What happened
On February 10, 2009, a Bell 206L-3 helicopter, registration JA6055, was performing a transmission line inspection flight in the mountains of Gunma Prefecture. The aircraft departed from a temporary helipad in Numata City, carrying a pilot in command and a transmission line inspector.
During the mission, the pilot was attempting to locate an overhead intersection where two different power grids meet. After deviating from the planned route to avoid a ranch, the pilot became uncertain of his exact position near tower 36. While attempting to climb and maneuver to find the intersecting lines, the aircraft struck a transmission line belonging to a separate grid. This contact caused the aircraft to become uncontrollable, leading to a crash in a mulberry field. The impact resulted in two serious injuries to the crew members. The aircraft was destroyed, and the collision with the power line caused a significant electrical outage affecting approximately 17,000 households.
The investigation
The Japan Transport Safety Board (JTSB) examined the wreckage, the flight history, and the cockpit communications. Investigators analyzed the flight path, noting that the pilot was flying along a descending slope, which likely distorted his perception of altitude during the climb. The investigation also reviewed the maintenance of the transmission towers, noting that some identification plates were weathered and difficult to read. Furthermore, the investigators looked into the coordination between the pilot and the inspector, specifically regarding the use of call-outs for obstacles and position verification.
Findings
Several contributing factors led to the accident:
- The pilot continued the flight progression without successfully confirming his current position relative to the towers.
- The aircraft contacted an overhead transmission line during an attempt to climb and maneuver.
- Inadequate communication occurred in the cockpit, as the pilot did not ask the inspector for assistance in identifying tower numbers, and the inspector was distracted by inspecting foliage near the ground.
- The pilot's focus was heavily concentrated on spotting the intersecting lines, which prevented him from noticing the supporting towers that could have served as navigational landmarks.
- The visual background of the transmission lines made them difficult to spot against the landscape.