What happened
On September 22, 2005, a Eurocopter AS 350 BA was engaged in a forest liming operation in the Pfahlberg woodland area near Oberwiesenthal, Germany. After a refueling stop and approximately one hour of flight time, the pilot attempted to depart westward with a full external load of lime.
As the helicopter reached an altitude of approximately 140 feet in a vertical climb, the aircraft suddenly began rotating to the left around its vertical axis. The pilot attempted to correct the rotation using right tail rotor pedal inputs and by reducing engine power, but these actions were ineffective. The aircraft entered a series of yawing motions, lost altitude, and struck trees approximately 25 meters tall at the edge of the loading site. The helicopter subsequently fell vertically into the forest floor. The pilot sustained serious injuries, and the aircraft was destroyed. The accident also caused damage to the surrounding forest.
The investigation
The BFU investigation focused on the mechanical state of the aircraft and the environmental conditions during the departure. Investigators found that the two-part tail rotor shaft, which featured a flexible coupling between the front and rear sections, had become separated. The shaft cover near the flexible coupling was found heavily damaged.
At the time of the accident, the helicopter was operating at its maximum allowable takeoff weight. The investigation also examined the meteorological conditions, noting that while the ground-level wind was perceived as calm, a 13-knot wind from the east was recorded at a nearby weather station, which could have created turbulence or lee-side effects near the terrain.
Findings
The investigation established that the primary cause of the accident was a loss of tail rotor effectiveness. This phenomenon was triggered by a combination of several critical factors:
- The helicopter was operating at its maximum takeoff weight.
- The specific configuration of the loading site required a vertical climb to clear the surrounding high trees, meaning the aircraft was operating outside of ground effect.
- High engine power was required for the climb, creating a strong torque effect that the tail rotor could not sufficiently counteract.
- An easterly wind at higher altitudes contributed to the loss of directional stability.
- The pilot'elected to depart toward the west to avoid the low morning sun, which meant departing with a tailwind rather than into the wind, further complicating the stability of the aircraft.