When operating in sandy environments, sand hitting the moving rotor blades erodes their surface. This can damage the rotors; the erosion also presents serious and costly maintenance problems.
The abrasion strips on helicopter rotor blades are made of titanium, which is very hard, but less hard than sand; so when a helicopter is flown near to the ground in desert environments abrasion occurs, and at night there is a visible corona or halo around the rotor blades, caused by the sand hitting the titanium and causing it to spark and oxidize.
In August 2009, Michael Yon, an American journalist attached to the British Army in Afghanistan, referred to the rotor-blade halo as the "Kopp-Etchells Effect", after two soldiers who were killed in Afghanistan
