On March 28, 2002, at 1731 Pacific standard time, a Bell OH-58A, N487BC, landed hard during an autorotation to a dirt field in Chico, California. During the landing, the main rotor flexed downward and severed the tail boom, resulting in substantial damage to the helicopter. The helicopter was owned and operated by the Butte County Sheriff's Department as a public-use aircraft under the provisions of 14 CFR Part 91. The commercial pilot and a sheriff's deputy observer were not injured. Visual meteorological conditions prevailed and a company flight plan had been filed. The local area search and rescue mission originated at 1620 from the Butte County Sheriff's Department Search and Rescue base at Chico and was returning to the helipad when the accident occurred. The pilot was interviewed by telephone on March 28, 2002, and reported that he was cruising at 1,000 feet above ground level (agl), at 90-100 knots, when he detected a left yaw. This was followed by a low main rotor speed warning. He noted that the rotor needle (rotor rpm) was below the bottom of the green arc, and lowered the collective. As a result, the main rotor warning light went out. He glanced at the N1 gage (gas producer tachometer) and noted it at 70 percent and decreasing; the N2 needle (power turbine rpm) showed a split. The pilot then rolled the throttle off and began the restart process by depressing the start switch on the collective, holding the start switch approximately 15 seconds during the autorotation. No other warning lights were visible on the caution panel. Engine power was not regained. The pilot performed an autorotative landing to a large dirt field in back of an unoccupied elementary school, and the touchdown was slightly harder than normal. The main rotor then contacted and severed the tail boom. The pilot noted that the engine was running at idle after the landing and he performed a manual shutdown. He had flown this helicopter for the last 2 days with no discrepancies noted. The pilot was recontacted regarding the accident on April 2, 2002. He reported that the helicopter was topped off with fuel the night prior to the accident and the quantity was verified at 400 or more pounds. The total duration of the flight on the Hobbs meter was 1.1 hours. After the accident, the fuel gage read about 350 pounds. While en route back to the base, the torque gauge was showing 70 to 80 percent during cruise. During the autorotation, he felt there may have been a surging of the engine but was unsure because he wore a helmet. On April 4, 2002, the pilot filed National Transportation Safety Board Form 6120.1/2, Pilot/Operator Aircraft Accident Report, which was received by the investigator-in-charge (IIC) on April 10, 2002. In the attached narrative, the pilot reported encountering the nose left yaw as discussed via telephone on March 28. He stated, in part, "...rolled the throttle to flight idle and began the restart process by depressing the start switch on the collective lever." On April 5, 2002, the IIC received an e-mail from the pilot with additional information regarding the accident. After verifying various times concerning the accident, the pilot stated that he did not relay that after the low rotor rpm light illuminated, he looked at the N2 gage (dual tachometer) and saw the rotor needle below the green arc indicator. He then lowered the collective without rolling off the throttle. In an additional phone conversation on September 10, 2003, the pilot was questioned regarding his emergency procedures training. He reported that his most recent emergency training, including autorotations, took place over 1 year prior to the accident. He further reported that pilots were not allowed to practice autorotations without the Chief Pilot on board the helicopter. The engine was examined at the Rolls-Royce Corporation facilities in Oakland, California on April 5, 2002, under the auspices of two Federal Aviation Administration (FAA) inspectors. Rolls-Royce issued a report on the results of the examination on April 7, 2002. The examination of the engine and engine components did not reveal any evidence of preimpact failures. Excerpts from the report (Rolls-Royce Engine Investigation Report, Engine CAE 406013) are included in the public docket for this accident. The IIC contacted the Butte County Sherrif's Aviation department regarding emergency procedures and training for department pilots. The department required all pilots to complete oral and flight emergency training annually. The tests were reviewed by check pilots with predetermined areas of review. There were no written requirements for the training. The accident pilot completed his last emergency training on March 23, 2001. In the Air Operations Handbook for the Butte County Sherrif's Department, it states that emergency procedures, including autorotations, may not be performed without the Chief Pilot on board the aircraft with the dual controls installed and operational, unless prior written approval from the Chief Pilot has been obtained. In the emergency procedures section of the OH-58A handbook (TM 55-1520-228-10), the indications for an engine malfunction are as follows: left yaw; drop in engine rpm; drop in rotor rpm; low rpm audio alarm; ilumination of the rotor rpm warning light; engine out warning light; and change in engine noise.