Many of the utilitarian tasks for which helicopters are uniquely suited unfortunately place them in dangerous proximity to power lines and guy wires. The annual fatality toll from helicopter collisions with these difficult-to-see obstructions attests to the seriousness of this particular aviation safety issue. This blogpost discusses some of the practical ways helicopter pilots (and ag-cat or ultralight pilots) can reduce the risk of an in-flight wire strike.
A recent fatal accident in Mentone, Texas on 9/10/2021 illustrates the danger. A Robinson R22 Beta helicopter (Part 27 Rotorcraft) operating under Part 91 flight rules collided with power lines in Class G airspace while rounding up cattle on a local ranch. The NTSB accident number CEN21FA410 Final Report concluded "... it is likely that the pilot did not see the powerlines while maneuvering at low altitude ...". At the point of impact, the power lines were about 45 feet AGL. From the electrical arcing damage, the NTSB concluded the helicopter nose probably hit the line first, severing it, whereupon the main rotor sliced the tailboom off before both pieces fell the ground 50 feet beyond the power lines. Meteorological conditions at the time of the accident were 70 degrees Fahrenheit, VMC, clear skies, 9 knot winds, and 10-mile visibility.
The See & Avoid approach to wire-strike risk mitigation applied in this case, especially since this model helicopter was not equipped with fuselage mounted cable-cutters or any kind of electromagnetic wire detection equipment. In the incident cited above, it's not likely that cable cutters would have precluded the tail boom-to-main rotor contact that ultimately doomed the aircraft because the helicopter forward fuselage itself severed the cable. Even so, while fuselage mounted wire cutters might prevent fatalities in some cases, there's almost certain to be substantial airframe damage in any imaginable instance.
Most antenna towers and power lines are depicted on hardcopy aeronautical charts and Electronic Flight Bag (EFB) e-charts, but the age-old wisdom always applies--plan your flight and fly the plan. Standard heights for most basic utility poles are 35-40 ft AGL, but some, particularly high-voltage transmission lines, may be 120 feet or more tall. In mountainous terrain, power lines spanning the crests of adjacent hills may be several hundred feet above the ground. Reckless pilots who are scud-running through valleys may not see the towers obscured by the clouds on the hilltops, and often won't see the wire strung across their flight path until it's too late.
Guy wires present another hard-to-see hazard. A safely conservative approach to avoiding them is to stay completely outside an airspace "cylinder" equal to the height of the stayed tower and of diameter twice as wide as the tower's height. Many radio transmission towers have unusual guy wire arrangements to avoid electromagnetic interference with the signals transmitted from the tower, so don't assume that all guy wire layouts are substantially identical. Always plan your flight to operate at an altitude safely above the tallest local obstruction.
I'd advise to maintain a healthy skepticism for electronic "power line detectors". Most only detect lines carrying AC high voltage, don't alert you until you’re nearly on top of them, and often don't provide much directional information toward the hazard. If the cable carries direct current, or no current (guy wires and fiber optic lines) most of these detectors are even more unreliable. Years ago, while a flight test engineer for a major helicopter manufacturer, I participated in testing a power line detector unit developed by the company’s R&D team. Although this device could detect live power lines, the useful results varied widely depending on the azimuth and elevation of the helicopter heading relative to the line orientation. Although that company has no doubt improved its power line detector performance since then, at the time we tested it, the detector design required improvement to be practically useful as a hazard alert and avoidance tool.
Thanks for reading this blogpost, and best wishes for continued safe flying!
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