You have to check these photos to believe it – Blades of helicopter cause sparks:  http://www.michaelyon-online.com/the-kopp-etchells-effect.htm?beautiful

"Basically it is a result of static electricity created by friction as materials of dissimilar material strike against each other. In this case titanium/nickel blades moving through the air and dust. It occurs on the ground as well, but you don’t usually see it as much unless the aircraft is landing or taking off. The most common time is when fuel is being pumped. When large tankers are being fueled they must be grounded to prevent static electricity from discharging and creating explosions."