On July 12, 2001, at 1330 Pacific daylight time, a Bell 206B, N474SF, lost engine power and made a hard landing from an autorotation in an open field near San Francisco, California. The helicopter sustained substantial damage. The certificated commercial pilot and three passengers were uninjured. San Francisco Helicopter Tours was operating the helicopter as a for-hire sightseeing flight under the provisions of 14 CFR Part 91 when the accident occurred. The local flight originated from San Francisco International Airport at 1317. Day visual meteorological conditions prevailed at the time, and a company VFR flight plan was filed. The pilot reported that he was inbound to the airport when he saw an engine chip light illuminate, and heard a whining sound. He was looking for a suitable landing area when the "engine out" caution light and aural warning activated. An engine failure followed at approximately 400 feet agl (above ground level). The helicopter had been descending to land over a large, flat, open field at the time. The collective was lowered to enter an autorotation and aft cyclic applied to slow the forward airspeed. As he neared the bottom of the maneuver, he pulled collective pitch to cushion the landing; however, the helicopter abruptly spun 90 degrees to the left. The main rotor blades struck the tail boom and the ground. The tail boom was separated from the helicopter. All the occupants were able to exit the helicopter without assistance. A postcrash examination of the engine was conducted with the assistance of Rolls-Royce/Allison. It was disassembled and both the compressor and turbine were reported to have rotated freely. The accessory gearbox was found fractured, however. There was damage to the 2 1/2 bearing, fuel control/generator idler gear, and spur adapter gear shaft. The 2 1/2 bearing had (3) fractured separator rails. Individual bearing rollers displayed uneven wear. Further metallurgical examination revealed an overheated microstructure in the roller path of the 2 1/2 bearing, and that the bearing itself had failed in fatigue. Aircraft logbooks did not reveal any recent maintenance inside the accessory case. The accessory gearbox was last opened for a 2 1/2 bearing replacement in 1990. The manufacturer's guidelines for a teardown inspection of the accessory gearbox and 2 1/2 bearing are on condition only. An engine chip light history existed on the helicopter. The Daily Flight Sheet record indicated another warning had been encountered on a flight 4 days prior. According to San Francisco Helicopters, maintenance personnel followed the published procedure after this occurrence. There was a paste buildup found and mechanics performed a cleaning and inspection of the plugs. Then the engine received a ground test run, at which time no warning lights were observed. The engine and helicopter were returned to service and a corresponding notation was recorded in the helicopter's logbook. The published maintenance manual procedure calls for a nondestructive inspection/cleaning of the magnetic chip detectors and then an engine ground test run for 30 minutes. The chip light would illuminate if metal particles collected on the magnetic plugs in the accessory gear drive housing. According to the Allision 250 Series Operation and Maintenance Manual, when either a chip light is encountered after a gearbox flush and a second ground test run has been performed or when a total of (4) chip lights have been encountered within 50 hours of engine operation, the engine must be removed from service and sent to the manufacturer's authorized maintenance center.