What happened
On May 27, 2007, a Eurocopter AS350 B1 Astar helicopter, registration C-GZCN, departed a mining camp in the Otish Mountains, northeast of Chibougamau, Quebec. The aircraft was performing a routine crew change for a mining exploration project. Shortly after takeoff, witnesses observed smoke trailing from the aircraft before it descended rapidly toward the ground.
The helicopter crashed in a swamp approximately 8 nautical miles from the camp. The impact was catastrophic; the main rotor blades struck the forward fuselage, causing the aircraft to break apart. The tailboom separated from the main cabin, and the wreckage was scattered over a 7/00-foot area. The pilot, who was the sole occupant, sustained fatal injuries.
The investigation
Investigators examined the wreckage and the aircraft's maintenance history. The helicopter had recently undergone extensive maintenance, including a 3500-hour inspection of the main gearbox (MGB) epicyclic reduction gear module. During this process, components were removed and sent to a facility for overhaul before being reinstalled.
Technical examinations of the MGB revealed that the main rotor shaft retaining bolts and a self-locking center bolt had unscrewed. The investigation focused on the installation sequence of the internal components. Specifically, the team looked at the relationship between the snap ring and the spacer assembly within the gearbox. Metallurgical analysis of the tailboom showed that while the area had recent structural modifications, the failure was caused by loads exceeding design limits during the in-flight breakup, rather than defective riveting.
Findings
- The aircraft maintenance engineers failed to follow the correct installation sequence for the snap ring relative to the spacer assembly.
- The incorrect installation sequence allowed the snap ring to slip, which prevented the mast retaining bolts from being properly secured, leading them to loosen and allow vertical movement of the main rotor shaft.
- Maintenance personnel did not consult all applicable sections of the work cards, likely due to the complexity and length of the provided instructions.
- Existing troubleshooting procedures and symptoms—such as low rotor RPM indications, a low-frequency hum, and metal particles in the oil—were not previously linked to this specific assembly error.
- The maintenance manual's borescope inspection instructions were insufficient, as they did not specify the minimum number of threads required to confirm a secure installation, providing a false sense of security.