What happened
On November 29, 2011, a Hiller UH-12E helicopter was performing utility work near Stahlberg, Germany, to install new power lines along high-voltage masts. The mission involved using an external load sling, approximately 10 meters in length, with a 130 kg weight attached to the hook to assist in positioning cables.
After completing the placement of the third cable, the pilot intended to re-tension the final line because it was sagging too low. To achieve this, the pilot initiated a left rotation around the vertical axis at approximately 120 ft AGL without forward airspeed. During this maneuver, the helicopter suddenly began an uncontrolled rotation to the right. The pilot attempted to correct the movement using pedal inputs, but the rotation accelerated. During the descent, the external load sling made contact with the ground and was dragged along the terrain; the pilot successfully unhooked the load before the final impact.
During the uncontrolled descent, a main rotor blade struck a work trailer parked in a field, leading to the destruction of the rotor system. The helicopter struck a field below high-voltage lines, resulting in two persons slightly injured and heavy damage to the aircraft.
The investigation
The BFU examined the wreckage and the mechanical components of the tail rotor assembly. Investigators found that the tail rotor drive shaft had experienced a torsional break, and the tail rotor blades were destroyed, with pieces found scattered away from the main wreckage. The tail rotor pitch change mechanism was found jammed in the "high-power" position, though investigators concluded this jamming was likely caused by the impact of the crash rather than a pre-existing mechanical failure.
Technical inspections confirmed that the tail rotor was receiving power from the engine up until the moment of impact. The control cables were connected to the pedals and showed no signs of fraying or breakage prior to the accident; the breaks in the cables were consistent with the structural failure of the tail boom during the crash.
Findings
- The helicopter was properly maintained, and the weight and balance were within limits.
- There was no evidence of a mechanical failure in the tail rotor or the flight control system.
- The pilot had relatively low experience with this specific aircraft model and was performing his first flight operations in this specific utility role.
- While the phenomenon of Loss of Tail Rotor Effectiveness (LTE) was considered, investigators found it unlikely due to the low wind speeds and the fact that the aircraft was not in a high-risk flight configuration (such as high power with significant tailwind).
- The accident was most likely caused by a pilot error during a rotation around the vertical axis, leading to an uncontrolled loss of control.