Event Details
The pilot, who also served as the company's maintenance director, was conducting a sling load operation near the helicopter's maximum gross weight. All engine parameters were normal during lift-off. Shortly after takeoff, the pilot observed a high engine temperature followed by a significant power loss. The sling load was released, and the pilot performed an autorotation into a swamp. The helicopter touched down hard with the engine still running, resulting in substantial damage. Both crew members were uninjured.
Maintenance Background
The engine, an Allison 250-C20B (serial number CAE 832271), had undergone several maintenance events prior to the occurrence. A compressor replacement was performed 96 flight hours before the accident. A 300-hour inspection was completed approximately 54 flight hours prior, and the fuel control unit was replaced 5 flight hours before the accident. The 300-hour inspection includes tasks such as measuring oil flow from the scavenge passage, inspecting the scavenge oil strut, cleaning carbon deposits, and inspecting bearings and nozzles.
Examination Findings
An examination of the engine at the TSB Engineering Branch found no external faults. The N1 and N2 rotors initially appeared seized but rotated independently with a slight rubbing noise. The turbine assembly revealed a damaged O-ring packing between the spur gear adapter shaft and the turbine-to-compressor coupling. The oil filter contained a moderate collection of carbon particles and several small metal particles, though not enough to significantly affect oil flow. The turbine-to-compressor coupling shaft was heavily carboned internally and externally, deformed due to torsional stress, and showed areas of blueing indicative of heat distress. The No. 6 and No. 7 bearings were blackened and gritty, with some silver plating "melt out". The No. 8 bearing outer race could turn, but the stationary lab seal was cracked and contained significant carbon, indicating imminent bearing failure.
Cause
The engine lost power due to interference between the turbine-to-compressor coupling and the turbine inner shaft, caused by carbon accumulation between these components. The buildup of carbon in the turbine assembly was attributed primarily to operation and maintenance procedures that did not prevent the formation of carbon or apply adequate corrective measures.