On June 7, 2000, at 1236 central daylight time (cdt), a Boeing 767-222, N603UA, operated by United Airlines, Inc., experienced aileron control difficulties while in cruise flight at flight level (FL) 390. The flight diverted to the O'Hare International Airport, Chicago, Illinois where an uneventful landing was made. There were no injuries reported to the 154 passengers, 9 cabin attendants, or 2 flight crew members. The 14 CFR Part 121 flight was operating on an instrument flight rules flight plan. Visual meteorological conditions prevailed at the O'Hare International Airport at the time of the event. The flight originated from the John F. Kennedy International Airport (JFK), New York, New York at 1030 cdt, and the San Francisco International Airport, San Francisco, California, was the intended destination. The captain reported, in a written statement, that while in cruise flight at FL 390, with the center autopilot engaged with lateral navigation (LNAV), vertical navigation (VNAV) and autothrottles engaged, "...the center autopilot made an uncommanded disconnect." According to the captain, the engine indicating and crew alerting system (EICAS) autopilot disconnect message illuminated and the aircraft master caution was cancelled. The captain said that autopilot disconnect warning light and aural warning activated and were subsequently cancelled. The captain said that he took control of the aircraft and found that the rudder and elevator systems appeared to be functioning normally but "...the control wheel was jammed in the straight and level position." The captain said that approximately 15 pounds of force was applied in order to free the control wheel. The captain said that the autopilot and autothrottles were not engaged for the remainder of the flight, an emergency was declared, and the flight diverted to O'Hare where an uneventful landing was made. A postaccident examination was conducted and several components of the autopilot system were examined. No anomalies were found that could be attributed to the aileron control anomaly. Further examination of the aircraft revealed that the wheel well canted pressure deck drain lines were obstructed by debris. A Boeing service bulletin was found that recommends changes to the drain system. Boeing service bulletin 767-51A0020 states that, "Incorporation of this service bulletin will help ensure that fluid entering the canted pressure deck area will be drained out of the airplane and not leak into the wheel well area where it could freeze on the aileron control cables or the landing gear doors during flight." The service bulletin also states that, "Three operators reported instances of ice accumulating on the aileron cables. In two of the instances, the ice on the aileron cables caused the control wheel not to move when on autopilot. The autopilot was disengaged and the pilot had to operate the aileron system manually. Higher than normal control wheel input force was required to free the cables and restore normal aileron control. The ice-build up on the aileron control cables was attributed to fluid from the sloping pressure deck leaking into the wheel well and freezing." It was determined that the Boeing service bulletin had not been complied with on this aircraft. According to United Airlines records, the aircraft arrived at JFK on June 6, 2000 at 1956 eastern daylight time. Weather reports for the JFK airport show rain during the night of June 6, 2000 and during the morning of June 7, 2000. Parties to the investigation were the Federal Aviation Administration, United Airlines, Rockwell Collins, and Boeing.