B3/2006L Aircraft accident at Hirsijärvi on 8 August 2006

On 8 August 2006, at Lake Hirsijärvi in Kisko, at 20:16 local time, an accident took place in which a privately owned Ikarus C42S ultralight aircraft OH-U396 equipped with floats struck a 20 kV power line and crashed into Lake Hirsijärvi. The pilot who was alone in the aircraft perished. On 10 August 2006, the Accident Investigation Board Finland (AIBF) appointed in its decision no. B3/2006L an investigation commission with accident Investigator Ismo Aaltonen as chairman and investigator Tuukka Takala as a member of the commission.

The aircraft had taken off about 35 minutes earlier from the Inkoo archipelago with intention of visiting Lake Hirsijärvi. The aircraft flew between 220–320 metres above mean sea level. At Lake Seljänalanen in Karjalohja, the pilot apparently inspected the landing site, flying as low as 6 m above the lake surface (56 m above sea level). From there he continued the flight towards Lake Hirsijärvi approaching the south part of the lake from the east. The pilot turned right over the lake passing on the north side of a power line. When over the lake, he descended to a height of 20–30 m above the lake surface, flying northwest over the prospective landing site. At 20:16:41, flying at a height of 20 m, the aircraft struck the cables of a 20 kV power line at a speed of 62 knots (115 km/h). After hitting the cables, the aircraft crashed into the lake remaining afloat in an inverted position.

No technical defects preceding the accident were found in the aircraft. The pilot had prepared the flight using a GPS unit and maps. The power line did not appear in these. It was the habit of the pilot to fly the first landing site inspection at a low altitude. The altitudes used by the pilot for inspecting the landing site were not accordant with the altitudes given in the training instructions of the Finnish Aeronautical Association (FAA). At the time of collision, the pilot was making a very low altitude landing site inspection below treetop level. According to the view of the investigation commission, the power line crossing Lake Hirsijärvi was visible only from the pillars supporting it. The pillars were located on each side of the lake at a distance of 270 m apart. The pillars were hidden by trees and their tips were below the treetops, causing the power line to be almost impossible to detect from the air. There were no warning markings on the power line.

The pilot did not see the power line while flying low over the lake. In spite of his flight planning, the pilot did not know about the power line crossing the lake, because it was not marked on the maps he had used. According to his habit, the pilot had not made the first inspection of the landing site at a safe altitude before descending to a low altitude. The power line was extremely difficult to detect from the air. There were no warning markings on the power line and the pillars supporting it were hidden by trees.

The investigation commission made no safety recommendations. However, the commission emphasizes that, when operating from places not specially arranged for flight operations, pilots should check the safety of the takeoff and landing site by checking it from the ground before commencing the flight, and that pilots should fly the landing site inspection at a safe altitude above obstacles.

B3/2006L Report (pdf, 0.73 Mt)