The Kopp–Etchells Effect

It’s named after Cpl. Benjamin Kopp (1.20.1988 - 7.18.2009 - U.S. Army Ranger) and Cpl. Joseph Etchells (3.23.1987 - 7.19.2009 - British Army), two soldiers killed in combat in Afghanistan in July 2009.

 
The Kopp-Etchells effect is a result of dust striking helicopter blades as they take off or land at night, causing a bright 'halo' effect around the spinning rotor disk. When helicopters pass through dust storms, contact of the particles with the rotating blades produces either sparks or static electricity.
The phenomenon has been described as ‘electro-luminescence’, halo effect’, and ‘corona effect’.

 

Helicopter rotors are fitted with abrasion shields along their leading edges to protect the blades. These abrasion strips are often made of titanium, stainless steel, or nickel alloys, which are very hard, but not as hard as sand. When a helicopter flies low to the ground in sandy environments, sand can strike the metal abrasion strip and cause erosion, which produces a visible corona or halo around the rotor blades. The effect is caused by the pyrophoric oxidation of the ablated metal particles. In this way, the Kopp–Etchells effect is similar to the sparks made by a grinder, which are also due to pyrophoricity. When a speck of metal is chipped off the rotor, it is heated by rapid oxidation. This occurs because its freshly exposed surface reacts with oxygen to produce heat. If the particle is sufficiently small, then its mass is small compared to its surface area, and so heat is generated faster than it can be dissipated. This causes the particle to become so hot that it reaches its ignition temperature. At that point, the metal continues to burn freely.
Abrasion strips made of titanium produce the brightest sparks, and the intensity increases with the size and concentration of sand grains in the air.
Sand particles are more likely to hit the rotor when the rotorcraft is near the ground. This occurs because sand is blown into the air by the downwash and then carried to the top of the rotor disk by a vortex of air. This process is called recirculation and can lead to a complete brownout in severe situations. The Kopp–Etchells effect, however, is not necessarily associated with takeoff and landing operations. It has been observed without night vision goggles at altitudes as high as 1700 ft.
The erosion associated with the Kopp–Etchells effect presents costly maintenance and logistics problems, and is an example of FOD.
Sand hitting the moving rotor blades represents a security risk because of the highly visible ring it produces, which places military operations at a tactical disadvantage when trying to remain concealed in darkness.
The light from the Kopp–Etchells effect can interfere with the pilot's ability to see, especially when using night vision equipment. This may cause difficulty with landing safely, and produce spatial disorientation.