On December 25, 2007, about 0554 Hawaii standard time, an Airbus A330-323, N819NW, encountered severe turbulence during cruise flight approximately 1,300 nautical miles west of Honolulu, Hawaii. The airplane, which was being operated by Northwest Airlines as flight 16, was not damaged. One flight attendant sustained serious injuries and another flight attendant and two passengers received minor injuries. The other 7 flight attendants, 281 passengers, and 3 flight crewmembers were not injured. Night visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the regularly scheduled international passenger flight. The flight departed from Osaka, Japan, at 0141, and the intended destination was Honolulu, Hawaii. Following the turbulence encounter, the flight continued to Honolulu and landed without further incident at 0851.

According to information provided by Northwest Airlines, the flight was in cruise at 38,000 feet in the vicinity of 34 degrees North latitude and 180 degrees East longitude when it encountered severe turbulence. The turbulence caused the autopilot and autothrust to disconnect. The flight lost approximately 1,000 feet of altitude during the turbulence encounter. After exiting the turbulence, the flight crew contacted the flight attendants to check on the status of the passengers and cabin crew. At this time, they learned that two flight attendants and one passenger had been injured. The two flight attendants had been standing in the aft galley and were knocked to the floor during the turbulence encounter.

The remainder of the flight was uneventful. After landing and during deplaning, an additional passenger reported injury. The four injured persons were taken to the hospital. The two passengers and one of the flight attendants were treated for minor injuries and released. The other flight attendant was found to have two fractures in her neck.

The flight crew consisted of the captain and two A330 type-rated first officers. In written statements the flight crewmembers indicated that they had reviewed the preflight weather document, plotted the applicable Turbulence Plot (TP) advisories prior to departure, and obtained a briefing from dispatch. After departure the flight was cleared to 36,000 feet due to traffic at 35,000 feet, the filed altitude. According to the flight crew the flight had encountered occasional light to moderate turbulence for a good period of the flight. Later in the flight after experiencing constant light to moderate turbulence, they requested 38,000 feet and were cleared to climb to 37,000 feet, where they indicated the turbulence generally subsided. The seat belt sign had been on a good portion of the flight up to this point because of the turbulence.

The crew changed at approximately 0419, and the captain left the flight deck for his scheduled break. One first officer then occupied the left seat as the pilot monitoring and the other first officer took the right seat as the pilot flying.

At approximately 0519, the flight approached an area of scattered thunderstorms. The pilots requested and received permission to deviate approximately 30 nautical miles (nm) right of course around the area. The seat belt sign was turned on. At 0520 the flight requested higher altitude and was cleared to 38,000 feet. The pilots indicated that at this level conditions smoothed out and they were on top of a thin layer of cirrus clouds. At 0535 they reported back on course and then requested and received permission to deviate 40 nm left of course around another area of scattered thunderstorms. The pilot monitoring called the lead flight attendant (LFA) on the interphone and advised her to make sure all the flight attendants took their seats and remained seated until further notice as the flight deviated around some areas of weather and that it should not last much longer than 15 minutes.

The pilots indicated that night visual meteorological conditions, with unrestricted visibility, with a nearly full moon existed so they turned the cockpit lights off to better see with the moonlit sky. The pilot monitoring also indicated that they were using the weather radar extensively in both auto and manual modes, tilting the radar antenna down to various degrees of tilt. They observed only light intensity echoes (green), which were not organized and poorly defined, and no returns were depicted at their flight level. Both pilots indicated that the only lightning activity they observed was far off in the distance, south of their route when they first approached the area of echoes.

At 0553 the pilots reported back on course and believed they were clear of any weather and were discussing allowing the flight attendants to resume their duties when they encountered a brief area of moderate to severe turbulence without warning. Both pilots indicated that they were clear of any clouds and were well clear of any echoes when the turbulence occurred. Both described losing approximately 1,000 feet in altitude during the turbulence encounter. After the flying pilot regained control of the aircraft and recovered the lost altitude, the monitoring pilot checked with the LFA to determine if anyone was injured and was advised of some injuries. He then made a pilot report to warn other aircraft of the turbulence. A Korean Airlines B-777 behind their flight at 39,000 feet at this time had also encountered severe turbulence.

In a written statement, the LFA reported that it had been "bumpy" most of the flight. When she received the call from the flight deck, it was smooth, flight attendant breaks had just started, and the seat belt sign was on. The instructions were to be seated for the next 15 minutes. She walked to both galleys and told the flight attendants to be seated for the next 15 minutes. About 12 to 13 minutes later, the flight encountered severe turbulence.

The two flight attendants who were injured were both located in the aft galley. In a written statement, the flight attendant who sustained serious injuries reported that the LFA "came and informed us that it was going to be bumpy in about 15 minutes or so to sit and buckle up." She further reported that she completed cleaning up the galley and then began to prepare her crew meal. When the turbulence occurred, she was "still standing trying to get ready for [her] meal."

In a written statement, the flight attendant who sustained minor injuries reported that the LFA "did walk back to us to tell us that we will be having some turbulence in fifteen minutes." She further reported that she and the other flight attendant in the aft galley "did not sit down and buckle in once the information was provided by the flight deck crew because the turbulence was supposed to occur in fifteen minutes."

A Safety Board meteorologist gathered and reviewed weather data from government sources and the operator. Satellite imagery depicted the accident site within an area of cumulonimbus clouds. The operator's meteorology department issued multiple TP messages on the thunderstorm activity in the vicinity of the accident site. For detailed weather information, see the Meteorology Factual Report in the public docket for this accident.

The airplane was equipped with a flight data recorder (FDR), which was sent to the Safety Board's laboratory in Washington, DC, for readout. The FDR data indicates that the airplane experienced a vertical acceleration of -0.4 G to +1.8 G, or 2.2 G total change. The National Weather Service uses the following changes in vertical acceleration to quantify the intensity of turbulence: Less than 0.5 G, light turbulence; 0.5 to 1.0 G, moderate turbulence; 1.0 to 2.0 G, severe turbulence; greater than 2.0 G, extreme turbulence. For further details of the data recorded by the FDR, see the public docket for this accident.

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