What’s That Comma Doing in That Engine?

At our Social Media department, we book thousands of flights and let you know if you’re allowed to take a huge toy giraffe on board. We, social media agents, know a lot (a whole lot!), but we actually also learn quite a lot from the many curious passengers who ask us questions via Facebook, Twitter and LinkedIn! Today I found an answer to the question, “What’s that comma doing in that engine?”

Twitter message to KLM

A passenger wondered on Twitter what that curling, comma-shaped symbol was doing in the engine. It’s actually called the spiral on the fan spinner cone. But what exactly does it do, besides looking rather cute? The spiral has a fairly straightforward function, alerting ground staff to a running engine and ensuring that no one comes too close to it. 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.

“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.

Photo: Mark Wagtendonk

Warning: Stay clear of hazard areas

There’s also a hazard area around every engine. For example, an idle engine of a Boeing 787 Dreamliner has a hazard area of 15 ft/4.57 metres. This radius is measured from the centre of the engine inlet. You can just imagine the immense power of such an engine and what it could do to anything that got too close to the fans. So that’s why the visual cue of a moving spiral is such an important safety measure. It enables us to see that an engine is running and to stay well clear of the engine inlet. Next time you’re at an airport, you might like to grab your binoculars to see if you can spot moving spirals in our blue birds’ engines!

Older engines

Spirals have been around for about five or six decades now. In older planes, the big fan blades in the front of the engines can spin the other way when it’s very windy. And in some engine types, the fan needs to spin before fuel is added to start the engine. You can imagine the spiral is quite useful in these engine types as well.

The ground crew or the maintenance technicians can easily see from the direction of the spiral whether the fan rotor is rotating in the correct direction and communicate this to the flight crew, so they can continue their start procedure. Of course, this procedure is no longer necessary with newer engines, because an auto-start engine does this on its own. By the way, the spiral on the spinner cone can only be seen if you’re in front of a plane, so most propeller aircraft also have markings on the propeller blades. When they rotate at high speed, the markings create lines in the air. Handy, if you want to avoid losing your head!

Shapes and sizes

We are talking about a spiral here, but in fact, the spinner markings come in a variety of shapes and sizes. Latest generation engines are delivered with a spiral applied by the engine manufacturer, there is a slightly different design in spirals between for example General-Electric engines and Rolls Royce engines, although the purpose is still the same.

aircraft engine spirals

What about the birds?

It’s not a scientifically proven fact, but the spiral may also encourage birds to fly clear of the engine during taxiing, take-off and landing. Instead of a black hole, they see the spinning spiral as a white disk, which might warn them to get away from the black hole that would lead to their untimely demise.

Several studies have been done, but none have been conclusive. Boeing and Rolls Royce, for example, say the spiral does not prevent birds from flying into the engine, as the rotation would be too fast for the birds to see the spiral. Other studies have suggested painting spiral shapes on the spinner cones do help prevent bird strikes. I guess the answer to this question can only be “maybe”, at this point. If only we could ask a bird:)

– Renée –

Photo featured image: Mark Wagtendonk

Sounds familiar?

It’s quite possible you’ve heard or read this before. We’ve posted this blog in January 2016. So this actually is a repost. But let’s be honest: you can never know too much about jet engines, right? ;-)

Posted by:   Renée Penris  | 
Join the conversation Show comments





Thank you Mehmood, safety first indeed! :)

Jan Kruizinga

If birds see the spinning enging as a black hole, why not add colored dots (white, yellow) to the fan blades, so that birds won’t see a black hole? Or even paint the blades entirely white? I doubt, however, if this story makes sense, since when birds can see the tiny white dot, why don’t they avoid the entire plane.


Hello Jan, as mentioned above, there is no clear answer to the question whether any markings on the engine spinner or blades have an effect on birds. I like your idea of coloured dots though! Thank you for reading.


I like Jan’s suggestion. Also a big may be..but may be it may just work!

Bob Howie

I discovered when driving, that, if I saw a bird/animal in my path I would flash my headlights about once per second, it seemed to work as the birds or animal would move out of the way. Perhaps a slow pulsing light just inside the cowling would help?


Thanks for the insight! I have another question: back in the days, airplanes often had black noses. Why was that and why not anymore?


Pieter! This is a great question that deserves its own blog! ;) But to answer your question, the black nose was indeed seen a lot in the 60s and 70s and for a reason. Behind the nose cone, there’s the weather radar. Back in the days, the paint that was used, contained elements like lead, which would interfere with the radar. So they used different material on the nose cone. I looked up some aircraft and I think it looks pretty cool too! Thanks Pieter!


well yes and no, all of the above is true but it was also to prevent glare in the pilots eyes. Then one day a pilot just put on sun glasses and the problem was solved…


Thanks for the addition, anti-glare was indeed also one of the factors. Go sunglasses!


I think the black paint was also used to cut down reflections from the sun shining into the cockpit. Airtractor spray planes have the top of the engine compartment and hopper lid painted black for that reason. I think with paint technology now, they can use other low sheen colours instead of just black.


Luckily, because we do like our blue noses now! :D


I would like to know what happens to the engine if a bird is pulled into it.

Thanks for this information


Hello Themba! Most bird strikes happen during take-off or landing, when the plane is at a low altitude. Depending on the size of the bird and the rotation speed of the engine, the blades and the inside structure can be damaged, which could possibly lead to engine failure. :(


The hudson river landing is a good example of what can when the engines are eating the birds flying in front of them.


why not in Dutch KLM ?


Maybe next time Miep! We do have quite some Dutch blogs too, though! You can select ‘Nederlands’ in the menu, then only Dutch blogs will be shown. Thanks! :)


KLM why not in Dutch ?


You guys are doing a great job with this blog. It’s well written and gives an interesting insight in all aspects of airline business. It’s always a delight to find an announcement of a new story on Twitter. Greetings from Stuttgart ;-)


Thank you so much Steph! That’s very nice to read. I hope 2016 will bring a lot of wonderful blogs for you to read. Greetings from Amsterdam!


Someone told me when engine work that shape become like eagle and the birds dont come close


That would be very wise of those little birds, Hesam. ;)


Thank you for sharing our post! It felt more powerful to pesrlnaoly give the staff great feedback than it would have been to fill out an anonymous customer survey. The Office Depot employee who helped me that day really proved that customers are important, even to big companies.


Hi Aisha and Renée,
Being a digital marketing professional…let me take this opportunity to congratulate you on the amazing behind the scenes work you guys are doing!! Mad respect!!


Leica! Wow, thank you very much! I’m very glad you like our blogs, hopefully we will be able to write a whole lot more this year. Cheers!


After a long time I get to know the purpose of the comma shaped representation. Thank You


That’s great! You’re very welcome Philemon!


Why Isn’t there a cage over the inlet?
To prevent anything entering except air??


Hello André! You could indeed build some kind of cage over the inlet. However, a cage with super small openings wouldn’t allow enough air to pass through to keep the engines running. A cage with bigger holes would let too much through. Plus, keeping in mind that 99% of the time an aircraft is flying above 10.000 feet (cruise level is 30.000-40.000 feet) where, as you can imagine, not a lot of birds fly. So it would be a bit useless to build this kind of cage for this reason. But I like how you think! Thank you!

Rose ruiz

Eu preciso de quatro passagens para Suecia…
Mas o peço aí aí aí. Como vocês poderiam me ajudar?
Minha mãe já viajou com vocês e adorou, agora ela quer ir novamente.


Olá Rose! How lovely your mum enjoyed it so much! I or any other agent from the Social Media Team would be happy to assist you with this booking! You can post a message directly on our wall (also in Portugese!) or send us a private message via the ‘Message’ button here: https://www.facebook.com/KLM. Hope to see you and your mother on board soon! :)


As far as I know from the way the jet engine works, compressor (those front blades stages) always has to achieve certain speed before the fuel is introduced into the engine. The airflow and pressure/temperature it creates is needed for the engine to enter the stage of self sustained combustion.
But anyway great article guys and I hope more people will be aware of interesting facts about aviation!


Correct! The engine must first come up to speed. On jet engines on larger aircraft (which is all of the KLM fleet), the Auxiliary Power Unit (APU) blows air into the engine first, so that the engine runs at low speed. Then the engine fuel valve is opened. The motor is further helped along by the APU until it’s revved up enough and keeps running at idle speed. Interesting stuff! And thank you for the nice compliment! :D


Can a man be sucked into the turbine cone if gets close to it while the engine running?
I’ve seen pictures and read stories of man sucked into the turbine and came out on the other side of the cone in the form of ground beef! How true is it?


Hello there AppWorld. This is indeed possible. :( The suction power of a running engine is very strong, even if it’s running at idle speed. That’s why the spiral and other safety measures are so important! As an example, on the engines of our Boeing 737s, there’s a red line visible on the sides of the engines. If the motor is rotating, you may absolutely not pass that red line, otherwise you can be sucked into the engine. Safety first!

Alex Graham

I wonder if there is a way to create a barrier in front of the engine intake that could prevent large debris or birds from entering the fan blades while minimally disturbing the air flow. Maybe a strong thin layer of mesh or chain link fence type structure that covers the intake? If there’s one glaring problem with jet aviation, I would say that objects entering the engine are the number one vulnerability!


Hey there Alex. It would really be a perfect solution if there would be some kind of fence or cage in front of the engine which would not disturb the airflow. Engines are indeed very sensitive to “Foreign Object Damage” or “FOD”. On Schiphol Airport, you can find a lot of containers labeled “FOD” in the areas around the aircrafts, to keep the platforms as clean as possible. Although we like birds (especially blue ones), there are also a few cars that make sure the birds are scared away, as we like our engines better! :)

Ricardo Ramacciotti Armando

Though I know the reason for the spiral I think the question was quite interesting and the whole explanation was excellent! Very technical and accurate as well as in a simple language and easy to absorb. I have worked for 10 years for KLM at the main ticket office in São Paulo and some of the technical explanation would never come to us but only for those in close contact with the aircraft.
Cheers for 2016!


Wow Ricardo, that’s so nice to read! Thank you, I’m happy that you found it interesting and well written. 10 years, amazing! You must have so many stories! :) All the best for 2016 Ricardo!


Hi Ken,
Thank you for sharing this great knowledge! Could you pls explain more about the material used by the spinner cone? Someone said it is soft. Is this true? Thanks alot!


I don’t think it is soft, as soft materials tend to have lower tensile strength, and considering that the spinner does over 2000 rotations per minute any soft material would shred from the vast amount of air being sucked in (1.25 metric tons per second) and the outward force (at blade tip it is around 90 tons (or like hanging 9 London buses of each blade)). The spinner is also there to keep foreign objects from entering the core of the engine and sending it outwards into the bypass.

Charles Lwanga

Thank you for educating me I had no clue at all what the use was.


No problem, Charles!

Gheccian Lammers

Great insight. Grazie mille



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