Boeing has implemented Engine chevrons, or serrated chevron ducts, on some of its more recent aircraft designs, a feature that can reduce noise emissions and help aircraft abide by noise regulations. Since incorporating chevrons onto the Boeing 787 Dreamliner, the planemaker has also equipped this feature to the Boeing 747-8 and Boeing 737 MAX.

Given that this was a new technology for Boeing to include on a commercial passenger jet, the design and implementation process did not exactly run smoothly. The manufacturer encountered several hurdles during testing and manufacturing that forced it to make some key changes to its chevron designs. It has since dropped chevrons from its 777X, but they remain a key feature on the thousands of 737 MAX, 747-8 and 787 Dreamliners in service today.

Early Problems Boeing Faced With Chevrons

Back in the 2000s, Boeing began working on the development of serrated chevron ducts as a solution to engine noise. The company teamed up with space agency NASA, which had identified the V-shape of chevrons as the optimal design for noise reduction by optimizing the mixing of air around the engine. With the FAA’s Stage 4 noise regulations coming into effect by 2006, aircraft manufacturers had to take noise-reduction seriously to adhere to the stricter requirements.

However, it was a long process to iron out the mistakes and develop an optimal design, and the team found that some of its tweaks actually made the noise problem worse. NASA researcher James Bridges noted that the researchers « didn’t have the advanced diagnostics, instrumentation, and insight to know what we had done to make it worse instead of better. » Bridges added,

« Sometimes the kernel of the idea might have worked out, but the way you did it wound up causing more noise. »

Structural durability issues were another major problem encountered during testing. As the chevrons are exposed to such high levels of heat and vibrations, early designs experienced cracking and fatigue. Enginemaker Rolls-Royce also got involved with Boeing on a Quiet Technology Demonstrator (QTD) – this sophisticated series of flight tests involved 100 microphones attached to a Boeing 777-300ER, as well as 200 microphones on the ground to measure the effectiveness of different noise reduction packages.

Chevrons Caused Thrust Loss

The biggest concern for Boeing was that, while chevrons were certainly effective at lowering engine noise, they also had the undesirable effect of reducing aerodynamic efficiency. This is caused by the creation of vortices around the engine which disrupt airflow and increase drag, ultimately diminishing the amount of thrust each engine can deliver.

As explained by aviation YouTube channel Mentour Pilot, the chevrons reduced engine thrust by approximately 0.5% – while it was only a small efficiency loss, every percentage point matters when it comes to designing efficient aircraft. In an industry where the margins are incredibly fine, a 0.5% efficiency penalty can take a significant chunk out of an airline’s profits.

The effect is more pronounced on aircraft conducting multiple flight movements per day, meaning 737 MAX operators usually pay a greater penalty. This is because the takeoff and climb phase of a flight is the most demanding on the engines, impacting planes like the 737 MAX, which typically perform several flights per day.

What Is The Point Of Engine Chevrons?

The primary purpose of chevrons is to reduce engine noise, which works by smoothing the mixture of exhaust flows. Boeing’s decision to explore and ultimately implement chevrons was driven by changing regulations around airport noise. As it became more apparent that noise emissions were a valid environmental concern, the rules got tighter around emissions, and the FAA’s Stage 4 standards had come into effect by the time chevrons debuted on the 787 Dreamliner.

For airlines, this meant restrictions or bans on certain aircraft, or higher airport fees for operating noisier models. Environmental compliance is a key factor that airlines keep in mind when considering their aircraft purchasing options. Not only do they need a plane that meets the noise standards of today, but one that will also be compliant with future standards.

There is another key benefit that chevrons can offer, and that is weight reduction, which can lead to major efficiency gains by reducing fuel consumption. This is because the presence of chevrons negates the need for heavier noise insulation inside the aircraft, allowing manufacturers to reduce an aircraft’s empty weight. For example, Boeing was able to drop 600 lbs (272 kg) of weight on the 787 Dreamliner by removing sidewall insulation.

Which Boeing Planes Have Chevrons?

Boeing has equipped engine chevrons to three of its aircraft families – the Boeing 737 MAX, 747-8, and 787 Dreamliner. Chevrons are a part of all 787 Dreamliners regardless of engine, while the 737 MAX and 747-8 only come with one engine option.

The 737 MAX and 787 Dreamliner both feature chevrons on the nacelles, but the 747-8 is unique in having them both on the nacelle and engine nozzles. However, Boeing’s next aircraft design – the 777X – removed the concept of chevrons entirely, opting for other noise-reduction methods (explored later in this article).

You can easily spot chevrons on an aircraft – they look like jagged, sawtooth-like patterns around the trailing edge of the engine nacelle, a feature that distinguishes them from the typical smooth and round edges you see on other commercial jet engines. Looking at data from ch-aviation, there are over 3,000 Boeing aircraft in service today that are fitted with chevrons.

Are There Better Ways To Reduce Engine Noise?

NASA conducted a series of computer simulations in the 1990s that explored different methods of engine noise reduction, concluding that serrated edges on the nacelle were theoretically the most effective design addition. But plane and enginemakers have a variety of tools and techniques at their disposal to reduce noise, and will typically combine these in their noise-reduction kits.

Nozzle designs are one of the most effective ways. With the Boeing 777X and its GE9X engine, an advanced nozzle design was able to achieve noise reduction on a level similar to a chevron but without incurring any thrust penalties. Engines with a higher bypass ratio will also emit less noise by allowing more air to flow around the engine core and at a lower velocity.

Then there are acoustic linings, which are sound-absorbing materials placed inside the nacelle to soak up some of the noise. This is also a feature on the upcoming 777X, which has incorporated a honeycomb-like lining consisting of thousands of little holes that absorb noise. As for the effectiveness of each choice, it depends on the specifics of each engine and airframe.

How Chevrons Got Their Name

According to Etymonline, the word chevron comes from the French language and translates as « rafter » or « roof beam, » essentially referring to the V-shape of two rafters coming together.

The word was then adopted to refer to the V-shaped military insignia denoting rank and remains in use today for that purpose. The V-shape of the serrated ducts that make up engine chevron designs made the word an ideal fit, and Boeing has run with it ever since.

The aviation industry has a strong heritage of using dated terms, particularly with nautical or military connotations, in describing parts of an aircraft. For example, this is where words like nacelle and fuselage come from, both of which have a French origin, while rudder is a nautical term.

How Boeing Solved The 777X Noise Problem

Boeing abandoned chevrons for the 777X, a decision heavily influenced by the thrust penalties caused by their inclusion. Together with GE Aerospace, more optimal noise-reduction solutions were implemented for the aircraft’s GE9X engine, the largest commercial jet engine ever built, without compromising on engine performance.

At the heart of this is its advanced low-drag nozzle system, which is also around 20% lighter than those found on other engines. As explained by Terry Beezhold, chief project engineer of the 777X program,

« We are replacing the chevrons with a new nozzle design technology. It provides equivalent levels of noise for the cabin and community but is lighter in weight and has lower drag. »

As such, GE claims that the engine is the quietest ever built on a per-pound-of-thrust basis. The GE9X will be one of the most powerful jet engines ever made with a thrust rating of 110,000 lbf (490 kN). Although this is slightly lower than the GE90 on the Boeing 777, the GE9X has far exceeded this during testing and currently holds the Guinness World Records title at 134,500 lbf (597 kN).