What happened
On August 18, 2010, a Bell 412EP helicopter, registered JA6796, was conducting a multi-purpose flight in the eastern Seto Inland Sea. Operated by the Japan Coast Guard, the mission included a patrol, a demonstration flight, and a survey of abandoned vessels. The flight departed from Hiroshima Airport at 13:47 JST.
While navigating near the islands of Sanagijima and Oshima in Kagawa Prefecture, the aircraft struck overhead power lines spanning the gap between the two islands. The impact severed three high-voltage wires belonging to Shikoku Electric Power Co. The helicopter subsequently crashed into the sea at approximately 15:10 JST. All five persons on board, including the pilot in command, co-pilot, and three crew members, suffered fatal injuries. The aircraft was destroyed upon impact, though no post-crash fire was reported.
The investigation
The Japan Transport Safety Board (JTSB) conducted an investigation involving on-site examinations and interviews with witnesses, including the captain of a nearby ferry. Investigators analyzed flight data from the aircraft's power line steel unit and examined the severed overhead wires.
Technical assessments focused on the visibility of obstacle lights on the power line towers. The investigation looked into the structural limitations of the towers, which prevented the use of standard obstacle markings, necessitating the use of medium and high-intensity lights instead. The team also evaluated the flight path and the visibility of the Sanagi Lines from the aircraft's trajectory.
Findings
- The aircraft struck the Sanagi Lines, which consisted of three overhead wires connecting Oshima and Sanagijima.
- The collision caused a significant power outage affecting 577 households in the region.
- Trees located near the Sanagijima tower obstructed the field of view for the medium-intensity white obstacle light, making the wires difficult to detect from certain approach angles.
- While the aircraft was flying under Visual Flight Rules (VFR), the combination of the aircraft's descent and the obscured visibility of the hazard led to the impact.