What happened
At approximately 1559 Pacific daylight time, an MD500D helicopter, registration C-GWPQ, departed Terrace Airport, British Columbia, to retrieve a geological survey crew from a mountain location 35 nautical miles northwest of the airport. The mission required the pilot to perform a toe-in procedure on a steep, 25-degree slope within a mountain cirque.
During the maneuver, a loud bang was heard, followed by the helicopter's tail dropping low. The aircraft entered an uncontrolled right turn and struck the terrain roughly 30 yards downhill from the intended pickup point. The impact ruptured the fuel cell compartments, triggering an intense post-crash fire that destroyed approximately 50 per cent of the aircraft by weight. While the geological crew on the ground assisted the pilot, the pilot sustained serious injuries. The helicopter was destroyed by the impact and subsequent fire.
The investigation
Investigators examined the wreckage and various components to identify the cause of the sudden loss of control. While the engine, transmission, and rotor head were heavily damaged by fire, detailed inspections of the engine, fuel control unit, and gearbox revealed no mechanical defects. The main rotor blades showed evidence of being in a positive pitch position at the time of impact, and no ground scars suggested a main rotor blade strike occurred at the initial pickup site.
Analysis of the tail rotor components revealed that the tail rotor drive shaft had twisted and fractured, indicating the tail rotor experienced a sudden stoppage under power. Ground marks were consistent with a tail rotor strike to the right and rear of the aircraft. Investigators also used X-ray analysis on the severely burned cyclic trim actuators, which showed the lateral actuator was near neutral and the longitudinal actuator was near full-forward trim. This finding suggested that a trim runaway was unlikely to have contributed to the accident.
Findings
- The tail rotor strike and subsequent clockwise yaw were consistent with a reduction in engine torque.
- The sudden tail drop and the cause of the initial loud bang could not be determined due to the extensive destruction of physical evidence by the post-crash fire.
- The fuel tank rupture during the crash sequence led to the intense fire that caused the pilot's injuries and the loss of critical evidence.