When you’re at a US airport, you may have noticed that some of the newest 737s in United’s, American’s, or Southwest’s fleet have serrations on the back of the engines. This feature is new to the Boeing 737, and it’s one of the few visual differences between the MAX and its predecessors. This distinctive design almost looks like shark teeth, leading some to wonder why there are spikes at the back of the plane’s engines.

These aren’t for show, but instead, the chevrons are designed to reduce noise. In short, the triangle bits that stick out generate vortices that help mix the two primary exhaust flows from the engine, which reduces the exhaust noise. The Boeing 737 MAX is the first 737 to include this innovative feature, and it’s also been used on the Boeing 787 as well as the 747-8. However, Boeing’s next jet, the 777X, will not include chevrons because, as it turns out, they come with some minor complications.

Taking A Small Hit On Performance

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Modern jet engines work by passing air through the front of the engine through a fan, and it enters one of two areas. The combustion chamber compresses the air and mixes it with fuel, while most of the air goes through the bypass duct, which is sped up by the fan to generate a majority of the thrust in high-bypass engines. The triangular shapes create vortices that control how the two exhaust flows mix, reducing exhaust noise.

This is a clear aeroacoustic benefit, although there is no aerodynamic benefit. In fact, the problem that Boeing ran into is that about 0.5% of the thrust was being wasted, and the Boeing 737 MAX was impacted more by this because it spends more time during takeoff and climbs, where the wasted thrust would be more pronounced. This 0.5% of inefficiency may seem minuscule, but commercial aviation is a game of percentage points and fine margins.

The other main benefit of the chevrons is that, because the exhaust is quieter, sound insulation can be removed. This is heavy, so it led to weight savings that improved efficiency. For smaller aircraft like the 737 MAX, however, this has less of an impact as the plane spends less time overall in the air compared to a widebody, and more time is spent running the engines at high power. In short, the 737 MAX was getting the same drawbacks as other aircraft, but benefiting less from the upside.

An Aircraft Optimized For Short-Haul Flying

The Boeing 737 MAX now has an incredible range of 3,800 NM (7,000 KM) in its MAX 7 form, and the MAX 8 has already seen transatlantic service with carriers such as Air Canada and (previously) Norwegian. However, this is still about half the range of the 787. Both planes will spend a similar amount of time climbing in a typical flight, but the 787 will spend more time cruising, where the weight savings from reducing the noise insulation have a greater impact.

Designing airliners involves compromises, and consideration has to be given for the plane’s intended purpose. Although the MAX does occasionally fly five, six, or seven-hour routes, most 737 MAX aircraft will operate multiple short flights every day compared to one long flight flown by a widebody. Although cruise efficiency still matters, a greater focus has to be placed on how to reduce fuel burn during climbs in particular.

The slight thrust reduction means that climb thrust settings need to be higher during this period of high fuel burn. For a plane that spends so much time climbing, even a minor loss of thrust was a concern. Meanwhile, the 737 is also notorious for long takeoff runs, and reducing the actual thrust available would only exacerbate the issue, even if only marginally.

Examining The CFM LEAP Engine

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The Boeing 737 has flown with CFM engines since the 1980s. The 737 Classic and 737NG used two generations of the CFM56, so when it was time to reengine the 737 again, it was natural that Boeing would select the company’s new LEAP engine. The LEAP-1B derives much of its technology from the General Electric GEnx, but has been refined to suit short-haul operations. The LEAP family was first developed for the Airbus A320neo family, dubbed the LEAP-1A.

However, the LEAP-1B has several changes that optimize it for the needs of the Boeing 737 MAX. Perhaps the most obvious difference is that it’s smaller, with a lower bypass ratio and less power. Despite this, it manages to achieve similar fuel burn figures as the LEAP-1A, largely due to its lighter weight.

To achieve its incredible efficiency, CFM is primarily upping the internal temperatures of the engine core. This boosts fuel efficiency, but the core also needs to be able to withstand the heat, which is why the new LEAP incorporates a wide variety of exotic materials, such as Ceramic Matrix Composites. Two low-pressure stages were removed for the 1B as it has lower thrust requirements, while the LEAP engine is generally known for having excellent reliability.

How Other Aircraft Are Addressing Noise Concerns

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It’s important to note that most of what you see on the outside is not actually the engine, but rather the nacelle. Companies such as Rolls-Royce or CFM International provide engines, while Airbus and Boeing design nacelles that cover most of the motor. For the 737 MAX, engineers weren’t able to develop an alternative solution to reduce noise, and the aircraft went out with the chevrons.

Meanwhile, the Boeing 777X’s initial 2013 concepts featured chevrons, but when its configuration was finalized, these were removed, as Aviation Week reported in a 2014 article specifically highlighting the change. Boeing first tested chevrons in 2005, so they’re ultimately old technology. With the 777X, the GE9X incorporates a new nozzle design that should have the same level of noise reduction while being lighter and more aerodynamic.

Airbus planes have never featured chevrons, and the company has also publicly rejected the concept. In 2006, Airbus stated point blank that it found no noise advantage by using chevrons on the A350, while also highlighting the fuel penalty. Instead, the European OEM focused on refining the A350’s nacelle construction to lower noise. Some CFM56-powered Airbus A320ceo aircraft do feature chevrons, but only on the inner nozzle.

The Future Of Engine Noise Reduction

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The Airbus A350 achieves noise reduction primarily through a “zero-splice acoustic inner barrel” in the nacelle, which reflects the company’s focus on sound insulation. The Boeing 777X, meanwhile, has a new nozzle design, and, while details are scarce as to how this has been accomplished, new composite materials are likely to have played a part in it. This means that the 737 MAX is likely the last commercial airliner that will feature chevrons in its current form.

The future of the turbofan engine appears to be a geared design. Pratt & Whitney debuted the PW1000G, the first geared turbofan engine on an airliner, and the engine has been a smash hit. P&W has faced serious reliability issues with the motor, but almost no issues are connected to the gearbox. Geared turbofans allow components to run much more slowly, greatly reducing noise. Rolls-Royce is also working on the UltraFan, a scalable geared design that is expected to eventually power a hypothetical A350neo.

High-bypass turbofan engines intake more air through the bypass ducts than through the core, which also reduces noise levels. The General Electric GEnx has a bypass ratio of 10:1, while the PW1100G has a 12.5:1 ratio, and the UltraFan is expected to feature a 15:1 bypass ratio. Furthermore, it appears that manufacturers are focusing on refining existing components and technologies to make engines less loud, rather than searching for radical changes like the chevrons.

The Bottom Line

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In the 2000s, Boeing was working to break conventions. The 787 that debuted this technology was filled with a wide array of innovations, some of which caught on and some of which remain unique to the Dreamliner. The chevrons were reused on the 747-8 and the 737 MAX, but it appears that their time has passed.

Commercial aviation is a conservative industry, and it’s a game of percentages and decimal points. Although chevrons brought an aeroacoustic benefit, the fuel burn penalty meant that the concept was always living on borrowed time. Today, improvements in other areas have rendered these obsolete, and although they’re a distinctive, modern design feature today, it won’t be long before they’re looked at as a quirky relic of the past.