A simple way to prevent small planes and helicopters from running into overhead power lines is being patented by Safe Flight Instrument of White Plains, New York (GB 2 343 956). Almost all the world's overhead power lines carry alternating current, at a frequency of 50 to 60 hertz. Because the long, high power lines act as very efficient aerials, aircraft can carry a low-frequency receiver that tunes into the hum. When the receiver picks up the signal from the power lines, an audible or visual warning will go off in the cockpit.
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