The Kopp-Etchells effect is the name given by Michael Yon to a phenomenon he witnessed in Afghanistan.
As a helicopter comes in to land a great cloud of dust is stirred up; the impact of this dust on the rotor blades would quickly wear the blades down, making them unsafe. The rotors are coated in titanium to resist this damage; the sparks you see are this titanium coating being worn away (a process physicists call ablation).
