Incident Overview
A helicopter pilot reported that while flying in support of a utility company crew installing line guards on an energized 161 kilovolt (kV) electrical transmission line, an electrical arc struck the main rotor. The transmission line consisted of three phases (power lines) arranged vertically and staggered horizontally, running parallel to the ground. The lower phase line was positioned approximately 9.5 feet horizontally further out from the middle phase line.
During the approach to the lines, the clearance between the main rotor blade and the middle phase line was about 2.5 feet. This inadequate separation resulted in an electrical arc to the main rotor. The pilot promptly landed the helicopter away from the transmission line without further incident.
Damage and Examination
A postaccident examination revealed substantial damage to the main rotor hub and two main rotor blades. The pilot reported no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.
Regulatory Context
According to the Occupational Safety & Health Administration (OSHA) standard 1910.269(l)(10), the Minimum Approach Distances (MAD) are intended to "ensure that workers do not approach or take any conductive object closer to the energized parts as set forth in the table below (excerpt)." For nominal voltages between 161 and 169 kV, the phase-to-ground exposure distance is 4.00625 feet. In this incident, the actual clearance of 2.5 feet was less than that prescribed distance.
Conclusion
The event highlights the critical importance of maintaining proper clearance from energized power lines during helicopter operations. The pilot's actions in landing safely prevented further escalation.