If you look up close inside many aircraft engines, you will see a spiral design painted in the center. These are not just for appearance; they serve a couple of practical purposes. Want to know what they are? Let’s find out.
Painting spirals
Most modern jet aircraft have a white spiral design painted on the spinner. There is no industry standard or legal requirement for these and designs vary between manufacturers and airlines. They are not present on all engines (many Embraer aircraft, for example, often do not have them). There are many different designs in use, including different thicknesses of spiral, comma or apostrophe symbols, balls and even a hurricane symbol in use by Philippine Airlines.
It is not just modern jet aircraft that use these. There have been appearances of spiral designs on several earlier aircraft, including German wartime Junkers and Messerschmitt aircraft.
So, why are they there? There are two mains reasons – to show crew when the blades are stationary and to help keep birds away from the engines.
Safety on the ground – alerting people to an active engine
Staying away from a running jet engine is very important in airfield safety. Even a smaller engine at idle has a hazard zone of around 9 to 10 feet, within which there is the risk of being sucked into the engine. Larger engines or those with greater power increase this hazard zone.
Of course, a running engine makes a lot of noise. And ground crew should also be aware of aircraft movements and active engines from communications as well as active aircraft lights on the fuselage. But being able to see when an engine is running is also important.
This is not always easy. When spinning at speed the blades become translucent. Adding a distinct white spiral to the center creates a much more visible target, especially in low light or dark conditions.
Keeping the birds away
The other reason cited for adding spirals is to help prevent bird strikes. Bird ingestion into aircraft is a dangerous, and expensive, problem. According to data reported by Wikipedia, there are 13,000 bird strikes annually in the US.
At worst these can lead to crashes, but these are fortunately rare (one of the most recent was with a Ural Airlines A321 in mid-2019). More commonly they lead to diversions or emergency landings.
And whilst very few strikes directly lead to aircraft accidents, the estimated cost of damage from these, according to a report from the University of Nebraska, is $400 million each year in the US (and up to $1.2 billion annually worldwide).
Keeping birds away, from the engine, especially during taxi, take-off, and landing is very important. And the white spiral is thought to help with this. Without a spiral, the engine would just appear as a black circle or hole. The spiral creates a white circle that birds, with better detection of motion than us, will avoid.
Rolls Royce quote in a 2004 brochure covering ‘a century of aviation’ that:
“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.”
There is however little real evidence to support this, and some doubt its effectiveness. There have been some studies, however. These include a year-long study by ANA in 1986 (reported in the New York Times), found evidence of a reduction in strikes when using spirals.
What did you know about the spirals on aircraft engines? Share your thoughts in the comments!