Incident Overview
On November 29, 2011, a single-engine Hiller UH-12E helicopter operated by a German company was involved in an accident near the locality of Stahlberg. The aircraft, manufactured in 1962 with serial number 2039, was engaged in industrial work involving the transport of pre-cables for high-voltage power lines. During the operation, an external load of approximately 130 kg was suspended from the helicopter's hook by a 10-meter cable.
Sequence of Events
After completing the third and final cable placement for the day, the pilot initiated a maneuver to tension the last cable, which had sagged excessively. The pilot rotated the helicopter to the left around its vertical axis at an altitude of approximately 120 feet above ground level without forward movement. According to witness accounts, the aircraft unexpectedly began rotating rapidly to the right. The pilot reported being unable to counteract this rotation using pedal inputs. The external load cable made contact with the ground and was released by the pilot before impact. The helicopter struck a field beneath high-voltage lines, resulting in severe structural damage.
Investigation Findings
The German Federal Bureau of Aircraft Accident Investigation (BFU) examined the wreckage and found that both main rotor blades were destroyed, and the tail rotor drive shaft had fractured. The tail rotor pitch control mechanism was found jammed in the "high-power" position, a condition attributed to external impact during the crash rather than pre-existing mechanical failure. Technical inspections revealed no prior defects in the tail rotor system or control cables.
Probable Cause Analysis
The investigation concluded that the accident was likely caused by an unintended rotation in the direction of torque during a hover maneuver outside the ground effect. The pilot's reaction to this unexpected behavior was deemed delayed or insufficient, leading to a complete loss of control. Factors such as low forward speed and high power settings may have contributed to the aerodynamic instability, although classic Loss of Tail Rotor Effectiveness (LTE) was considered less likely given the aircraft's performance margins.