On June 25, 2005, at 1302 eastern daylight time, a Bell 206B, N1212N, received substantial damage during a hard landing following an autorotation to Burke Lakefront Airport (BKL), Cleveland, Ohio. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local airborne signal relay flight, which was conducted under 14 CFR Part 91. According to the operator, the purpose of the flight was for the helicopter to act as an airborne relay station for television signals during an automobile race. The pilot reported in a written statement that he was hovering the helicopter at 5,000 feet above mean sea level (MSL), when he heard a loud "bang." The helicopter then yawed in a nose right direction, and pitched nose down. The pilot regained control of the helicopter about 1,500 feet MSL, and subsequently performed an autorotation to runway 24R. The helicopter landed "hard," and the main rotor contacted the tail boom, separating it from the helicopter. A witness, who was also a helicopter pilot, observed the accident helicopter. The witness first heard the helicopter hovering over the racetrack about 5,000 feet. He subsequently heard a loud "bang," then looked up and saw the helicopter "dropping at a fairly good rate." The helicopter was in a level attitude, and it appeared that the pilot flew the helicopter in an easterly direction, and then entered an autorotation. The helicopter arrested its high rate of descent, and turned left 180 degrees, to the west. It then sounded as if the engine power had been reduced to idle. About 3,000 feet above the ground, the helicopter continued to descend at a rate that was "normal" for an autorotation. The helicopter turned to the north, and flew straight for about 1/4 to 1/2 mile, then turned southerly toward runway 24R. About 75 to 100 feet above the ground, the helicopter flared, and started to slow. The helicopter pitched in a nose down direction, and descended to about 15 feet above the runway, then pitched in a nose up direction again, and slowed. The rear portion of the skids contacted the ground first, at a speed "that a person could run." The helicopter quickly pitched in a nose down direction, leveling the skids on the ground, while the main rotor system "stayed back," severing the tail boom at about its mid-point. The helicopter yawed in a nose left direction about 90 degrees and came to rest. As the main rotor system slowed, the witness noted that it was moving fore, aft, and site-to-side in an unstable manner. The witness then proceeded to the helicopter as the pilot exited it. The two individuals then discussed the accident, and subsequently opened the helicopter's engine cover. Inside the engine bay, they found pieces of the main drive shaft coupling. Another witness was directly under the helicopter, when he heard the "loud bang." He looked up and saw the helicopter "veer right" and "nose dive." About this time he also saw a puff of white smoke. The helicopter appeared out of control for a moment, and as it regained control, he lost sight of it. The weather reported at the airport, at 1253, included winds from 340 degrees at 6 knots, clear skies, temperature 88 degrees Fahrenheit, dew point 69 degrees Fahrenheit, and an altimeter setting of 30.08 inches of mercury. The pilot held a commercial pilot certificate with a rating for rotorcraft helicopter. The pilot also held a mechanic certificate with ratings for airframe and power plant. The pilot's most recent FAA second-class medical certificate was issued on August 8, 2005, and on that date the pilot reported 1,000 total hours of flight experience. The helicopter was placed in a hangar at the airport where the accident occurred, and was examined by Federal Aviation Administration (FAA) inspectors. In addition to the damage that was consistent with a hard landing, the inspectors found that the isolation mount had separated from the transmission. Additionally the inspectors found that the main drive shaft had separated. The components of the drive shaft were forwarded to the Safety Board for examination. The Safety Board materials laboratory conducted an examination of the main drive shaft and its associated couplings. All of the failures observed in the components were consistent with an excessive angular displacement between the rotational axis of the drive shaft and its couplings. An FAA inspector conducted a second examination of the helicopter after it was transported back to the operator's facilities. During the examination, the inspector noted the separated transmission isolation mount, and that all four of the bolts that attached the mount to the transmission were broken. The broken bolts on the isolation mount attachment pad were polished and rubbed from motion. Ripped and torn metal was present on the lower deck of the compartment, and damage to the structure around the main drive shaft was consistent with rotation prior to its failure. Examination of photographs taken of the area around the main transmission and the engine inlet revealed grease sprayed on numerous surfaces consistent with rotation. Additionally, photographs of the transmission deck revealed a significant amount of metal debris spread over the area which included small pieces of metal and metallic dust at the very front of the deck.