United Airlines Flight 896

While in normal cruise flight, the crew experienced lateral control problems. An emergency was declared. The aircraft landed safely at the intended destination. Evidence of a water leak was identified by the cabin crew approximately 5 hours from the destination. Efforts to control the leak were effective, however it was not completely stopped. Water was reported coming from the upper deck and flowing through the main deck ceiling. The flight was subsequently directed by air traffic control to make an enroute course change. However, when this command was entered in the Flight Management Computer (FMC) the aircraft began a shallow left turn instead of a right turn as required. Initial attempts to disconnect the autopilot were not successful and the autopilot was manually overridden. The relief first officer at the controls stated the controls "felt unusual" and "stiff." The captain reported elevator and rudder were normal, but bank angle was limited. The landing was accomplished smoothly and safely according to the captain, and the aircraft was taxied to the gate without incident. After landing, the captain noted the controls felt normal. Ramp personnel reported a significant amount of water draining from the fuselage and the drain masts at the gate. A post-incident examination of the aircraft revealed that areas of the main deck carpeting was saturated. The canted pressure deck overboard drains were not obstructed. A 6-inch long by 0.125-inch wide gap was located along the outboard edge of the canted pressure bulkhead on the right side of the aircraft. The seam was not sealed as required. Immediately aft of the canted pressure bulkhead were aileron and flight spoiler control cables. Four (4) circuit breakers common to the external drain line heaters were found open. The external drain lines route wastewater from the cabin overboard. The breakers were pulled in conjunction with routine cleaning of the drain lines prior to departure. Ground functional testing of the aileron controls, the aileron trim and the autopilot did not find any anomalies. A flight test was completed to verify in-flight operation of the flight controls and potable water system. No anomalies were noted. Airline procedures related to the routine inspection and cleaning of the external drains were reviewed. Resetting of the drain heater circuit breakers was the last item. A service bulletin had been issued which recommended testing, cleaning and inspection of the canted pressure deck drainage system (overboard drains), general visual inspection of the deck structure a pressurization test. Service bulletin instructions included a visual inspection for loose, missing or cracked sealant. The airline was in the process of incorporating the service bulletin into its maintenance program. As a result, the initial service bulletin procedures had not been completed prior to the incident. An airworthiness directive (AD) which required cleaning of "the cavity aft of the wing center section" and verification that all drains were open and clean was in effect at the time of the incident and had been complied with. A new AD was issued following the incident which mandated full compliance with the existing service bulletin.

Verbatim NTSB analysis and probable cause from the NTSB dataset