When American war correspondent and photographer Michael Yon accompanied US troops to Afghanistan in 2009, he noticed an unusual – and very spectacular – phenomenon while photographing them. Whenever a chopper was taking off or descending and there was a lot of sand in the air, the vehicle’s rotor blades began to glow like “distant galaxies”.
“The halos spark, glitter and veritably crackle, but in still photos the halos appear more like intricate orbital bands,” Yon wrote in a blog post. “The halos usually disappear as the rotors change pitch, dust diminishes and the ramp touches the ground.”
When asked about the phenomenon, none of the pilots seemed to have more than theories. According to one of them, the halos could be “a result of static electricity created by friction as materials of dissimilar material strike against each other.”
But there is a problem with this speculation. A static discharge does not produce the hail of sparks seen in these photos. Instead, the real explanation is as follows:
In order to protect the blades, the leading edges of helicopter rotors are fitted with abrasion shields that are often made of titanium, stainless steel, or nickel alloys. When a helicopter flies very near the ground in desert environments, these blades stir up an enormous amount of dust and sand into the air. In turn, the sand, which is harder than titanium and nickel, can strike the metal abrasion strip and cause erosion, gouging out tiny grooves on the blades’ surface. This sends bits of metals flying into the air, similar to the sparks generated when grinding metal. Since the blades turn around in a circle, a visible corona or halo around the rotor blades is produced.
Sand is more likely to hit the rotor when the rotorcraft is near the ground, because sand particles are blown into the air by the downwash and then a vortex of air carries them to the top of the rotor disk. This process is called recirculation and can lead to a complete brownout in severe cases. That said, the halo phenomenon has been observed without night vision goggles at altitudes as high as 1700 ft, so it is not necessarily associated with takeoff and landing operations.
By the way, rotor tip lights produce a visually similar but distinct effect.
When Yon discovered the phenomenon, to his surprise he found that it didn’t have a name yet. “How can the helicopter halos, so majestic and indeed dangerous at times, be devoid of a fitting name?”, he asked himself.
So, he went ahead and named it the Kopp-Etchells Effect, in memory of two soldiers killed in combat in Sangin, Afghanistan in July of 2009— Benjamin Kopp, a US Army Ranger, and Joseph Etchells, a British soldier.
“It’s not hard to imagine the two Corporals have already linked up and regrouped, and in sense they have. Knowing combat soldiers, it’s easy to imagine them laughing away at the idea,” Yon said.
Sources: 1, 2, 3