What happened
On July 4, 2009, a Bell 206L4T helicopter, registration OK-AHD, operated by ALFA HELICOPTER, was performing a private flight near the village of Pavlov. The flight originated from the Olomouc heliport, with a stop at the Brno Tuřany LZS heliport to pick up additional passengers, including one adult and three preschool-aged children.
During the initial approach to a football field in Pavlov, the pilot noticed the aircraft began to vibrate and tend to yaw to the right while hovering at approximately 15 meters. The pilot aborted the landing and performed a second approach, which initially appeared successful. However, during a subsequent attempt to land at a nearby open field, the aircraft experienced an uncontrolled rightward rotation at an altitude of just 2 meters. Despite the pilot's efforts to counteract the movement with full left pedal input, the aircraft completed a 360-degree turn. Upon touchdown, the rotation stopped abruptly, causing extensive damage to the tail boom, tail rotor drive components, and the main rotor blades.
There were no injuries to the pilot or the four passengers on board.
The investigation
The ÚZPLN investigation examined the aircraft's technical condition, the pilot's experience, and the environmental conditions. The investigation confirmed that the aircraft was airworthy, within its maximum takeoff weight, and that the pilot held valid licenses and medical certification. The investigation also reviewed manufacturer warnings regarding Loss of Tail Rotor Effectiveness (LTE) in specific flight regimes, particularly during high-power, low-speed operations with certain wind conditions.
Findings
- The primary cause of the accident was the pilot's failure to follow recommended flight manual procedures during the final phase of the landing maneuver.
- The pilot, who had limited experience on this specific aircraft type, did not reduce engine power to decrease the main rotor's reactive torque when the uncontrolled rotation began.
- Adverse environmental factors contributed to the event, including high ambient temperatures and potential thermal updrafts that could have introduced vortices from the main rotor into the tail rotor area.
- The aircraft was susceptible to LTE, a known aerodynamic characteristic of the Bell 206L4T during low-speed flight under specific wind conditions.