Hughes 369 N500TN accident near Peach Orchard, Missouri

Probable cause

The helicopter pilot’s failure to maintain sufficient separation between the main rotor blade and a power line during line guard installation, which resulted in an electrical arc to the main rotor.

NTSB analysis

The helicopter pilot reported that he was flying in support of a utility company crew that was installing line guards for a conductor damper on an energized 161 kilovolts (Kv) electrical transmission line. The transmission line had three phases (power lines) that were separated vertically and staggered horizontally from one another and ran parallel to the ground. The lower phase power line was about 9.5 ft (horizontally) further out from the middle phase power line. While the helicopter was approaching the lines, its main rotor blade clearance from the middle phase line was about 2.5 ft, which resulted in an electrical arc to the main rotor. The pilot landed the helicopter away from the transmission line without further incident.

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.

According to Occupational Safety & Health Administration 1910.269(I)(10), the minimum approach distance (MAD) is 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 a voltage of 161 Kv, the MAD for this operation was equal to or greater than 4.00625 ft.

Text from the NTSB. Open the NTSB docket for GAA16CA480 for the full record.