What happened
On February 5, 2009, at 12:46, a Hughes 369D helicopter, registration OH-HWH, crashed in the Rasikumpu area of Pyhäselkä during a power line trimming mission. The aircraft was equipped with a 302 kg topsaw attachment extending 17 meters below the helicopter.
Prior to the flight, during refueling with the engine idling, the pilot noted that the N1 idle RPM was 61%, which was below the flight manual's required range of 64–65%. Following an engine run-up and a change of pilot, the decision was made to proceed with the flight. After approximately ten minutes of hovering for treetop trimming, the engine suddenly lost power. The pilot was forced to maneuver the aircraft into the woods to avoid a more catastrophic impact. Upon hitting the ground, the helicopter overturned onto its left side. The pilot, who was the sole occupant, sustained minor injuries and managed to exit the wreckage through a broken windshield. The engine continued to run after the impact, and the pilot was unable to shut it down due to the aircraft's position and minor injuries.
The investigation
The investigation focused on the sudden loss of engine power and the technical state of the aircraft prior to takeoff. Investigators examined the engine's Gas Producer Fuel Control Unit and conducted laboratory tests on the components. The investigation also reviewed the operator's decision-making process regarding the N1 idle RPM deviation and the maintenance of flight manuals.
Findings
Technical analysis revealed that the bearing of the Gas Producer Fuel Control Unit failed due to insufficient lubrication. While the grease used and the materials in the bearing met all required standards, the investigation identified four potential causes for the lubrication failure: degradation of the lubricant due to long-term storage, a crack or defect preventing the lubricant from staying in the bearing, wear caused by continuous power adjustments during hovering operations, or a manufacturing error resulting in insufficient initial lubricant. None of these specific causes could be definitively confirmed or ruled out.
A contributing factor to the accident was the decision to commence the flight despite the N1 idle RPM being outside the limits specified in the flight manual. The investigation concluded that the low idle RPM was likely a symptom of the mechanical defect that had already begun to develop before the flight.