Ever noticed those white swirls in the center of a plane’s engines beneath the wings? What purpose would a white swirl on a turbofan serve? As it turns out, a whole lot is going on there!
The white swirls are formally called nosecone spirals, spinner spirals, spinner swirls and even nosecone swirls. According to Boeing’s spokesman,
…the nosecone spirals serve two purposes: one is to scare birds and the other is to identify when the engine cone is spinning.
Same is the case with Rolls Royce—a leader in manufacturing jet engines, who says,
Our aerospace engines have swirls painted onto their spinners in order to indicate when the engine is rotating while on the ground. In flight these swirls flicker as the engine rotates at high speed, scaring birds and allowing them to fly clear of the engine.
But some people contradict the “fending off birds” theory. For example, this article in Boeing’s own Aero Magazine which was co-written by a Ph.D. in Aviation System Safety and a safety pilot says in its “Common Misconceptions About Bird Strikes” section that
“airplane colors and jet engine spinner markings do not help in repelling the birds.”
An online aviation blog going by the name of AeroSavvy narrates the following contradictory statement from Rolls-Royce:
The nose cone (both in terms of strength and angle) is designed to reduced bird impact damage to the engine and reduce ice build-up. The spirals are there as a warning to ground crew when taxiing… In flight, the spirals could not be seen by birds as the rotation of the engine would be too fast.
A Japanese airplane company has, in fact, reduced bird strikes by painting “eyes” on Boeing 747 and 767 aircraft, as reported by the New York Times. In another study, zebra-patterns were painted on the propellers which reduced bird strikes in Norway, and in comparison, there is no conclusive evidence on the spinner swirls worth preventing birds from hitting the engine.
So what’s the reality?
Almost everyone agrees that the spinner spiral’s primary purpose is to warn the ground crew that their engine is still rotating so that the person de-icing or performing any maintenance on the airplane doesn’t get into the dangerous territory and sucked into the intake as we saw with this poor guy.
Dutch airline KLM also state the same in its article about spinner swirls,
“Can’t ground staff hear the deafening roar of a running jet engine?” I hear you wonder. Well, there could be several engines running at once near ground crew, plus they wear hearing protection. If five engines are singing in your ears, it isn’t always obvious which is running and which isn’t.
The post also expounds on how the spiral helps solve that problem:
If an engine is running, you see a white blur or a hypnotising twirl, depending on the rotation speed of the engine. This visual cue is extremely clear and warns everyone on the apron to stay away from the huge jet engines.
To build the case even further, AeroSavvy talks about how dangerous it is if someone gets too close to a turbofan,
Working near a running jet engine is extraordinarily dangerous. A Boeing 737 engine, running at idle power, has a hazard zone of 9 feet to the front and sides of the engine.
This means that, even at idle thrust, a human that walks in the hazard area runs the risk of being sucked inside and consumed by the engine. When the engine is above idle thrust, the hazard zone increases to 14 feet or more. Engines on larger jets, like the 777 have much larger hazard zones. It is absolutely critical that ground crews can identify a running engine and stay away from it.
So in contrary to the common myth, the spiral of the engine is more related towards keeping the ground crew safe and less about keeping the birds out-of-the-way, and “almost” every expert agrees!