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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.