What happened
On July 7, 2018, an MBB-BK 117 C-2 helicopter, registration HB-ZRE, operated by REGA, was conducting a medical rescue mission from Lausanne Airport toward Roc de Veyges. The crew, consisting of a pilot, a flight paramedic, and a physician, arrived at the site to find a clearing that was suitable for an approach but presented challenges due to the sloping terrain.
After an initial unsuccessful landing attempt, the pilot performed a series of maneuvers to find a more stable position. This included climbing to a higher altitude and executing a 180-degree anti-clockwise rotation in a hover. The pilot then moved closer to the forest edge and performed a second 180-degree turn in a clockwise direction. During this second maneuver, the pilot heard a noise lasting approximately one to two seconds through the cockpit window, though no vibrations were felt through the pedals.
The pilot successfully landed the aircraft in the clearing. However, once the engines were shut down and the main rotor had stopped, an inspection revealed that the tail rotor blades had been damaged. Due to the damage, the mission was aborted, and the patient was transferred to an ambulance for evacuation by another medical helicopter.
The investigation
Investigators examined the aircraft and found surface damage on both tail rotor blades. The damage was located on the underside of the blades, specifically between 5 and 10 cm from the blade tips and at the two-thirds chord position from the leading edge. Following a detailed inspection by certified mechanics, the tail rotor blades and the mechanical positioning system were replaced on-site. No other damage to the aircraft was identified.
Findings
The investigation established that the contact with a tree branch caused the damage. The branch was flexible enough that it bent under the aerodynamic forces generated by the airflow from both the main and tail rotors during the final clockwise rotation in a hover.