Before Hawaiian Airlines flight 35 departed, the captain called the flight dispatcher to discuss the flight release, at which time the dispatcher advised the captain of potential turbulence and potential embedded convective activity over the Hawaiian islands. While flying over the Hawaiian islands at flight level (FL) 400, the airplane, an Airbus A330, was in visual meteorological conditions and above a cloud layer between FL370 and FL380. A cloud that the flight crew described as a “plume” appeared vertically in front of the airplane. The flight crew called the lead flight attendant to advise her about the anticipated turbulence. Within about 1 to 3 seconds, the airplane encountered severe turbulence that resulted in multiple injuries in the cabin. Data from the airplane’s quick access recorder showed that the airplane experienced vertical accelerations greater than 2 Gs during the turbulence encounter. After the encounter, the airplane continued to the destination airport and landed there without further incident.The National Weather Service (NWS) forecast products for the time and route of the accident flight indicated that a storm was approaching the Hawaiian islands that was expected to support an unstable atmosphere with abundant moisture and isolated thunderstorms. As a result of these conditions, the NWS issued a SIGMET (significant meteorological information) that was valid for an area of embedded thunderstorms to FL380 that included the turbulence encounter location. This SIGMET, which implied the potential for severe or greater turbulence, was provided to the flight crew. The NWS had also forecast moderate-to-severe clear air turbulence that was expected to develop behind the main area of precipitation. Thus, the conditions along the airplane’s flight route were favorable for convectively induced severe turbulence, and the flight crew was aware that thunderstorms and turbulence could occur along the flight route.Page 2 of 16 DCA23LA096The flight crew noticed the plume-like cloud shortly before the airplane was to begin its descent into Honolulu. The captain then changed the weather radar display mode to determine the intensity and elevation of the weather formation directly in front of the airplane, but he and the first officer saw “nothing” displayed; weather radar has reduced sensitivity at higher altitudes. In addition, the Hawaiian Airlines A330 Flight Crew Operating Manual, Quick Reference Handbook, stated that weather radar does not detect weather that “has small droplets,” such as clouds.According to the cockpit voice recorder (CVR) transcript, the flight crew stated that the plume was “moving fast” and “building fast” but that the airplane should “clear most of it.” The meteorological evidence for this investigation showed that the plume that the crew observed outside the airplane comprised cumulonimbus clouds with severe-to-extreme convectively induced turbulence. The operator’s A330 flight crew operating manual/quick reference handbook also stated that “the flight crew should plan to fly above or around areas of severe turbulence.” However, the crewmembers did not anticipate the likely severity of the rising plume, even though they had been provided with sufficient meteorological information (including information from the dispatcher and the air traffic controller) to make such a determination. The CVR recorded the crewmembers stating, after the turbulence encounter, that they should have “gone around,” indicating that sufficient time should have been available to deviate away from the plume. The operator’s flight crew operating manual recommended that flight crews “consider storm cells above 35,000 ft as highly hazardous” and that a flight crew should not overfly a storm cell with its top at or above 25,000 ft because the airplane might encounter “stronger than expected” turbulence. Thus, the flight crew’s decision to overfly the active storm cell was a factor in the severe turbulence encounter. Once the flight crew notified the lead flight attendant about the expected turbulence encounter, the flight attendant attempted to convey this information to the other flight attendants via interphone. However, the turbulence occurred before she could complete the call. One flight attendant received serious injuries, and three flight attendants received minor injuries due to the turbulence encounter. The seatbelt sign had been turned on before the turbulence event, but the captain did not make an announcement over the public address system about the anticipated turbulence. The lack of a verbal warning directly from the flight deck might have contributed to some of the flight attendant and passenger injuries.When the turbulence encounter ended, the cabin crewmembers who were able and medically trained passengers assisted those with injuries. Cabin crewmembers and an off-duty company pilot secured loose interior panels for landing. The flight attendants made a list of passengers with serious injuries, and the lead flight attendant informed the flight crew that more than a dozen passengers were injured and that the airplane would need to be met by medical personnel upon landing. The flight attendants turned on the call lights in the cabin for those passenger seats so that the passengers could be readily identified for attention by emergency Page 3 of 16 DCA23LA096services upon landing. Thus, the in-flight response to the turbulence encounter was timely and effective.