Aircraft Engine Spirals & Swirls

Swirls and lines on engine spinners.  The spiral shapes can been seen on jet engines everywhere and come in several different designs and variations. They look cool on a spinning engine, but do they have a higher purpose?

There doesn’t seem to be an official, industry name for these things, so let’s call them “spinner spirals.” The spinner is the cone or dome shaped part on the front of many jet engines. Engine and aircraft manufacturers have been painting spinners with the catchy swirls for decades. The practice goes back at least to World War II when Messerschmitt painted spirals on a few of their aircraft.

 

Spinner Styles

Spinner spirals and swirls come in a several styles. Different airlines seem to prefer certain designs. I named a few of the styles in the slideshow below. Hands down, my favorite is the “Typhoon,” found on Philippine Airlines aircraft. The design resembles the meteorological symbol for a hurricane or typhoon. To my knowledge, Philippine Airlines is the only carrier that uses this design.

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What is the purpose of aircraft engine spirals?

The reason for painting designs on engine spinners is for the safety of ground personnel. 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.

Spinner Spirals Are Attention Getters

Even though jet engines make incredibly loud, whining noises, a running engine may not be obvious to ground crews. Airport aprons often have several airplanes in close proximity with engines screaming. Ground crews wear hearing protection to suppress the deafening noise. Making matters worse, it can be hard to see that an engine is running.  Just like the blades on a window fan, engine fan blades become translucent when they are spinning, especially in the dark. Aircraft engine spirals make it easy to identify a running engine. A quick glance is all it takes.

Engine Starting & A WWII Legend

The big fan blades in the front of an engine can spin backwards in windy conditions before it is started. A few engine types require that the fan spins forwards before fuel is added to start the engine. Maintenance technicians can watch the markings during engine start to determine the direction of the large fan disk. When the fan begins to spin in the proper direction, the technician can alert the flight crew by radio/interphone so that the start can be continued. This function is becoming less important with the advent of “autostart” engines. Newer engines take care of this sort of thing by themselves.

There’s a bit of a legend that the Germans used spinner spirals to confuse or distract allied gunners. At high RPM, the markings are more of a blur than an optical illusion. Lighting conditions and viewing angle would have to be perfect for an enemy pilot to see any spinner markings. While many allied aircraft had painted propeller tips, the Germans painted their spinners. The purpose for the markings in World War 2 was the same as now: safety of ground personnel.

Do the spirals scare away birds?

Maybe. Birds and aircraft engines are a bad combination. Bird ingestion and impacts cause expensive damage to engines (the birds aren’t happy about it, either).  For years, it has been speculated that spinner spirals might startle or frighten birds away from engines during taxi, takeoff, and landing.

A few studies on the effectiveness of spinner spirals for deterring bird impacts have been done. None of them come to an overwhelming conclusion one way or the other. One study, by the University of Oslo, was conducted on a very small scale with inconclusive results.

Japan’s All Nippon Airways (ANA) completed a well known, year-long study in 1986. The airline painted “Wobbly Ball” styled “eyes” on the engine spinners of several aircraft. The airline found a small reduction in bird impacts on the airplanes with the “engine eyes.” Soon after, ANA added the markings to all their aircraft spinners. AeroSavvy reader, Amanda, sent me this link that includes a photo. Interesting stuff!

Boeing and Rolls-Royce Say: No.

Engineers at Boeing have taken the position that engine spinner spirals do not reduce bird strikes. Boeing’s safety newsletter, Aero, states it is a misconception that airplane colors and jet engine spinner markings help to repel birds.

A representative from engine manufacturer Rolls-Royce in Derby, UK was nice enough to contact me about this issue. He stated:

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. 

Are Engine Spinner Spirals Going Away?

Not every jet engine has spinner markings. Airplanes are popping up here and there without spirals. The Embraer E-Jets (170, 175, 190, 195) have naked spinners. A few Boeing 757, 787 Dreamliners and 747-8 aircraft have been spotted without the swirls. Manufacturers and operators are questioning the value of the markings.

Line maintenance engineers that I have spoken with are quite vocal about the safety benefits of the markings and are disappointed many engines are now spinning naked.

Regardless of their value as a safety enhancement or bird repellent, the spinning swirls are part of aviation history and tradition. I’m sure AvGeeks everywhere would be sad to see them disappear.

Have you seen other engine spinner designs? 

I’d love to know if you’ve seen other spinner markings. What’s your favorite? Have you seen airliners without the markings? Feel free to post links to pictures of unusual spinner markings in the comments! Bonus points to whoever can find a photo of an ANA engine with an eye painted on it!