C6/2005L Helicopter accident in the vicinity of the Vuomaselkä in Sodankylä on 21 September 2005

In the vicinity of Vuomaselkä reindeer round-up corral in Sodankylä municipality, about 7 km south-west from Kakslauttanen occurred an accident for a Hughes 259C helicopter, registration sign OH-HAY, owned by a commercial operator, on Wednesday September 21st 2005 at 10.25 Finnish time. The helicopter fell down on the ground from low altitude and was substantially damaged. The pilot and the accompanying observer were slightly injured. Accident Investigation Board Finland set September 26th 2005, through its decision number C6/2005L, an investigation commission to investigate the accident with investigator Ari Huhtala nominated as chairman and investigators Hannu Mäkeläinen and Arja Holopainen as commission members. The investigation commission was assisted by Juhani Mäkelä in the investigation of the accident site.

The crew of the helicopter that had been on a reindeer management flight, noticed that some of the reindeer at the right part of the flock turned around and begun to run away from the incoming opening of the reindeer fence. The helicopter moved to the right to turn back the escaping reindeer towards the opening. The reindeer however did not stop and the pilot descended in front of the reindeer to hover at 5 to 8 meters between sparsely growing high pine trees and simultaneously reduced the ground speed close to zero. During the hover the pilot suddenly felt a strong vibratory motion and the helicopter was rapidly swung to the left. The main rotor blades were struck to a tree on the left rear side of the helicopter and the helicopter fell on the ground simultaneously turning upside down. The pine, cut by the rotor blades, fell on the helicopter. The reindeer caretaker and the pilot left the helicopter through the broken windshield on right hand side. On the ground the reindeer herders, that had driven the reindeer, heard a powerful bang simultaneously when the helicopter main rotor blades hit the tree and the helicopter fell on the ground. The reindeer caretakers immediately started the rescue actions and simultaneously reported the accident to the Emergency Response Center of Lapland.

It was found out in the investigation, that the intake outer valve spring seat of the helicopter engine cylinder number 1 was broken in two pieces. Fractures were also noted to emerge in the spring seats of the three other cylinders. According to the investigations the fracture of the spring seat was caused by an unsuccessful carbon case hardening, with an outcome that the spring seats did not endure a longer usage.

The outer valve spring seat of the helicopter engine intake valve broke in two pieces and caused a rapid loss of the engine power. The helicopter rapidly turned around to the left around its vertical axis and drifted against a tree that was on the left rear side. As a consequence of the main rotor strike, the helicopter fell on the ground.

The accident investigation board recommends, that The European Aviation Safety Agency, EASA, and the Federal Aviation Administration, FAA; should take action so that all intake outer valve spring seats P/N LW10077 of production lot 17328 33-04 of Textron-Lycoming engine HIO-360-D1A shall be replaced by corresponding spring seats of another production lot.

C6/2005L Report (pdf, 2.28 Mt)