Schweizer 269 C-1 N2011A accident near Sunol, California

Probable cause

The pilots' failure to maintain adequate clearance from transmission wires during cruise flight. Contributing to the accident were the dark night and low fog.

NTSB analysis

The certified flight instructor (CFI) and student pilot were on the return portion of a night cross-country flight to the airport where the helicopter was based. After refueling, the helicopter departed and flew a path over an interstate highway. Radar data showed the flight continued along the interstate for 3 miles varying between altitudes of 300 to 700 feet mean sea level toward the accident site. The last recorded altitudes of the helicopter were about 250 feet above the interstate. The tops of the surrounding hills were about 150 feet above the interstate, with transmission towers that extended an additional 120 feet above the hills. Witnesses observed the helicopter maneuvering low along the interstate under a heavy fog layer that was about 200 feet above ground level until it collided with the power lines. The CFI had most likely flown a similar flight path in night conditions about 3 months prior to the accident. The accident site was located in a small canyon where the power lines crossed the interstate at a near 45-degree angle, with colored balls on the highest wires. These were the only wires that cross over the interstate in the stretch of the canyon. The two transmission towers on either side of the interstate were not lit or marked. Examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. The relevant Terminal Area Chart pictorially depicted a tower and the power lines, showing them crossing the interstate in the area of the accident site.

Text from the NTSB. Open the NTSB docket for LAX08LA213 for the full record.