HISTORY OF FLIGHT On September 9, 1995, at 1217 eastern daylight time, a Bell 230, N232UM, registered to Corporate Jets, Inc., of West Mifflin, Pennsylvania, and operated by an ATR pilot, while in cruise flight at approximately 1,400 feet above ground level, suffered the separation of the tail rotor assembly and the 90 degree gear box. The pilot was able to maintain partial control of the helicopter and conduct an emergency landing at the Ann Arbor (MI) Airport. The helicopter sustained substantial damage at the time of the departure of the tail rotor assembly; however, no additional damage occurred during the emergency landing. The pilot and two passengers (medical personnel) reported no injuries. The flight was a positioning flight for medical transportation operating under the provisions of 14 CFR Part 91. The flight had just departed the University of Michigan Hospital at Ann Arbor, Michigan, with the intended destination of Toledo, Ohio, at the time of the accident. The pilot stated that after he heard a loud noise and realized that he had lost yaw control, he elected to make an emergency landing. He said he was able to maintain directional control as long as the airspeed was high; however, when slowing for landing, he lost control. The helicopter spun around three or four times prior to coming to a stop on runway 24. The two passengers completed witness statements which are attached to this report. They stated essentially the same as the pilot. PERSONAL INFORMATION The pilot was born October 2, 1960. He was the holder of an airline transport rating. He had accumulated a total flight time of 5,200 hours with 385 hours in this type of helicopter at the time of the accident. He held a first class medical certificate issued on June 27, 1995 with no restrictions. His most recent biennial flight review was on June 22, 1995, in the accident helicopter. AIRCRAFT INFORMATION The helicopter was a Bell 230, N232UM, serial number 23013. The helicopter had accumulated 1,188 hours time in service. The most recent inspection occurred on September 5, 1995, and the helicopter had flown 10 hours since that inspection. The maintenance records recorded no pervious mechanical failures of the tail rotor system prior to the accident. WRECKAGE AND IMPACT INFORMATION The helicopter was initially examined in a hangar at the Ann Arbor, Michigan airport. Damaged sections of the tail rotor system were retrieved from a golf course nearby the airport and shipped to the NTSB laboratory in Washington, DC. A diagram of where the tail rotor components were found is attached to this report. The diagram was supplied by the Federal Aviation Administration, Flight Standards District Office, Detroit, Michigan. TESTS AND RESEARCH The factual examination report completed by the NTSB, Office of Research and Engineering is attached as an addendum to this report. The report indicated that the failure of the tail rotor blade occurred as a fatigue fracture in the skin. Bell Helicopter indicated that the skin thickness required was in a range between 0.007 inches and 0.009 inches. The fractured piece measured 0.004 inches in thickness. Bell Helicopter also conducted tests and their engineering laboratories report dated September 22, 1995, is attached as an addendum to this report. Bell Helicopter indicated that they used a manufacturing procedure whereby the thickness of the skin was reduced below that required in the specifications by grinding material away from the metal tail rotor skin. Subsequently on October 9, 1995, Bell issued an "Alert Service Bulletin," No. 230-95-8, effecting Model 230 helicopters Serial number 23001 through 23038 requiring inspection of all the affected helicopter tail rotor blades prior to the next flight and every three (3.0) flight hours thereafter. ADDITIONAL INFORMATION Parties to the investigation were the Federal Aviation Administration, Flight Standards District Office, Detroit, Michigan; Bell Helicopter, Fort Worth, Texas; and Corporate Jets, Inc., Hurst, Texas. The tail rotor components of the helicopter were retained for inspection and were returned to representatives of the owner at the conclusion of the laboratory examination.