitude. The flight attendant was securing a duty free cart when struck by it during the turbulence.
Flight Conditions
Prior to the event, the captain reported a smooth ride over a flat broken cloud layer. As the aircraft passed 25 degrees south, cloud tops gradually rose. Weather radar showed minimal returns, with a few green returns to the right. The area had no forecast turbulence or cumulonimbus buildups. The captain activated the seatbelt sign and made a passenger announcement.
Digital Flight Data Recorder (DFDR) data indicated a series of oscillating vertical and lateral accelerations lasting nearly two minutes. Vertical accelerations ranged from +0.31g to +1.7g, while lateral accelerations varied from 0.119g left to 0.115g right. During the encounter, the Flight Management Computer reported wind changes: from 92 knots to 50 knots, then back to 101 knots. Indicated airspeed, averaging 315 knots before, peaked at 344 knots before returning to 315 knots.
Weather Information
The dispatch and flight release package included forecast winds at 31,000–35,000 feet near 173 degrees east, 25 degrees south: from 290 degrees at 102–113 knots. The package noted possible turbulence between longitudes 164°E and 172°E along the planned track. A National Weather Service Significant Weather Forecast Chart showed a 120-knot jet stream, occasional moderate or lesser clear air turbulence south of the accident location, and isolated cumulonimbus clouds with tops to 40,000 feet to the north. The only SIGMETs for turbulence covered an area about 900 miles west and south of the accident location.