What happened
On September 21, 2005, a Hughes 269C helicopter, registration OH-HAY, was conducting a reindeer management flight near the Vuomaselkä reindeer corral in Sodankylä. During the operation, the crew observed a portion of the reindeer herd breaking away from the fence opening. In an attempt to redirect the animals, the pilot maneuvered the aircraft to the right and then descended to a hover at an altitude of approximately 5 to 8 meters among sparse pine trees to intercept the fleeing herd.
While hovering, the pilot experienced a sudden, violent vibration, and the helicopter rapidly yawed to the left. The main rotor blades struck a pine tree located at the rear left of the aircraft, causing the helicopter to lose stability and crash upside down. A tree severed by the rotor blades fell onto the wreckage. The pilot and the accompanying observer sustained minor injuries and were able to exit the aircraft through the broken right windshield. Reindeer herders on the ground witnessed the impact and immediately initiated rescue efforts.
The investigation
An investigation by the Finnish Safety Investigation Authority (OTKES) focused on the mechanical failure of the engine. Technical analysis of the Textron-Lycomming HIO-360-D1A engine revealed that the outer valve spring seat of the intake valve in cylinder number 1 had fractured. Further inspection showed that the spring seats in the other three cylinders also exhibited developing cracks.
Laboratory testing conducted by the University of Oulu determined that the fractures were caused by an unsuccessful carbon case hardening process. The hardening layer was too thin and the hardness levels did not meet minimum requirements, resulting in a failure to withstand prolonged operational use. The investigation also noted that similar engine failures had occurred in other aircraft using components from the same production lot.
Findings
- The primary cause of the accident was the fracture of the intake valve spring seat in cylinder number 1, which triggered a rapid loss of engine power.
- The loss of power caused the helicopter to yaw sharply to the left and drift into a tree.
- The failure of the spring seats was attributed to improper carbon case hardening during manufacturing.
- All damaged spring seats belonged to the same production lot (Lot 17328 33-04).
- The pilot's decision to hover in a confined area of trees contributed to the severity of the impact, though the engine failure was the initiating event.