N500TNHughes Helicopters HUGHES 369D · 1979

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 feet (horizontally) further out from the middle phase power line. During approach to the lines, the main rotor blade clearance from the middle phase line was about 2.5 feet, 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 the Occupational Safety & Health Administration (OSHA) 1910.269(I)(10) the Minimum Approach Distances (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). "

Nominal voltage in kilovolts Distance: Phase to ground exposure (feet)

161 to 169 4.00625

The MAD for this operation was a distance equal to or greater than 4.00625 feet.

Prior to departure on a positioning flight, the pilot of the helicopter checked the weather and determined that visual meteorological conditions existed at both his departure and destination airports. However, while in cruise flight at 400 feet above ground level while following a highway, the ceiling rapidly became lower, and the pilot encountered instrument meteorological conditions. Moments later at an airspeed of 65 knots, marker balls appeared which indicated that power lines were directly in front of him. He lowered the collective and entered a right diving turn in order to avoid the wires but one of his main rotor blades made contact with a power line. The rotor speed remained within limits but the helicopter began to vibrate. As a result, the pilot decided to land in a nearby field. Post accident examination of the helicopter revealed that the main rotor blade that had made contact with the power line was substantially damaged.