On September 20, 2010, at 1720 central daylight time, a Schweizer 269C single-engine helicopter, N34LB, sustained substantial damage during a forced landing after experiencing a mechanical malfunction while hovering prior to takeoff at the Schaumburg Regional Airport, Schaumburg, Illinois. The helicopter was owned and operated by Bachman Aero, Skokie, Illinois. The certificated flight instructor and pilot receiving instruction were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the 14 Code of Federal Regulations Part 91 instructional flight. The local flight was originating at the time of the accident. According to the pilot and registered owner, while hovering prior to departure for a routine training flight, a bolt and its associated hardware for the left aft cluster fitting of the tail boom assembly departed the helicopter. The helicopter experienced a vibration and subsequently yawed to the right when the instructor initiated a forced landing. The skids contacted the ground, the helicopter rotated 180 degrees and came to rest upright. Examination of the helicopter revealed the tail boom was partially separated and the tail rotor driveshaft had separated. The bolt and its associated hardware were located on the ramp near the helicopter and were retained for further examination. In addition, the right aft cluster fitting bolt with its associated hardware, which remained attached, were removed and retained for examination. A review of the maintenance records revealed the aft cluster fitting modification kit (Schweizer P/N: SA-269K-0106-1) was installed on the helicopter on June 1, 2010, at a total aircraft time of 1,512.8 hours. On September 10, 2010, 25/50/100/300/400-hour/Annual inspections were performed with no anomalies noted with the aft cluster assembly at a total aircraft time of 1,693.9 hours. The owner stated that since the installation of the aft cluster, the cluster and its components had not been removed or replaced. The airframe had accumulated 1,729.1 total hours at the time of the accident. According to Schweizer Service Letter L-177, AFT CLUSTER FITTING MODIFICATION KIT, dated September 13, 2002, the company developed and approved the cluster modification kit as an alternate means of compliance (AMOC) for the repetitive inspections mandated by the Federal Aviation Administration (FAA) Airworthiness Directive (AD) 2001-25-52. AD 2001-25-52 mandated a repetitive dye penetrant inspection of the aft cluster fittings used on certain Schweizer helicopters. The letter states, "The installation of this kit is recommended at owner's/operator's convenience to provide enhanced aft cluster fitting durability." On December 9, 2010, at the facilities of IMR Test Labs, Lansing, New York, under the supervision of a NTSB senior air safety investigator, the accident nut and bolt assemblies were examined. In addition, two new bolt and nut assemblies that were supplied by Schweizer were examined for comparison. It was noted that the bolts were Avibank-5 adjustable diameter bolts that were secured with a MS-21043 lock nut torqued to 23-inch pounds (lbs). According to the IMR report, examination of the bolts removed from the accident helicopter showed no anomalies to suggest that they contributed to the loosening. Examination of the lock nut from the loosened left side fastener revealed incomplete threads at the pinch point of the locking stake. Torque testing showed the loosened left side lock nut had no prevailing torque. The prevailing torque should have been between 2 to 18 in-lbs. On December 20, 2010, the FAA issued Emergency AD 2011-01-52, which required before further flight, unless previously accomplished, the removal of the lock nut. The lock nut should then be reinstalled while determining the lock nut drag torque. If the drag torque is a minimum of 2 in-lbs, retorquing the locknut to 23 in-lbs is required. If the drag torque is not at least 2 in-lbs, the replacement of the lock nut with an airworthy lock nut is required.