CEN13LA378

The flight instructor reported that, shortly after he and the pilot-rated student departed on a night training flight in the turbine-powered helicopter, they heard a loud bang, and the helicopter yawed left. The flight instructor chose to perform an autorotation with forward airspeed onto the airport's taxiway. Examination of the turbine engine revealed several holes in the compressor case, consistent with engine blades exiting the engine. Further examination revealed that the stages 3 through 6 compressor blades had separated at the roots. Examination also revealed that the 3rd-stage wheel blade root had fractured due to progressive cracking, consistent with fatigue; 54 percent of the fracture surface exhibited fatigue features. However, the fracture initiation surface was destroyed, so it was not possible to determine the cause of the fatigue failure. Indications of fatigue were also found on the stages 3 through 6 blades' fracture surfaces. The engine's compressor had 13,874 hours time since new (TSN) and the compressor rotor stages 1 through 6 wheels had 1,185.3 hours TSN and were Parts Manufacturer Approval replacement parts.

On May 23, 2013, about 2100 central daylight time, a Bell 206B helicopter, N297CA performed an autorotation to a forced landing at the Spirit of St. Louis Airport (KSUS), St. Louis, Missouri. The flight instructor and commercial rated pilot were not injured and the helicopter was substantially damaged. The helicopter was registered to GM Leasing Co., LLC and operated by Air-Evac Lifeteam, O'Fallon, Missouri, under the provisions of 14 Code of Federal Regulations Part 91 as a training flight. Night visual meteorological conditions prevailed for the flight that operated on a company flight plan. The flight instructor reported that he was demonstrating a normal takeoff. As the helicopter accelerated to about 50 knots and was 20 feet in altitude, there was a loud bang. The helicopter yawed left, and then the instructor elected to perform an autorotation landing with forward airspeed on the taxiway. During an initial visual inspection of the airframe, no observable damage was noted. Inspection of the engine, revealed a case breach on the axial compressor. Further examination of the helicopter's airframe revealed substantial damage to the aft fuselage and main rotor system.A review of the helicopter's Rolls-Royce 250-C20B engine maintenance records, revealed that the engine had 15,179.6 hours TSN (time since new) and 4,948 cycles TSN. The engine's compressor had 13,874 hours (TSN) and the compressor rotor stage 1-6 wheels had 1,185.3 hours (TSN) and were Parts Manufacturer Approval (PMA) replacement parts. The examination of the engine revealed several areas where the compressor case had been breached by fragments exiting the engine. Further examination of the stage 1 compressor blades revealed the blades were in place and in good condition. The stage two blades were present, but exhibited varying degrees of trailing edge damage. All stage 3 through 6 blades had separated at the blade root. The 3, 4, and 6 compressor stator vanes were completely separated and ingested. Stage 2 and 5 vanes were present, but bent and/or torn in the direction of rotation. The compressor impeller was intact, but had leading edge damage 360-degrees around the disk. Several impeller blades exhibited blade tip bending, consistent with debris ingestion. The axial compressor rotor and stator was sent to the National Transportation Safety Board (NTSB) materials lab, in Washington, D.C. Examination of the 3rd stage wheel found a blade root that had about a 54% progressive cracking area, consistent with fatigue. The examination also found indications of fatigue on stage 3 through 6 blades.