What happened
On a mission involving seismic contract work in northeastern British Columbia, a Bell 212 helicopter, registration C-FHDY, crashed into a mountain slope near the Pink Mountain area. The aircraft was performing vertical reference sling operations, transporting a 2700-pound compressor using a 100-foot longline. While hovering at an altitude between 50 and 100 feet above a drilling site, the helicopter experienced a sudden loss of tail rotor authority. This caused the aircraft to begin rotating rapidly to the right. Although the pilot managed to release the load and clear a nearby ridge, the rotation continued as the helicopter descended into a mountain cirque. The aircraft struck a slope at approximately 5200 feet, resulting in one fatality and substantial damage to the helicopter.
The investigation
Investigators examined the wreckage and found that the tail-rotor drive shaft transflex coupling had failed. The examination revealed that the coupling's splines were worn, discolored from overheating, and lacked necessary lubrication. While a grease leak had been detected in the transmission compartment 34 flight hours prior, it had been misdiagnosed as originating from the main drive shaft coupling rather than the tail-rotor coupling.
Further analysis of the daily inspection revealed that the overheating was not detected. The inspection was limited to a visual check with a flashlight, which was insufficient because the heat-sensitive indicators were obscured. Additionally, the use of a zinc chromate primer stripe made detecting temperature changes difficult. The investigation also noted that the pilot was sitting on a pillow to facilitate visibility through the bubble window and was not wearing the shoulder harness, a common practice in vertical reference operations. This lack of restraint allowed centrifugal forces to throw the pilot's upper body into the window, preventing him from closing the throttles to regain control.
Findings
- The helicopter lost tail rotor authority due to a loss of lubrication in the transflex coupling.
- The overheating of the coupling went undetected during the daily inspection because the inspection was not thorough enough to view the temperature indicators.
- The pilot's failure to use the shoulder harness prevented him from maintaining control of the aircraft's throttles during the rapid rotation.
- The use of a zinc chromate stripe as a heat indicator was unreliable and did not meet manufacturer recommendations.
- A previously reported grease leak was misidentified, masking the deteriorating condition of the tail-rotor coupling.