What happened
During an early morning herbicide spraying mission, a Bell 206B helicopter, registration C-GLRA, was departing from a logging road intersection. The aircraft had been loaded with 340 litres of herbicide and 25 US gallons of fuel, reaching its maximum allowable takeoff weight of 3200 pounds. As the helicopter lifted off at an altitude of approximately 10 feet and an airspeed of less than 30 miles per hour, it experienced an uncommanded yaw to the right.
To avoid a grove of trees located to the right of the flight path, the pilot increased collective pitch. This maneuver caused the aircraft to enter a rotation. While the pilot was attempting to regain control, the tail rotor struck a single spruce tree. The impact caused the tail-rotor gearbox to separate from its mounts, resulting in a total loss of heading control. The helicopter subsequently struck the ground and rolled onto its left side. The aircraft sustained substantial damage, and the pilot sustained minor injuries.
The investigation
Investigators examined the flight controls and the tail rotor drive system at the accident site. No mechanical anomalies or pre-impact failures were identified in the flight control or drive systems that could have caused the loss of tail rotor authority.
The investigation also looked into the cockpit configuration, specifically the placement of the Ag Nav remote switch. This switch was located between the cyclic stick grip and the spray control box, making the herbicide dump switch difficult to reach while the pilot was manipulating the cyclic.
Findings
- The helicopter was operating in a high-power, low-airspeed environment with a left, quartering tailwind, which created conditions conducive to unanticipated right yaw and loss of tail rotor effectiveness (LTE).
- The takeoff was performed downwind to avoid a complex hovering turn, which contributed to the unstable flight conditions.
- The physical positioning of the Ag Nav remote switch likely hindered the pilot's ability to quickly activate the spray dump switch, which might have otherwise assisted in the recovery.
- The pilot's attempt to clear the trees by increasing collective pitch exacerbated the aircraft's rotation.