Why are there spirals painted on an airliner’s jet engines? What is their purpose?
To answer simply, the engine spirals are hazard indicators that, when spinning, alert people nearby to get as far away from the engine as possible.
The spiral is a prominent visual cue that accentuates the rotating motion of a jet engine’s front air intake fan, so airport ground staff can easily identify a running engine at a glance.
An active engine can ingest objects and humans in close proximity, causing damage and fatal injuries. Therefore, a rapidly rotating spiral serves as a clear warning for people nearby to stay clear from the engine.
Here’s a spinning jet engine, with a spiral painted on:
Now here’s the same engine, but without the spiral:
As you can see, the spiral marking makes the running engine look significantly more obvious. This is especially vital during nighttime operations, as the spinning fan blades are difficult to see in low light conditions.
The airport workers can also deduce the approximate power setting of the jet engine based on how blurry the spiral appears to be, and determine when it’s safe to approach the engine.
While the spiral pattern is ubiquitous nowadays, it isn’t the only design used by airlines. In fact, there are quite a number of variations, depending on the engine manufacturer, and they’re all designed to be very eye-catching when rotating at lower speeds.
The classic spiral is common on Rolls-Royce engines, such as the A350’s Trent XWB.
The “g-swirl” is a simpler spiral design that is widely seen on modern General Electric engines, like the Boeing 787’s GEnX shown below, along with CFM engines.
The “apostrophe” is also relatively common, and primarily used on Pratt & Whitney engines, including the A320neo’s PW1100G.
The “comma”, “wobbly ball”, “wobbly moon”, and “horseshoes” (rare) are older designs that were mainly applied on early high-bypass turbofan engines, such as the 747’s Pratt & Whitney JT9D and the 767’s General Electric CF6.
The “hurricane” is used on aircraft operated by a few carriers, such as Philippine Airlines and Qantas.
A jet engine’s suction force is lethal, even at low power settings. For instance, at idle thrust, the Boeing 737’s engine can ingest items up to 2.8 meters (9 feet) in front and 1.2 meters (4 feet) to the side of the air inlet.
At breakaway thrust, which is the thrust needed for a plane to begin taxiing, the engine can suck in items up to 4 meters (13 feet) ahead, and 1.5 meters (5 feet) to the side.
Hence, airport workers must be vigilant at all times on the apron and taxiways. Visual warnings, such as the spiral, are essential to safe ground handling operations.
The spiral pattern was first implemented on German fighter aircraft back in World War II, in part to help alert ground personnel of rotating propellers. Here is the Messerschmitt Bf 109 with a large spiral:
Can the engine spirals prevent bird strikes?
There are speculations that the hypnotising movement of the spiral and other designs may be able to scare off birds and help reduce bird strikes. However, this has not been confirmed, despite several research studies.
In the mid 1980s, All Nippon Airways conducted an experiment by painting the “wobbly ball” markings on their engines. The airline observed a slight reduction in bird strike occurrences, but ultimately did not yield any conclusive results.
Boeing has stated that there’s no scientific evidence to prove that the engine markings can effectively deter birds.
Why don’t the ground crew just listen for engine noise instead of looking at the spiral?
The apron is often a very chaotic place, with engine noises coming from various airplanes that are parked and/or moving. In such a noisy environment, it’s difficult for the ground staff to differentiate between the running engines and the inactive engines by merely listening for engine noise.
Besides, the workers are required to wear earmuffs to protect their hearing, which reduces their ability to perceive sound direction.
Therefore, a spinning spiral is simply a more reliable, clear-cut indication of a running engine than engine noise.
In conclusion, the spiral’s main objective is to warn airport ground workers to keep clear of a running jet engine to avoid the risk of being ingested into the engine.
Thanks for reading.
