What happened
The flight originated at Williams Pipeline in Minneapolis, Minnesota, where the pilot picked up a passenger. The aircraft then departed, heading eastward along Interstate 90 with the destination being a pipeline site near Wausau, Wisconsin. During this leg of the journey, the aircraft was operating at an altitude of approximately 400 feet above ground level.
While in flight, both the pilot and the passenger reported that the engine began emitting a grinding or knocking noise. This auditory warning was followed almost instantaneously by a complete loss of power. In response to the emergency, the pilot immediately dropped the collective pitch control and executed a turn of approximately 40 degrees toward nearby sandy ground that sloped upward.
During the descent and impact sequence, the nose of the aircraft remained in a low attitude while the tailboom was raised significantly. As the aircraft settled onto the terrain, the main rotor blades made contact with the tailboom structure.
The investigation
A physical examination of the wreckage provided critical mechanical insights into the cause of the power loss. Investigators found that the piston rod for the number four cylinder had protruded completely through the engine case. This catastrophic failure resulted in a crack extending from the hole created by the rod all the way to the base of the number four cylinder.