Date: 201409
Local time of day: 0601-1200
Location: ZZZZ.ARTCC, FO
Aircraft: B777-200
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Descent
ATC / advisory: Center ZZZZ
Aircraft component: Pilot Seat
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Relief Pilot; Pilot Not Flying; First Officer
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; General Physical Injury / Incapacitation
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: During our initial descent (through approx 30;000 feet); the aircraft experienced a turbulent jolt resulting in numerous Flight Attendant and passenger injuries. The seatbelt sign was on as we began our descent out of FL380. We were descending to FL160 when I let the First Officer back into the cockpit. As the First Officer moved to sit in the seat; the seat began to prematurely move forward; trapping him just below the knees. The First Officer's legs were trapped between the seat and the pedestal as the seat continued to move forward electrically. He appeared to be in a great deal of pain. Just then; the aircraft dropped and the autopilot disconnected. The Captain flew manually; and I moved to free the First Officer with the electrical seat switch. I called back and discovered a number of flight attendants (3) and passengers had received injuries from the turbulent drop. I was able to stay on my feet and thought we had experienced severe turbulence (something I have never experienced before). We declared an emergency with ATC and were cleared directly to the field. Approach and landing were uneventful. We requested 7 ambulances to address injuries to flight attendants and passengers.


Second narrative: We had been cleared for a descent from FL380 to FL200. The aircraft was on autopilot being operated by myself. At about FL300 the Relief Pilot got up out of his seat to swap with the First Officer returning from his break. As the First Officer entered his seat I heard a cry of pain. I looked over at him and his seat was electronically moving forward pinning his legs between the left inboard of the seat and the center console. The seat pushed him forward into the yoke and this resulted in an autopilot disconnect with a pitch over (down). I manually took over and applied pressure to stop this as gently as I could. The Relief Pilot got to the seat control from behind and ran the seat to the aft outer position releasing the First Officer. Calls to the flight attendants confirmed there were injuries. A medical emergency was declared and we continued to destination while coordinating to have medical personnel meet the aircraft.


Synopsis: B777 flight crew described an inadvertent activation of the First Officer's electric seat control by the seat belt buckle; while he is attempting to get into the seat. His legs were pinned between the pedestal and the seat and he fell onto the yoke; causing an abrupt pitch over injuring passengers and flight attendants.