Sequence of Events
On May 27, 2007, a Heli-Transport Services Inc. Eurocopter AS350 B1 Astar helicopter (registration C-GZCN, serial number 2207) was supporting mining exploration for Melkior Resources Inc. in the Otish Mountains northeast of Chibougamau, Quebec. The helicopter departed a mining camp at 0800 eastern daylight time en route to a drill site 20 nautical miles southeast. Approximately four minutes after takeoff, the helicopter broke up in flight and descended rapidly. Witnesses observed a trail of smoke from the helicopter. The aircraft was later found partially inverted in a swamp 8 nautical miles from the camp. The main rotor blades had struck the cockpit in flight, fatally injuring the pilot, the sole occupant. The tailboom separated from the fuselage, and wreckage was scattered over 700 feet.
Maintenance and Testing
The helicopter had undergone extensive maintenance at Heli-Transport's base in Trois-Rivières between March 15 and May 14, 2007, including a 3500-hour inspection of the main gearbox (MGB) epicyclic reduction gear module. During this work, the MGB epicyclic reduction gear module was removed and sent for overhaul. After reinstallation, ground and flight tests revealed a low rotor rpm indication (later confirmed as an indicator problem), ground resonance after landing, and a humming noise. The MGB chip warning light illuminated once, and fine metal fuzz was found on the magnetic chip detector. The special inspection procedure was performed, and no further warnings appeared. The aircraft was released for flight on May 15, 2007.
Over the following 10 days, the pilot and an aircraft maintenance engineer (AME) monitored the low Nr indication and addressed the ground resonance and chip light incidents. On May 27, before the accident flight, the low rotor rpm warning horn sounded, and a low-frequency hum was heard at idle power. The hum disappeared when power was increased.
Post-Accident Examination
Transportation Safety Board of Canada (TSB) investigators examined the wreckage. The engine was producing power at impact. The main rotor system had impact marks indicating rotation during the strike. The MGB was split at the main rotor shaft lower casing assembly line. The six main rotor shaft retaining bolts and the self-locking centre bolt were found unscrewed in the bottom of the sun gear. Snap ring segments were found above the phonic wheel in the rotor shaft upper casing.
Examination revealed the snap ring within the MGB epicyclic reduction gear module had been installed before the spacer assembly, contrary to the correct sequence. This allowed the snap ring to slip from its groove, preventing the locking tabs from holding the mast retaining bolts. The bolts loosened and fell out, permitting vertical movement of the main rotor shaft and causing the rotor blades to strike the forward fuselage.
The tailboom had separated at the rear fuselage bulkhead rivet line, where a recent modification had been performed per Airworthiness Directive F-2004-035. Metallurgical analysis found no deficiencies in the rivets or installation; the failure was due to overload during the in-flight breakup.
Probable Cause
The accident was caused by incorrect installation of the snap ring relative to the spacer assembly in the main gearbox epicyclic reduction gear module, which allowed the snap ring to slip from its groove. This prevented the locking tabs from securing the mast retaining bolts, which subsequently loosened and fell out, leading to vertical movement of the main rotor shaft and a rotor blade strike on the forward fuselage.