My earliest memory of boarding a Boeing 737-300 from Philippine Airlines dates back to the early 2000s. At that time, something about the aircraft caught my attention – the engines weren’t entirely round, they had a flat bottom. Parked adjacent to our 737-300 was a 747-400, its engines noticeably rounded. This observation stirred my curiosity about the unique design of the Boeing 737 engines, the flat bottom. After some time spent on research, I eventually found the answer. I’m excited to share that discovery with you now.
The unique flat bottom design of the Boeing 737’s engines has piqued the curiosity of many, from aviation buffs to everyday passengers. Is this an error in design or a purposeful feature? What could have led a reputable aircraft manufacturer like Boeing to choose such a distinct structure? But before we delve into the reasons, let’s first take a brief journey through the history of the 737.
Early Days and the Birth of an Icon
The story of the Boeing 737 dates back to 1965, with the introduction of the 737-100 and 737-200 models, both integral parts of the Boeing 737 original series. These aircraft utilized Pratt & Whitney JT8D engines, which were distinctly rounded.
One of the main characteristics that sets the 737 apart from other aircraft is its low-to-ground design. This was a deliberate decision on Boeing’s part to meet the specific requirements of the market it was targeting.
Designing for Accessibility
In the 1960s, many small airports lacked the extensive support equipment found in larger ones. They often didn’t have the jet bridges used for passenger boarding or the high-load baggage systems. Therefore, an aircraft that could operate in such conditions was needed.
The low ground clearance design of the 737 made it ideal for such scenarios. Passengers could easily board the aircraft via built-in airstairs rather than a jet bridge. Baggage and cargo could be loaded directly from the ground to the cargo hold without the need for specialized high-load equipment. The design also allowed for easier maintenance inspections and repairs.
The progression towards more advanced models demanded an extension of these original models, leading to the birth of the 737 Classic series in 1984. Comprising of the 737-300, 737-400, and 737-500, these enhanced aircraft models needed certain modifications to uphold the aircraft’s balance.
The Quest for Efficiency and Stability
To maintain the critical center of gravity (CG), which directly affects aircraft stability, the engines in the Boeing 737 Classic series were situated slightly forward of the aircraft wing. This careful repositioning was paramount in preserving the aircraft’s equilibrium, considering the extended lengths of these newer models.
However, the quest for greater efficiency led to another significant shift – the replacement of Pratt & Whitney JT8D engines with the CFM International CFM56 engines. The CFM56 engines boasted a higher bypass ratio, resulting in increased efficiency, which was an enticing proposition for the aircraft giant.
Overcoming Design Challenges
The switch to CFM56 engines, while advantageous in terms of efficiency, presented a formidable design challenge. The CFM56 engines were substantially larger than their predecessors, the JT8D engines. Without modification, these bulkier engines would collide with the ground.
Some may propose the simple solution of lengthening the landing gear to mitigate the ground clearance and center of gravity complications. However, several reasons prevent Boeing from easily implementing this change for the 737. One significant deterrent is the need for substantial redesign and modification of the aircraft, a process that would be both costly and time-consuming.
The design places the landing gear almost centrally, just inside the nacelles, retracting inwards to nearly converge at the fuselage centerline, leaving the wheels doorless.
In essence, without a total overhaul of the wing design and subsequent recertification of the newly resulting aircraft, there is no feasible way to extend the main gear’s length.
Boeing aimed for an inventive yet simpler resolution. The company opted to adjust the engine’s form, which led to the creation of a flat bottom. This minor change proficiently augmented the distance between the engine and the ground, eliminating the necessity for significant alterations in aircraft design or operation. There was no requirement to increase the length of the landing gear.
Preserving the Flat-bottomed Legacy
Boeing continued this unique flat-bottom engine design with the introduction of the next-generation series in 1993. This series, which includes the 737-600, 737-700, 737-800, and 737-900, all sport the CFM56 engines with the signature flat bottom, demonstrating the successful integration and longevity of this design solution.
The 737 MAX Engines Have Flat Bottoms
When Boeing decided to introduce the 737 MAX, the company aimed to further improve fuel efficiency. This led to the decision to equip the MAX with the LEAP-1B engines designed by CFM International. These new engines, larger in diameter than the previous CFM56 engines used in 737 Classic and Next Generation aircraft, promised significant reductions in fuel consumption and noise.
However, incorporating these larger engines into the design of the 737 posed a challenge, due to the 737’s traditionally low ground clearance – a hallmark feature since its first iteration. Just as they did with the 737 Classic and Next Generation series, Boeing needed to make modifications to ensure sufficient ground clearance.
With the MAX series, apart from maintaining the somewhat flat bottom engine design, Boeing opted to position the engines slightly more forward and higher on the wing. This allowed them to accommodate the larger diameter of the LEAP-1B engines, while preventing the engines from striking the ground during take-off and landing. It’s also worth noting that this adjustment had an impact on the aerodynamics of the aircraft, requiring Boeing to introduce the MCAS (Maneuvering Characteristics Augmentation System) to address these changes.
First love never dies. I fell in love with airplanes and aviation when I was a kid. My dream was to become a pilot, but destiny led me to another path: to be an aviation digital media content creator and a small business owner. My passion for aviation inspires me to bring you quality content through my website and social accounts. Aviation is indeed in my blood and blog!
