Why is there a spiral painted on aircraft engines?

We sometimes answer questions from our readers, but this one comes…from a seatmate. The other day, as I was getting ready to board, I saw a little boy pointing at a plane through the bay window and asking his father “Daddy, Daddy, why is there something painted in the middle of the engine?

The “thing” in question is the spiral you’ll see painted on the cone at the center of the turbojet engines.

And then I feel the slightly embarrassed father stammer and say… “It must be to scare the birds away from the engine“.

An answer that satisfied the little boy. End of the story.

However, if I’d been asked the question, even though I’d never delved into the subject specifically, that’s not the answer I’d have instinctively given.

Yes, Dad was right, but only partly. Let’s have a closer look.

To ensure the safety of personnel on the runways

The primary purpose of the spiral is to show ground staff that the aircraft engine is running. If the spiral is visible, the motor is stopped; if it’s moving, it’s running at low speed; if it’s no longer visible, it’s running at high speed.

Remember that even at idle, an aircraft engine can suck in a person standing too close. This is why a safety distance is imposed on personnel working around the aircraft, and why a “safety cordon” is deployed around the engines when passengers disembark directly on the tarmac.

For example, people working on the tarmac near an idling Boeing 737 engine must keep a minimum distance of three meters in front of and to the sides of the engine to avoid being sucked in. When the engine is pushed to its maximum, the danger zone increases to more than five meters, or even more, depending on the type of aircraft.

But an engine that runs can be heard, can’t it? Well, not obviously. Firstly, because the staff wear noise-cancelling headsets. Secondly, airports are often very noisy, with many aircraft lined up side by side. At this point, knowing which aircraft has its engine running can be quite complicated.

But the system is not without its faults. For example, at night, if the lighting frequency is the same as the motor rotation frequency, then the spiral may appear to be stationary when in fact it is rotating. More than unlikely, but technically possible. In mechanical workshops, however, fluoride lamps should always be used in pairs: a single lamp could, again for reasons of frequency, make a saw or milling machine look like it’s not running when it is. All because of the strobe effect of the lamp.

To protect birds

Yes! Dad was right. The spiral also serves to keep birds away! At least in theory.

The avian risk is taken very seriously in the industry: the aspiration of a bird by an engine can have dramatic consequences.

But the impact of the famous spirals on avian risk seems to be purely statistical. It’s been reported that there have been fewer accidents since the introduction of the spirals, but it’s not clear whether this is due to the spirals or just a coincidence.

It’s said that seeing the spiral spinning at full speed would scare the bird and make it change course. However, for this to happen, it has to be able to see the spiral at this speed… and I don’t know enough about the visual abilities of birds compared to humans to be able to say. Moreover, many scientists are skeptical, attributing this fear to noise.

Bottom line

The real purpose of the spiral is to protect the people on the ground. Moreover, when aircraft engine manufacturers are asked, they never mention the birds.

Image : aircraft engine by Artistic Ahry via Shutterstock

Bertrand Duperrin
Bertrand Duperrinhttp://www.duperrin.com
Compulsive traveler, present in the French #avgeek community since the late 2000s and passionate about (long) travel since his youth, Bertrand Duperrin co-founded Travel Guys with Olivier Delestre in March 2015.

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