Why Jet Engines Have a White Swirl (It’s Not for Birds)

Why is there a white spiral on a jet engine's spinner? KLM explains it as a warning to ground staff: when an engine is running, the marking turns into a white blur or a hypnotic twirl depending on rotation speed. Crews wear hearing protection and several engines can be running nearby, so sound alone doesn't tell them which one is live ("sound won't tell you" is our short summary of those two facts). KLM gives a 15-foot (4.57 m) hazard area around the inlet of an idling Boeing 787.
The idea that the spiral scares birds is widespread. Boeing's AERO magazine lists "jet engine spinner markings help to repel birds" among common misconceptions, saying it is not scientifically proven. KLM notes that studies have been inconclusive. Designs vary, from commas to hurricane shapes and "wobbly balls".
Notes: illustrated. The bird's reaction is a dramatization of the myth, not an observation. The twin jet is generic, the blur is stylized, the sound rings are a metaphor, and two engines are shown perfectly still for clarity. The 15-foot zone is drawn to scale but is schematic, and applies to a 787 at idle only; hazard areas differ by aircraft and power. We did not find a manufacturer or regulator document stating the spiral's purpose. Not affiliated with or endorsed by Boeing or KLM.

Sources:
– Boeing AERO Q3 2011, "Strategies for Prevention of Bird-Strike Events" (archived):
– KLM Blog, "What's That Comma Doing in That Engine?" (archived):
– AeroSavvy, Aircraft Engine Spirals: