What happened
During an approach to a prepared landing site in the Monashee Mountains near Mica Creek, a Bell 212 helicopter carrying a pilot, a ski-guide, and 11 passengers experienced a sudden engine failure. Approximately 200 feet above the ground, an explosion occurred and the number 1 engine ceased functioning. As the pilot attempted to maintain the approach, the rotor RPM dropped, causing the aircraft to drift right and land in an unprepared area of waist-deep snow.
Upon touchdown, the helicopter tipped onto its left side. The impact caused the main rotor blades to strike the ground, resulting in the main transmission being torn from the airframe and a rotor blade striking the cockpit roof. While most passengers evacuated through the side door, the ski-guide had to exit through a roof window and subsequently assist in extracting the pilot. Although the number 2 engine continued to operate for 15 minutes following the initial impact, it eventually failed after a second explosion. Two small fires ignited in the engine compartment but were extinguished quickly. There were no fatalities or other injuries.
The investigation
The investigation focused on the mechanical failures of both engines. It was established that both engines suffered uncontained power turbine failures. The primary cause was identified as the failure of the number 1 engine clutch coupling shaft, which sheared due to extreme loads generated when a worn clutch slipped and then abruptly re-engaged. This caused the power turbine wheel to overspeed, leading the blades to fracture and penetrate the containment rings.
The subsequent failure of the number 2 engine was attributed to a loss of lubrication while the aircraft was resting on its side. Investigators also examined recent maintenance history, noting a tail rotor torque overload and a C-box chip light warning that had occurred in the weeks prior to the accident. While these events were linked to the clutch wear, maintenance staff had not connected the two incidents or identified a clutch malfunction.
Findings
- The number 1 engine clutch coupling shaft sheared because of excessive loads placed on it by a worn and slipping clutch.
- The resulting overspeed of the power turbine caused the engine to lose power, leaving the pilot unable to arrest the descent rate.
- The failure of the number 2 engine was caused by a lack of lubrication during the period the aircraft was overturned.
- The aircraft weight and center of gravity were within legal limits, and the pilot was properly qualified for the operation.