Incident Description
A McDonnell Douglas MD-11 was descending from flight level (FL) 350 to FL240 in smooth air under visual meteorological conditions (VMC) with the autopilot (A/P) engaged. As the aircraft approached FL250, the captain became concerned that the airplane would not level off at the assigned altitude. He instructed the first officer (F/O) to reduce the rate of descent.
Sequence of Events
The F/O attempted to slow the descent by using the pitch thumbwheel on the autopilot control panel. Unsatisfied with the response, the captain took control of the aircraft and attempted to overpower the autopilot by pulling back on the control yoke. While maintaining back pressure on the yoke, he disengaged the autopilot. With the reduced control column resistance after disengagement, the elevators further deflected upward, subjecting the aircraft to a positive load factor of +2.28 G, as recorded by the flight data recorder.
Injuries
A passenger in the aft lavatory suffered a fractured ankle. One other passenger and two flight attendants received minor injuries.
Aircraft Systems and Procedures
The MD-11 Flight Crew Operating Manual (FCOM) advised against attempting to overpower the autopilot; however, this guidance was contained in a section titled 'SEVERE TURBULENCE AND/OR HEAVY RAIN INGESTION.' Each use of the pitch thumbwheel interrupted the automatic level-off function, and the system required a two-second pause after thumbwheel release before initiating a level-off sequence. Continued use of the thumbwheel prevented the autopilot from performing the level-off at FL240.
Corrective Actions
Following the incident, the manufacturer issued changes to the FCOM to warn pilots about the hazards of applying force to the control wheel or column while the autopilot is engaged and of adjusting the pitch thumbwheel during a level-off maneuver.