What happened
On a day with clear visibility and calm winds, a Boeing Vertol BV-107 II helicopter, registered C-GHFT, was performing helicopter logging operations in the South Bentinck Arm, approximately 20 nautical miles from Bella Coola, British Columbia. While returning to a log pick-up site, the crew reported experiencing vibrations. Shortly after, the pilots transmitted that the aircraft was behaving unusually and intended to return to their maintenance base. Within ten seconds of these reports, the aircraft suffered a catastrophic in-flight breakup, falling to the ground in several sections. The accident resulted in two fatalities.
The investigation
Investigators examined the wreckage, which was spread across a three-hectare area. While the main fuselage sections remained relatively close to one another, smaller debris including rotor blade segments and drive shaft components were found along the flight path. The investigation confirmed that the engines had not suffered power loss prior to the breakup.
Detailed analysis of the rotor systems showed that while the forward blades remained attached to the hub, the aft rotor blades had struck the helicopter's fuselage. This contact caused significant damage, including scuffing, paint transfer, and structural fractures. Reconstruction of the fuselage revealed that the aft blades had entered the cabin, severing critical components including the synchronizing shafts and control cables.
Technical examinations focused on the speed trim actuator. Investigators found that the limit switches used in the actuator did not meet manufacturer specifications for arm travel. This discrepancy prevented the retract limit switch from activating. Consequently, the end fitting adapter was driven into the torsion bar, creating a massive tensile load of over 1300 pounds on the jack screw nut. The investigation determined that the small thread-root radius of this nut acted as a stress concentrator, leading to fatigue failure.
Findings
- The failure of the retract limit switch to activate caused the mechanism to exceed its intended range of motion.
- This mechanical over-extension imposed a high tensile load on the jack screw nut.
- The fatigue failure of the jack screw nut was driven by a stress concentration at the 0.001-inch thread-root radius, which exceeded the material's endurance limit.
- The resulting failure of the speed trim actuator caused the aft rotor system to become unstable, leading the blades to strike the fuselage and causing the aircraft to break up in flight.