What happened
On a flight returning to Sudbury, Ontario, a Bell 204B helicopter, registration C-GRGY, was performing its second approach to the airport when the pilot experienced a sudden loss of directional control. During the initial approach, the pilot noted a rightward yaw that could not be corrected with left pedal, though the aircraft eventually returned to a normal flight state. During the subsequent attempt to land, raising the collective caused a loud thump and the left pedal moved to full deflection, forcing the pilot to declare an emergency.
The crew, consisting of a pilot and an aircraft maintenance engineer, managed to maintain heading by adjusting the throttle. The helicopter crossed the runway threshold at a low altitude and touched down at approximately 30 KIAS. The aircraft skidded roughly 90 feet before coming to a complete stop. There were no injuries resulting from the incident.
The investigation
Investigators examined the aircraft's tail rotor pitch change mechanism and discovered that a cable had detached from one of the two speed rigs. The inspection revealed that the threaded end of this speed rig had not been secured with lock wire, and the other speed rig was also found to be missing lock wire.
While the aircraft maintenance engineer was experienced with smaller helicopters, this was his first time working on this specific type of quick-disconnect turnbuckle. Furthermore, the inspection point for the lock wire was difficult to observe. The investigation also looked into the independent inspection process, noting that the pilot had performed the check but failed to detect the error. The pilot's maintenance training for this specific aircraft was found to be elementary and lacked the depth required for detailed control system inspections.
Findings
- The tail rotor pitch change cable speed rigs were not secured with lock wire according to approved procedures.
- The failure to lock wire the speed rig caused a cable to come undone, resulting in the loss of tail rotor authority.
- The independent control inspection did not meet the standards required by the Canadian Aviation Regulations, as the missing lock wire went undetected.