On August 19, 2004, approximately 0705 central daylight time, a Bell 412 twin-engine helicopter, N22347, sustained minor damage during a forced landing following a loss of tail rotor control near South Pass 65, an offshore platform located in the Gulf of Mexico. The helicopter was registered to and operated by Petroleum Helicopters Inc. (PHI), of Lafayette, Louisiana. The airline transport pilot-in-command, commercial co-pilot, and seven passengers were not injured. Visual meteorological conditions prevailed, and a company visual flight rules (VFR) flight plan was filed for the 14 Code of Federal Regulations Part 135 on-demand air taxi flight. The cross-country flight originated from Boothville, Louisiana, at 0635, destined for offshore platform Viosca Knoll 989. The 11,750-hour co-pilot, who was in the right seat and piloting the helicopter, reported in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) that while in cruise flight he heard a loud bang followed by an uncontrolled 30-degree yaw to the right and a 15-20 degree "nose tuck." The co-pilot stated that he attempted to correct the situation by lowering the collective and "trimming the yaw with the pedals;" however, the helicopter failed to respond to the inputs. Subsequently, the co-pilot reduced power and initiated an autorotation to the water. Prior to touchdown, the co-pilot successfully deployed the emergency floats. At an altitude approximately 10 feet above the water, the co-pilot "pulled pitch" until the helicopter settled onto the water. Both pilots and the passengers evacuated the helicopter into an inflatable life raft. When the helicopter was located in the water, the tail rotor blade assembly and a section of the 90-degree tail rotor gearbox were not attached to the fuselage. After recovery from the water, the helicopter and separated components were transported by truck to the PHI facilities near Lafayette, Louisiana. Examination of the helicopter was conducted by personnel from the Federal Aviation Administration (FAA), PHI, and Bell Helicopter. The cockpit voice recorder (CVR) was removed from the wreckage and forwarded to the NTSB laboratories in Washington, D.C., for review. The 90-degree input quill and pieces of the 90-degree tail rotor gearbox assembly, tail rotor link assembly, and the upper and lower hinge of the lower access door were sealed in a box and sent to the engineering laboratories of Bell Helicopter for further examination. The review of the CVR from the NTSB laboratories did not reveal any additional information or significant findings relative to the accident. On September 23, 2004, at the field investigations laboratory of Bell Helicopter, Fort Worth, Texas, the examination of the remaining section of the tail rotor gearbox and access door hinges was conducted under the supervision of the NTSB investigator-in-charge, FAA, PHI, and Bell Helicopter. The examination revealed that the input quill exhibited no damage other than saltwater corrosion. The 90-degree gearbox case assembly fracture surfaces were also corroded from saltwater, which hindered determination of the fracture mode, but the surfaces that could be determined were consistent with an overload condition. The other fractured parts that were sent for examination were also determined to be caused by an overload condition. The reason for the failure of the tail rotor gearbox could not be determined.