McDonnell Douglas 369E N554CP

The public-use helicopter was conducting a routine patrol flight when it experienced an uncontained engine failure. The pilot immediately performed an autorotation and run-on landing to an open parking lot area without further incident. The helicopter was damaged with several punctures with outward deformation consistent with projectile damage originating from an uncontained engine failure. Inspection of the engine revealed that portions of the 1st-stage gas producer wheel had departed the engine. The gas producer turbine tie bolt (part number 23068265, serial number NM81907) was identified as the primary failed component. The tie bolt fractured immediately adjacent to the torque nut. Further examination revealed fracture features consistent with fatigue, initiating at the thread root radius. The microstructure, chemistry, and hardness of the turbine tie bolt met engineering drawing specifications. No material anomalies were detected at the fracture surface. Damage to the fracture surface precluded the determination if the failure was associated with low or high cycle fatigue. The tie bolt had completed 3,427 cycles, about 38 percent, of its published life limit of 9,000 cycles. The failure was the first documented since the tie bolt was redesigned during the late 1990s. The redesigned tie bolt incorporated a thicker and stiffer cross-section that had a higher resonant frequency. The manufacturer reviewed the testing and analysis that was preformed when the tie bolt was redesigned. Their review confirmed that the current life limit of 9,000 cycles was within design guidelines. Additionally, the stress modeling and analysis performed during the component redesign was reviewed and no discrepancies were noted. The manufacturer also reviewed the reengineered tie bolt service history and several tie bolts with between 5,000 and 9,000 cycles were taken out of service for examination. Component examinations and database reviews failed to show any similar failures within the fleet. The tie bolt installation required that the bolt length be measured in order to determine the exact stretch of the bolt after the nut was torqued to specification. The original stretch requirement was used with the redesigned tie bolt to eliminate confusion during assembly. A subsequent service bulletin and FAA airworthiness directive mandated that the older design be removed from service, but the installation stretch requirement was not modified. To mitigate future tie bolt fractures, Rolls-Royce reduced the stretch requirement on the tie bolt during the assembly process. As a result, the mean stress on the tie bolt was reduced. The reduced stretch is also expected to increase both high and low cycle fatigue margins in the part. The assembly instructions were updated with the new stretch callout during the first quarter of 2008.

Quoted verbatim from the NTSB record.