Accident Overview
On September 20, 2010, at 1720 central daylight time, a Schweizer 269C helicopter, registration N34LB, experienced a forced landing at Schaumburg Regional Airport, Schaumburg, Illinois. The helicopter was on a local instructional flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed.
Sequence of Events
While hovering prior to takeoff, a bolt and its associated hardware for the left aft cluster fitting of the tail boom assembly separated from the helicopter. The helicopter subsequently developed a vibration and yawed to the right. The flight instructor initiated a forced landing. The skids contacted the ground, the helicopter rotated 180 degrees, and came to rest upright.
Damage and Examination
The helicopter sustained substantial damage. Examination revealed the tail boom was partially separated and the tail rotor driveshaft had separated. The separated bolt and hardware were found on the ramp near the helicopter and were retained for analysis. The right aft cluster fitting bolt and hardware, which remained attached, were also removed for examination.
Maintenance History
The aft cluster fitting modification kit (Schweizer P/N: SA-269K-0106-1) was installed on June 1, 2010, at a total aircraft time of 1,512.8 hours. On September 10, 2010, inspections were performed with no anomalies noted regarding the aft cluster assembly at a total aircraft time of 1,693.9 hours. The owner stated that the cluster and its components had not been removed or replaced since installation. The airframe had accumulated 1,729.1 total hours at the time of the accident.
Component Examination
On December 9, 2010, at IMR Test Labs, Lansing, New York, the accident nut and bolt assemblies were examined under NTSB supervision. New bolt and nut assemblies supplied by Schweizer were also examined for comparison. The bolts were Avibank-5 adjustable diameter bolts secured with MS-21043 lock nuts torqued to 23 inch-pounds. Examination of the accident bolts showed no anomalies to suggest they contributed to loosening. The lock nut from the left side exhibited incomplete threads at the pinch point of the locking stake. Torque testing showed the loosened lock nut had no prevailing torque, whereas the expected prevailing torque should have been between 2 and 18 in-lbs.
Subsequent Airworthiness Directive
On December 20, 2010, the FAA issued Emergency AD 2011-01-52, requiring before further flight, removal of the lock nut and reinstallation while determining the lock nut drag torque. If the drag torque is at least 2 in-lbs, retorquing to 23 in-lbs is required. If not, replacement with an airworthy lock nut is required.