Helicopter rotors (shown here a tiltrotor) are often made from titanium, stainless steel or nickel alloys. The effect is caused by the pyrophoric oxidation of metal particles. At the base is the so-called Kopp-Etchell effect.
When a helicopter descends into a sandy environment, the enormous downward thrust from the rotors inevitably kicks up a cloud of sand. Once in the air, the helicopter’s blades cut through this cloud, generating the halo.
But how? To prevent early degradation, most helicopter blades are coated with an abrasion strip. This strip, typically crafted out of a metal like titanium or nickel, prevents the leading edge of the rotor blade from being worn down too quickly by the various particulate hazards of the atmosphere.
This abrasion strip can handle a lot of wear and tear, but the desert is a harsh environment. Sand is harder than the titanium and nickel that make up the abrasion strip, so when the blades begin cutting through a cloud of sand, the particles hit the blades and send bits of metal flying into the air.
It turns out that the metals that make up the abrasion strips on helicopter blades, bracing themselves against the stinging force of the sand, can be pyrophoric. This means that the metals used are substances that can spontaneously ignite in air. But of course this doesn’t happen in normal circumstances; we don’t see bricks of titanium bursting into flames. Rather, the spinning blades of the helicopter generate a cloud of metal particles, just like the cloud of sand. Once in a powdered form, the metal particles can ignite and create the brilliant scenes above.
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