In 2003, the inadvertent brush of a tree branch against power transmission lines set off a cascade of failures that within hours grew into the largest blackout in US history, affecting more than 50 million residents of the northeast United States and Canada. These flying buzz saws are here to ensure that never happens again.
In response to this blackout, various government agencies and public utilities came together to develop the Transmission Vegetation Management Program, essentially a regional tree maintenance service with the specific aim to prevent vegetative overgrowth from interfering with high power (200 kV and higher) transmission lines. The problem was that many of these transmission lines run through treacherous and remote terrain, nearly impossible to navigate by foot or ground vehicle. Even if the area is accessible, climbing crews can only clear about 60km of brush in a good season -- and we're talking about lines that run for hundreds of kilometres. But that's where the helicopters with buzz saw pendants come in.
Known as aerial side-trimmers, these devices are essentially a string of rotating buzzsaws affixed to the bottom of a helicopter. The pilot navigates his aircraft into position between the power lines and overgrowth, engages the saws, and slowly makes his way down the line, clearing three to six miles of brush every day (that's on both sides of the power lines, natch).
The Haverfield Aviation system, for example, has been in use since 2008. According to the Haverfield website, the buzzsaw chain consists of "10 circular, carbide-tipped blades -- 60cm in diameter -- suspended 37m below the helicopter. A low-emission engine drives the saw, which a pilot operates from the cockpit. The length of the aerial saw can be increased or decreased by adding or subtracting sections of aluminium poles." A similar setup from Aerial Solutions in Tabor City, North Carolina, utilises 11 60cm blades strung along a 12m aluminium bar and spun up to 5800 RPM by a 45hp, two-cycle gas engine controlled by the pilot.
These systems not only reduce the amount of work-related injuries suffered by trimming crews as they clamber up and down tree trunks, it also better protects more of the power transmission system from shorts, reduces the environmental impact of these maintenance efforts (one guy in a helicopter vs 10 guys in two trucks plus support vehicles), and clears more lines for a fraction of the price of conventional methods. [Farmshow - Haverfield - Asplundh - ELP]