What happened
At approximately 0600 hours, a Bell 206 helicopter, registration C-GPOS, was performing a high hover roughly 150 feet above a forestry cut block. The aircraft, operated by Cariboo Chilcotan Helicopters Ltd., was carrying a pilot and two forestry workers. While scanning the area for hot spots, black smoke suddenly filled the cabin.
The pilot attempted an emergency descent toward a nearby road. During the maneuver, the pilot noted a drop in rotor RPM, indicating a loss of engine power. The helicopter struck the ground with significant force, causing the nose to dip and the landing gear to collapse. The aircraft subsequently rotated 180 degrees and rolled onto its left side. While the engine remained running after the impact, the pilot shut it down. The pilot and front passenger were briefly trapped in the wreckage because an aftermarket step on the landing gear shifted, jamming the pilot's door. The pilot sustained a back injury, and both passengers suffered minor injuries.
The investigation
Investigators examined the wreckage at a facility in Richmond, British Columbia. While the engine itself showed no external anomalies, the investigation focused on the input drive coupling between the engine and the main-rotor transmission. It was determined that the damage to the coupling and the transmission mounts was caused by the extreme forces of the hard landing and rollover, rather than being the initial cause of the incident.
Further analysis of the engine revealed that the loss of power was triggered by the separation of parts from the GP turbine blades. The investigation established that sulfidation corrosion had weakened the blades, leading to fatigue cracks and the eventual failure of a blade. This failure created an imbalance that likely damaged turbine seals, allowing oil to leak onto hot engine components and produce the smoke observed in the cabin.
Findings
- Sulfidation corrosion caused fatigue cracks in the GP turbine blades, leading to a blade separation and subsequent loss of engine power.
- The helicopter was operating in a high hover at a high density altitude and near maximum weight, making a successful landing with reduced power unlikely.
- The hard landing and subsequent rollover caused the landing gear to break and the transmission mounts to deform.
- The pilot was unable to exit through the door because an aftermarket step attached to the landing gear cross tubes moved into a position that blocked the door's opening.
- The aircraft was not equipped with emergency release systems for the doors or windows.
Safety action
Following the investigation, the TSB issued a Safety Advisory to Transport Canada regarding the risks of sulfidation corrosion in Rolls-Royce 250 engines. In response, Transport Canada indicated they would recommend that the engine manufacturer implement inspections for sulfidation on turbine blades.