C1/2009L Helicopter collision with the ground at Pyhäselkä on 5 February 2009

An accident occurred at Pyhäselkä on Thursday, 5 February 2009 at 12:46. A Hughes 369D helicopter, registration OH-HWH, collided with the ground on a power line sawing flight. The pilot, who was the sole occupant in the helicopter, sustained minor injuries. The helicopter was destroyed. On 16 February 2009 Accident Investigation Board of Finland appointed investigation commission C1/2009L for this incident. Investigator Juhani Hipeli was named Investigator-in-Charge, accompanied by Investigator Hans Tefke and Captain Juha Kepsu serving as members of the commission. Mr. Hannu Kokkonen, a teacher of aircraft maintenance, was designated as technical expert to the commission.

The helicopter was equipped with a topping saw (heli-saw) weighing 302 kg and hanging 17 metres below the helicopter. Following its previous flight the helicopter was refuelled while the engine was idling. At this time it was noticed that the N1 idle RPM was 61%, whereas according to the flight manual the desired value is 64–65%. After an engine runup and pilot change it was decided that the aircraft could take off again. After ten minutes of treetop trimming in hover the engine suddenly lost power. In the sudden, unexpected circumstances the pilot’s only option was to steer the helicopter into the woods. When the helicopter hit the ground it tipped over onto its left side. On the ground the pilot was unable to turn the engine off. The pilot reported the accident to the flight’s supervisor by radio after which he managed to get out the helicopter on his own.

When the helicopter’s engine was inspected, it was detected that the bearing of the Gas Producer Fuel Control Unit was broken. Laboratory tests concluded that the bearing failed due to insufficient lubrication of the spool bearing. Tests established that the grease that was used as the lubricant met the requirements and that the materials used in bearing components met their requirements. The investigation aimed at determining the factors that possibly contributed to the insufficient lubrication of the bearing. While four possible factors were determined, none of them could be definitively confirmed or eliminated. The investigation also called attention to the fact that the RFM’s N1 idle RPM requirement was not followed when the decision was made to take off again. The N1 idle RPM that was lower than the desired value was possibly a symptom of a defect that was already present before the flight.

The accident was caused when the bearing of the Gas Producer Fuel Control Unit failed due to insufficient lubrication. The malfunction of the Fuel Control Unit caused a sudden loss of engine power that resulted in an emergency landing in which the helicopter was destroyed. Possible causes for degraded lubrication could be attributed to shelf life, a flaw or impact damage that prevented the bearing from retaining its lubrication, wear due to continuous fuel scheduling variations due to hovering operations, or a manufacturing flaw resulting from an insufficient amount of lubricant. None of the aforementioned factors could be definitively confirmed or eliminated.

The decision to take off again even though the N1 idle RPM was not within the flight manual’s limits was a contributing factor. It is highly likely that the N1 idle RPM that was lower than the desired value was a symptom of a defect that was already present before the flight.

The investigation commission made no safety recommendations.

C1/2009L Report (pdf, 0.42 Mt)