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 power cut in US history, affecting more than 50 million residents of the Northeast and Canada. These flying circular 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 40 miles of brush in a good season—and we're talking about lines that run for hundreds of miles. But that's where the helicopters with circular saw pendants come in.
Known as aerial side-trimmers, these devices are essentially a string of rotating saws 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 circular saw chain consists of "10 circular, carbide-tipped blades-two feet in diameter-suspended 120 feet 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, NC utilises 11 24-inch blades strung along a 40 foot aluminium bar and spun up to 5,800 RPM by a 45 hp, 2-cycle 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]