Kopp–Etchells effect

The Kopp–Etchells effect is a sparkling ring or disk that is sometimes produced by rotary-wing aircraft when operating in sandy conditions, particularly near the ground at night.

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.

[9] Abrasion strips made of titanium produce the brightest sparks,[2][10] and the intensity increases with the size and concentration of sand grains in the air.

[3] Yet another incorrect theory is that the extreme speed of the helicopter blades pushes sand particles out of the way so fast that they burn up like meteors in the atmosphere due to adiabatic heating.

[11] 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 Kopp–Etchells effect produced by a CH-47 Chinook helicopter in Afghanistan
The Kopp–Etchells effect is created by metallic sparks, similar to the sparks generated when grinding metal.
Rotor tip lights produce a visually similar but distinct effect.
The Kopp–Etchells effect creates a halo around the rotors of a MV-22 Osprey in this long-exposure photograph .