What happened
On a clear day in British Columbia, a Eurocopter Lama SA315B, registration C-GXYM, was performing heavy-lift logging operations near Buhl Creek. While the aircraft was engaged in its third logging cycle on the slopes of Sawtooth Mountain, the pilot was attempting to lift a load of logs using a 150-foot longline. During the maneuver, the pilot encountered difficulties and released the load from the lower hook. Following a mechanical screeching sound, the helicopter briefly gained altitude before a rapid descent began. The aircraft struck a steep, 45-degree slope, resulting in the destruction of the helicopter and the fatal injury of the pilot.
The investigation
Investigators examined the wreckage and the mechanical state of the aircraft's transmission components. While the engine remained functional after the impact, the damage to the main rotor blades indicated that the rotor system was not being driven at the time of impact. The investigation focused on the input freewheel unit (IFWU) and the drive shaft assembly.
Technical reviews revealed that the IFWU had been subjected to intense operational stress, performing approximately 30 loading cycles per hour. Furthermore, the investigation found that a required 80-hour inspection of the IFWU had not been completed, which prevented the detection of internal wear and contamination. The aircraft was also operating at a density altitude of over 6000 feet, in a flight regime that placed it within a restricted area of the flight manual regarding continuous operation at low speeds and altitudes.
Findings
- The primary cause of the accident was the failure of the input freewheel unit (IFWU) and drive shaft assembly, which prevented the engine from driving the main rotor.
- The IFWU failed due to accelerated wear on internal surfaces caused by high-frequency loading cycles and the presence of abrasive metal debris trapped within the unit.
- The operator had not performed the mandatory 800-hour inspection of the IFWU, leaving the condition of the component unverified.
- The heavy-lift flight profile, involving frequent rapid descents and climbs, caused the IFWU to repeatedly disengage and re-engage, further accelerating component degradation.
- The aircraft was operating at a height and airspeed combination that left insufficient margin for recovery following the loss of rotor drive.