Incident Summary
While en route, the captain left the cockpit for a rest period, and a reserve first officer occupied the captain's seat. A flight attendant entered the cockpit to deliver a box of beverages for the flight crew. Directed by the reserve first officer, she attempted to place the beverages on the footrest of the center observer's seat but encountered difficulty due to the position of the first officer's seat.
Sequence of Events
Recognizing the obstruction, the reserve first officer reached across the cockpit and, without the first officer's knowledge, activated the horizontal movement switch for the first officer's seat, moving it forward. At that moment, the first officer had their legs crossed, which then contacted the control column, pushing it forward. The forward input exceeded the limitations of the autopilot system, causing the autopilot to disengage.
Result
With the autopilot off, the aircraft responded to the forward control column input and pitched down. This motion resulted in injuries to passengers and crew. The first officer took manual control of the aircraft after the autopilot disconnected and returned it to level flight.