The Purpose Of Jet Engine Spirals

You may have noticed those spinning white spirals in a jet engine at one point or another. But have you ever stopped to think about why they’re there? As cute and as comical as they look, those spirals on the spinner cones are more than just for aesthetics – they are an important safety measure for the ground crew and even for birds. This article will attempt to enlighten you on this matter!

Ground Crew Warning

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 critical that ground crews can identify a running engine and stay away from it.

hazard-placards Aircraft Engine Spirals
Hazard placards on a 737 engine. If you can read these – YOU’RE TOO CLOSE!
credit: Boeing Aero

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 near 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 glance is all it takes.

Working around running jet engines can be very dangerous if one is unaware that it is running. Just take a look at this video of a ground crew who got sucked into a Grumman A6 Intruder and lived:

credit: footage

Effect On Birds

Bird ingestion and impacts cause expensive damage to engines. For years, it has been speculated that spinner spirals might startle or frighten birds away from engines during taxi, takeoff, and landing.

Bird strikes severely damage an engine’s components and can cost thousands of dollars in repair costs

However, the spinning spiral effect on birds is highly controversial. A few studies on the effectiveness of spinner spirals for deterring bird impacts have been done. None of them conclude 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 airplanes with the “engine eyes.” Soon after, ANA added the markings to all their aircraft spinners.

Other spiral variations can be used on aircraft, including the “Wobbly Ball” that was added to Japan’s ANA aircraft engines.
credit: Wikimedia

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 scare away birds. In addition, a representative from engine manufacturer Rolls-Royce stated: “The nose cone (both in terms of strength and angle) is designed to reduce bird impact damage to the engine and reduce ice build-up. The spirals are there as a warning to the 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 Being Done 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.

There you have it! Regardless of their value as a safety enhancement or bird repellent, the spinning swirls are nonetheless a part of aviation history and tradition among Avgeeks alike.


Cover Photo:

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