Bell 206B N5016U accident near Auberry, California

Probable cause

The pilot's failure to see and avoid a wire while maneuvering during low-altitude operations.

NTSB analysis

The pilot was flying a deer-surveying mission under contract with a state government agency with three state employees onboard. About 2 hours into the flight, witnesses observed the helicopter flying along a valley and colliding with one of two “skylines,” or cables strung between the towers of power transmission lines. The transmission lines consisted of two parallel steel skylines on top and three power conductor lines mounted about 20 feet below. The helicopter appeared to be flying straight and level prior to the collision, exhibiting no indications of distress. Examination of the engine and damage to the rotor system indicated that the engine was producing power at the time of the collision. Examination of the airframe and engine did not reveal any evidence of preaccident failure or malfunction. The transmission lines were depicted on the applicable sectional map, as well as a printed survey map located in the debris field. The power conductor lines were observed to sag about 70 feet below the skyline in the area of the accident. The power conductor lines were about twice as thick as the skyline. Additionally, the position of the sun would have hindered identification of the skyline by the pilot. In the area of the collision a second set of power lines were located about 200 feet below the lines depicted on the maps. As such, it is possible that the pilot misidentified these as the lines depicted on the maps. Neither of the sets of power lines were equipped with spherical visibility markers or similar identification devices.

The helicopter was equipped with a wire strike protection system; however, examination of its cutting surfaces revealed that it did not make contact with any lines. The helicopter struck the second skyline along the direction of flight, indicating that it flew below the first line with the main rotor blades striking the second line from below; since the wire struck outside of the cutters’ capture envelope, the wire strike protection system would not have been in a position to protect the helicopter. Federal regulations require that any planned construction of structures over 200 feet above ground level (agl) be filed with the Federal Aviation Administration. While the power lines’ height exceeds 200 feet agl at the center of the span, their construction predates adoption of the regulation. At the time of the accident, the state agency did not have any formal safety or operational training systems in place for passengers who fly on surveying missions.

Text from the NTSB. Open the NTSB docket for WPR10GA097 for the full record.