On June 29, 1994, about 0023 eastern daylight time, a McDonnell-Douglas MD-11, N1752K, registered to American Airlines, Inc., operating as American Airlines flight 901, experienced altitude deviation in cruise flight over the Caribbean Sea just south of the Cuban coast, resulting in injuries to passengers and crew. The flight was a 14 CFR Part 121 scheduled international passenger flight from Miami, Florida, to Buenos Aires, Argentina. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The airplane received minor damage. One passenger and 1 flight attendant received serious injuries, 12 passengers and 3 flight attendants received minor injuries, and 67 passengers and 10 flight attendants were not injured. The airline transport-rated captain, first officer, and reserve first officer were not injured. The flight originated at Miami, Florida, on June 28, 1994, at 2333. While en route the captain went to the main cabin for a rest period and the reserve first officer took the captain’s seat. A flight attendant entered the cockpit to bring a box of beverages for the flight crew. She attempted to place them on the footrest of the center observer’s seat as directed by the reserve first officer. She had trouble doing this and the reserve first officer realized the first officer’s seat was in the way. He 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. The first officer’s legs were crossed and contacted the control column pushing it forward. The limitations of the autopilot system were exceeded and the autopilot turned off. The aircraft responded to the forward control column input and pitched down. This resulted in injuries to passengers and crew. The first officer took manual control of the aircraft after autopilot disconnect and returned the aircraft to level flight. Probable Cause: The reserve first officer’s unauthorized movement of the first officer’s seat during flight which resulted in the first officer’s legs being forced against the control column, pushing it forward, and disconnecting the autopilot. This resulted in the aircraft pitching down and subjecting the occupants to negative g forces, resulting in injuries.