Bell 206B N20620 accident near Eureka, Nevada

Probable cause

The pilot’s failure to maintain clearance from a wire while maneuvering during low-altitude operations. Contributing to the accident was task- and/or work-related fatigue.

NTSB analysis

The pilot was flying a helicopter as part of a Department of Interior (DOI) reseeding project when the helicopter struck the static line above a set of power transmission lines. Postaccident examination of the engine and airframe revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. The length and direction of the debris field was consistent with the helicopter traveling in the direction of, and at a rate of speed consistent with, a seed dispersal run.

The pilot was familiar with the area and was aware of the transmission lines, having performed a reconnaissance flight and flown over them multiple times during the previous flights that day. In the area of the accident site, the power lines began an ascent up an adjacent hill, with the static lines diverging such that they were about 40 feet higher than the power lines at the collision location. The static lines were of thinner gauge and could easily have been obscured by the surrounding hilly terrain. The lines were depicted on the relevant Federal Aviation Administration sectional chart. The helicopter was not equipped with a wire strike protection system nor was it required to be under the terms of the DOI contract. Neither the power nor static lines were equipped with visibility markers nor were they required to be due to their elevation above ground level.

The pilot had flown 40.5 hours over the last 5 days as part of the reseeding project, which was just short of the DOI contract maximum. A review his sleep history indicated that he received adequate rest the night before the accident. On the morning of the accident, he had flown 10 application runs in an area about 70 miles away. He took a lunch break about 3 hours before the accident, then flew 20 separate seed application runs and 10 reloads. His total flight time for that day was 6.6 hours. The low-level and "nap of the earth" flight operations required considerable concentration, most likely resulting in task-and/or work-related fatigue at the end of the day. By comparison, the United States Army recommends a maximum flight time of 8 hours per day or 37 hours per week, with a reduction of a factor of 1.3 to 1.6 for low-level and "nap of the earth" flying; thus the pilot would have exceeded their duty hour recommendations for each of the 5 days leading up to and including the accident day.

The pilot held an assisted special issuance second-class medical certificate based on a reported history of diabetes for which he was taking medication. His most recent medical examination revealed a new diagnosis for hypertension, as well as preexisting cataracts in both eyes, which exhibited no change since his last examination a year prior. In part due to the lack of an autopsy, the relevance of these findings, combined with his likely work-related fatigue, could not be determined.

Text from the NTSB. Open the NTSB docket for WPR13GA128 for the full record.