On September 13, 1996, at 0710 central daylight time, a Bell 206L-1/C-30P helicopter, N2654, was substantially damaged during a forced landing, following a loss of tail rotor control near Morgan City, Louisiana. The helicopter was registered to and operated by Mobil Administrative Services Company, Inc., doing business as Mobil Air, as a Title 14 CFR Part 135 non scheduled passenger flight. Visual meteorological conditions prevailed, and a company VFR flight plan was filed. The airline transport rated pilot and four passengers were not injured, and one passenger sustained minor injuries. The flight originated from Mobil's helipad, Morgan City, Louisiana, about 5 minutes prior to the accident. According to the pilot, he did a preflight of the helicopter, and then successfully completed a leak check and operational test flight for the previous days maintenance. After the aircraft was shutdown, it was fueled, and the passengers and their baggage were loaded. The flight departed the Mobil helipad, and was en route to South Marsh 243C in the Gulf Of Mexico. About 5 minutes into the flight, at an altitude of 500 feet AGL, "a loud continuous, chattering and violent vibration started without warning." The pilot did not suspect the tail rotor as the source of the vibration because he "still had tail rotor control and the pedals were not vibrating excessively." At the onset of the problem, he initiated a 180 degree turn and lowered the collective. During the emergency landing to a canal, about 50 feet above the trees, the vibration totally ceased; however, the helicopter began turning to the right. At this time he initiated an autorotation and deployed the floats. The helicopter touched down on the water in a level attitude at 20 to 25 MPH. The water rose half way up the wind screen, and the helicopter made a sharp 180 degree right turn. The helicopter was listing to the right approximately 20 degrees and submerging before the main rotor blades began striking the water. The helicopter came to rest on its side. Examination of the helicopter by the FAA inspector revealed that one main rotor blade was separated approximately 4 feet from the blade grip. The aft portion of the tailboom just forward of the tail rotor gearbox and vertical fin was separated. The four tail rotor gearbox attaching bolts were fractured. One tail rotor blade had a 1.25 inch piece sheared off from the tip of the blade. Examination of the tail rotor gearbox housing with the portions of the fractured studs and the tail rotor blade with the missing tip, was completed on February 12, 1997, under the supervision of the investigator-in-charge at the Bell Helicopter Textron Materials Laboratory. According to the Bell Helicopter metallurgist, the left front, left aft, and right aft gearbox mounting studs had fractured as a result of reverse bending low cycle fatigue. The right forward stud fractured as a result of overstress. The examination of the tail rotor blade revealed that the top side of the blade was sheared from the leading edge to the trailing edge at the outboard end. Examination of the tail rotor blade with the binocular microscope at 30X magnification revealed small particles of red paint at various locations along the sheared end. The smeared surface areas along the end of the tail rotor blade were scanned by energy dispersive x-ray analysis (EDXA) to determine if there were any detectable smears of another metal. An area of particles on the sheared surface identified as area 3, contained elements of lead and chromium. Another particle identified on the sheared surface identified as area 4, contained elements of titanium, iron, nickel, and chromium. Examination of area 3 with the optical microscope at 60X magnification revealed an area of particles of red paint. There were no distinguishable or recognizable particles or features visible at area 4 when observed at this magnification. See the enclosed metallurgist report for technical details.