What happened
On October 13, 2017, an MBB BK 117 helicopter, registration N140SF, was engaged in a night training session near Canton, Illinois. The flight instructor was training a new company pilot using night vision goggles. While the aircraft was in a hover, the instructor attempted to demonstrate the operation of the controllable search light to the pilot in the right seat.
To reach the light's control switch, the instructor moved his left hand from the left cyclic to the cyclic, while simultaneously using his right hand to grab the throttles of the right collective. During this maneuver, the instructor inadvertently pushed down on the collective, causing the helicopter to settle toward the runway. In an attempt to correct the descent, the instructor pulled up with his left hand, which caused the helicopter to move rearward. When the instructor pushed the cyclic forward to correct the rearward movement, the aircraft touched down heavily, causing the main rotor to impact the upper wire strike protection unit.
There were no injuries resulting from the event.
The investigation
The investigation focused on the flight instructor's actions during the light demonstration. The instructor reported that there were no mechanical malfunctions or failures present in the helicopter prior to the accident that would have prevented normal operation. The impact between the main rotor and the wire strike protection unit caused substantial damage to the main rotor.