Incident Summary
A flight attendant sustained serious injuries when the aircraft encountered turbulence during a descent from flight level (FL) 350 to FL 340. The flight crew stated in interviews that they were preparing for turbulence and wanted passengers and flight attendants seated with seatbelts fastened.
Flight Crew Actions
According to the investigation, the flight crew did not advise the lead flight attendant (LFA) of the turbulence level or its anticipated duration, nor did they use turbulence codes as required by Northwest Airlines' Flight Operations Manual. Additionally, the LFA did not relay turbulence information to all flight attendants, as stipulated in the company's Flight Attendant Manual.
Restraint System Examination
Examination of the restraint system from the injured flight attendant's jump seat revealed that the system did not fail during the turbulence upset. Investigators were unable to determine whether the lapbelt was improperly secured by the flight attendant or inadvertently disengaged during the event.
Flight Data Analysis
Analysis of flight data recorder (FDR) data indicated that the turbulence caused substantial vertical and lateral oscillations, which may have made it difficult for the first officer, who was the flying pilot, to maintain a constant pitch attitude. The FDR showed variations in pitch attitude during the turbulence encounter. Flight crew statements indicated they were attempting to maintain constant pitch attitudes, but FDR data showed that large and rapid pitch excursions were primarily the result of control column inputs made by the first officer.
Determination
The Safety Board's investigation determined that the main pitchover, which likely caused the flight attendant's injuries, occurred as a result of the first officer's control inputs.