What happened
During a sling load operation conducted near the aircraft's maximum gross weight, a Bell 206L helicopter experienced a sudden loss of engine power. The pilot, who was also a maintenance director for the operating company, noted high engine temperatures immediately preceding the power loss.
In response to the engine failure, the pilot released the sling load and performed an autorotation. The aircraft landed heavily in a swamp area. While the helicopter sustained substantial damage, the two crew members on board escaped without injury.
The investigation
Investigators examined the engine, an Allison 250-C20B, and reviewed recent maintenance history. Records indicated that the engine had undergone a compressor replacement 96 flight hours prior, a 300-hour inspection 54 flight hours prior, and a fuel control unit replacement just five hours before the accident.
Technical analysis by the TSB Engineering Branch revealed that the turbine-to-compressor coupling shaft was heavily carboned and deformed by torsional stress, showing signs of heat distress. The investigation found that the O-ring packing providing a seal between the spur gear adapter shaft and the turbine-to-compressor coupling was damaged.
Further disassembly of the gas producer and power turbine rotors showed that the No. 6 and No. 7 bearings were blackened and gritty, with evidence of silver plating melt-out. Additionally, the No. 8 bearing lab seal was cracked and filled with carbon, indicating that a bearing failure was likely imminent. The oil filter also contained a moderate amount of carbon and metal particles.
Findings
- The engine lost power due to interference between the turbine-to-compressor coupling and the turbine inner shaft.
- This interference was caused by the accumulation of carbon and oil sludge between these components.
- The build-up of carbon within the turbine assembly was attributed to a combination of operational and maintenance procedures that failed to prevent carbon formation or implement sufficient corrective measures.