Incident Overview
The aircraft was being operated for a pipeline patrol mission. The pilot had picked up a passenger at Williams Pipeline in Minneapolis, Minnesota, and then flew eastward along Interstate 90 en route to a pipeline near Wausau, Wisconsin. While cruising at an altitude of approximately 400 feet, both the pilot and passenger heard the engine begin to produce a grinding or knocking noise. Almost simultaneously, they experienced a complete loss of engine power. The pilot then lowered the collective and turned approximately 40 degrees toward a sandy, upward sloping area of ground. During the descent, the nose of the aircraft remained low, causing the tailboom to raise. The main rotor blades subsequently struck the tailboom.
Wreckage Examination
Following the accident, an examination of the wreckage was conducted. It revealed that the number four piston rod had protruded through the engine case. Additionally, the engine case was cracked from the hole where the piston rod exited to the base of the number four cylinder.
The investigation did not state a probable cause.