If you’ve ever looked closely at a Boeing 737, especially the newer models, you may have noticed something unusual: its engines aren’t perfectly round. Instead, they appear to have a distinctively flat bottom. This isn’t a design flourish—it’s the result of decades of engineering decisions tracing back to the 1960s. Here’s how this peculiar feature came to define the world’s most popular commercial jet.
The Origins: A Low-Slung Legacy
When Boeing first launched the 737 in the late 1960s, it was designed to be low to the ground. Back then, the idea was to create a short-haul aircraft that could operate at smaller airports without requiring specialized ground equipment like built-in staircases or cargo loaders. The 737’s engines—originally the narrow Pratt & Whitney JT8D—fit snugly beneath the wing, thanks to this low-slung design.
But as engine technology advanced and fuel efficiency became increasingly critical, Boeing faced a growing problem: newer engines were physically larger, and the 737’s low clearance left little room to fit them.
Adapting to the CFM56: The First Engine Workaround
In the 1980s, Boeing decided to upgrade the 737 with more efficient CFM56 turbofan engines for the new 737 Classic (-300/-400/-500) series. The problem? The CFM56 had a much larger fan diameter than the old JT8Ds and simply wouldn’t fit under the wing in its traditional form.
So Boeing engineers got creative. Instead of mounting the engines directly beneath the wing, they were moved slightly forward and higher—and the bottom of the nacelle was flattened to increase ground clearance. This distinctive shape allowed the 737 to retain its low stance while accommodating more powerful and efficient engines.
Bigger Engines, Bigger Problems: Enter the LEAP-1B
With the 737 MAX program, Boeing once again faced the challenge of integrating an even larger and more fuel-efficient engine: the CFM LEAP-1B. These engines offered major performance improvements and fuel savings, but their size further strained the limits of the 737’s design.
To make them fit:
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Boeing slightly raised the nose landing gear.
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The engines were again moved higher and farther forward.
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The nacelle maintained its now-iconic flat-bottomed shape.
While this worked from an engineering perspective, it changed the aircraft’s aerodynamics, contributing to the need for flight control software (MCAS) to counterbalance the altered pitch behavior. The MCAS system would later be central to the 737 MAX crisis, leading to two fatal crashes and a lengthy global grounding of the aircraft.
A Design Choice That Came Back to Haunt Boeing
Boeing’s decision to keep updating the original 737 airframe instead of designing a clean-sheet replacement eventually boxed the company into a corner. As competitors like Airbus launched the A320neo—a modern aircraft platform reengineered for new-generation engines—Boeing had to push the 737 design further than ever intended.
The modifications required to accommodate larger engines on a legacy frame led to compromises in design and systems integration, culminating in the issues surrounding the MCAS system on the 737 MAX.
Despite It All, the 737 Remains a Bestseller
Despite its design constraints and well-publicized setbacks, the 737 remains the best-selling commercial aircraft in history. The 737 MAX has rebounded with strong sales and is now flying for major carriers worldwide, thanks to software updates, regulatory oversight, and safety reforms.
Its popularity stems from:
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Proven operational history
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Commonality with older 737 models
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High fuel efficiency with new LEAP-1B engines
Fuel Efficiency Gains—But at What Cost?
The LEAP-1B engines on the 737 MAX deliver up to 15% better fuel efficiency compared to older models, allowing airlines to reduce costs and emissions. However, achieving these gains while sticking to an aging airframe came with engineering trade-offs and regulatory scrutiny.
In contrast, Airbus’s A320neo family—designed from the ground up for larger engines—was able to incorporate its upgrades more seamlessly.
Final Thoughts
The flat-bottomed engines on the Boeing 737 are a visual legacy of decades of adaptation. They reflect Boeing’s attempts to modernize a decades-old design in a competitive and cost-conscious aviation market. While this approach brought fuel efficiency and lower operating costs, it also exposed the limitations of continually modifying an aging platform.
As Boeing looks toward its next generation of aircraft, the lessons from the 737—and its uniquely shaped engines—will likely guide future decisions more cautiously.
