On January 29, 2000, at 1500 mountain standard time, a Bell 212 helicopter, N212HQ sustained substantial damage when it impacted the ground after the main rotor system separated from the aircraft during logging operations near Mount Pleasant, Utah. The commercial pilot, who was the sole occupant, received serious injuries. The flight was operating under Title 14 CFR Part 133 and no flight plan was filed. Visual meteorological conditions prevailed for this local area flight that departed Mount Pleasant at 1410. The helicopter was in level flight approximately 120 feet above the ground, over down sloping terrain carrying an external load of logs, when the main rotor mast failed. The fuselage descended nose down into trees with the load of logs attached. On impact with the ground, the fuselage inverted and broke into three pieces. The pilot remained in the aircraft and the seat remained attached. Examination of the main rotor mast was conducted at the facilities of Bell Helicopter Textron, Inc., (BHTI) Hurst, Texas, under the direction of a Safety Board investigator from the Arlington, Texas, office. The examination provided evidence that the main rotor mast P/N 204-011-450-007, S/N N9-19783, failed in fatigue at the lower ring groove of the damper splines. The origin was in a burr area left after machining of the part. (See attached NTSB and BHTI reports) According to information provided by the Federal Aviation Administration (FAA), BHTI, and the operator, the rotor mast had accumulated 5,644.4 hours since its manufacture in 1978, and had been overhauled four times, with the latest being at an accumulated time of 3,660.8 hours. The retirement requirement for the rotor mast at the time of the accident was either 15,000 flight hours or 300,000 RIN (retirement index number). The operator calculated the RIN to be approximately 264,000; the FAA calculated it to be 291,065 and BHTI calculated it to be 463,000. The operator and FAA used recorded data to calculate RIN and BHTI used data from service bulletins and past Airworthiness Directives pertaining to RIN count. For an explanation of RIN calculation requirements see Airworthiness Directives (AD) 98-24-15 and AD 20-00-08-52. Based on the results of the accident investigation, the FAA issued Emergency AD 20-00-08-52 on April 21, 2000. The AD reduced the allowable RIN life limit, clarified the RIN calculation method, and required a one time special inspection of main rotor masts on the BHTI models 204B, 205A, 205B, and 212. Compliance was required prior to further flight.