On June 23, 2014, about 1345 central daylight time, an Airbus Helicopters AS-350B2, N407EM, was substantially damaged following an autorotation near Texarkana, Texas. The commercial pilot, two crew members, and passenger were not injured. The helicopter was registered to and operated by EagleMed LLC under the provisions of 14 Code of Federal Regulations Part 135 as an emergency medical services flight. Visual meteorological conditions prevailed for the flight, which operated on a visual flight rules flight plan. The flight originated from Idabel, Oklahoma, about 1320, and was destined for Texarkana, Texas.According to statements provided by the pilot and medical crew, the helicopter was in cruise flight about 1,000 feet above ground level and five minutes from landing at the destination hospital when the pilot noticed the helicopter's rotor RPM rapidly increase with the associated high rotor aural warning. The pilot attempted to troubleshoot the malfunction before deciding to perform an autorotation to a farm field. During the run-on landing from the autorotation, the helicopter skipped and settled firmly to the ground. The main rotors contacted and partially severed the tail boom resulting in substantial damage. Damage to the front of the left skid was also found. The engine was removed from the helicopter and sent to Honeywell's laboratories in Phoenix, Arizona. Under the auspices of the Federal Aviation Administration (FAA), the engine was examined and placed on a test bed for the purpose of an engine run. The engine started and idled for 8 seconds before performing an uncommanded acceleration. The engine run was then terminated. Three subsequent engine runs were performed isolating various connections in order to troubleshoot the uncommanded acceleration command. Only when the pg line from the fuel control unit (FCU) to the power turbine (PT) was removed, did the engine not experience an uncommanded acceleration. The FCU was removed from the engine and inspected. The drive bearing appeared to be degraded and no longer provided axial or radial positioning of the FCU drive shaft. The splined fuel pump to FCU coupler spun freely on the FCU drive shaft. The fuel pump was inspected and debris and three bearing balls from the FCU were removed from the fuel pump overboard drain passage. The fuel pump was then sent to Triumph for additional testing. The additional testing found no defects with the fuel pump. Disassembly of the FCU revealed that the drive bearings failed. Metallurgy analysis of the FCU bearings and particulates found evidence of polyimide material in particles located during the FCU removal. The bearing separators are constructed of a porous polyimide material. Particles rich in silicon and oxygen were found imbedded in ball contact surfaces of the drive bearing inner and out races in addition to being found imbedded in undamaged bearing balls. Bearing balls that were recovered from the drain passage were dented and covered with aluminum rich material. No material anomalies were detected in the components. The reason for the failure of the drive bearing could not be determined. On December 18, 2014, Honeywell Service Bulletin LT 101-73-20-0272 was amended to change a onetime replacement of the main drive bearings to every 600 hours. A search of the FAA's Service Difficulty Reporting Site did not find any reports of part failure after the issuance of the updated service bulletin.