CFM Built A Jet Engine So Large That It Forced A Redesign Of The Jet It Powers

CFM International did not arbitrarily build an oversized engine. When Airbus announced the A320neo family featuring ultra-efficient, large-diameter engines, the CFM LEAP-1A and Pratt & Whitney GTF, airlines demanded a matching fuel-efficient option from the American maker. Boeing demanded a high-bypass turbofan to compete with Airbus, forcing the engine maker to deliver the LEAP-1B. Rather than spending years and billions of dollars building a clean-sheet, modern airplane from scratch, Boeing chose to modify the existing 737 Next Generation (NG) airframe. The original 1960s Boeing 737 was intentionally designed to sit very low to the ground. In the MAX, Boeing attempted to fit 2010s engine technology onto an airframe originally designed in the 1960s, in stark contrast to its competitor. While CFM actually shrunk the 737 MAX's engine fan to 69 inches (175 cm), compared to the 78-inch (198 cm) for Airbus, it was still far too large for the existing 737 airframe. Moving the heavy, wide engines upward and forward meant that during specific high-altitude, high-angle-of-attack maneuvers, the engine nacelles acted like mini-wings. To counteract the aerodynamics and ensure the MAX handled exactly like older 737 models, Boeing implemented the Maneuvering Characteristics Augmentation System software. MCAS was intended to operate in the background and autonomously push the nose back down, which ultimately became the focal point of the subsequent 737 MAX design crisis. America's Low-Slung Jetliner: Legacy And An Airframe Dilemma Credit: Wangkun Jia | Shutterstcok In April 1967, Boeing introduced the 737, which had a low fuselage and short landing gear for ease of ground crew access and was intended for tiny airports with minimal equipment needs. Pratt & Whitney JT8D engines, which fit neatly beneath the wing, were used in early variants. Wider fans were more cost-effective, but they also added weight and drag; thus, the need for greater fuel economy created problems. When Boeing began planning the 737-100 and -200 in 1964, the aviation world looked vastly different. Jet travel was transitioning to smaller cities, and the primary design objective was to build a self-sufficient aircraft. To pull off the ultimate trick of building virtually new planes while legally keeping them registered under the exact same 1967 type certificate, Boeing used a clever strategy for both the NG and the MAX. In the 1990s, Boeing engineered a larger modern wing, computer-controlled engines, and digital flight screens. However, to keep the planes operationally identical, engineers programmed those advanced digital screens to display retro, analog-style dials and preserved the mechanical overhead switch layout. Pilots could step into the advanced NG and fly it using the exact same muscle memory and procedures as the 1980s Classic models without setting foot in a simulator. The 2010s 737 MAX repeated this illusion but pushed it past physical limits. To fit massive, ultra-efficient LEAP-1B engines onto the low-slung frame, Boeing moved the engines higher and further forward on the wing. Boeing promised airlines that any pilot flying an NG could transition to a MAX by doing nothing more than an iPad course. To legally satisfy the Federal Aviation Administration (FAA) that the MAX was identical to the older 737 NG, Boeing had to present the new plane as having no major behavioral differences. The Hamster Pouch: Nacelle Cantilevers & Wing Geometry Credit: Coby Wayne | Shutterstock The 'hamster pouch' engine casing designed for the 1984 Boeing 737-300 is the exact mechanical ancestor of the MAX crisis. It represents the very first time Boeing chose to patch over a fundamental physical limitation with a clever, compromising workaround. When the Boeing 737-300 made its debut in February 1984 with the CFM56-3 engine mounted ahead of the wing, it represented a major engineering change. The engine's bypass ratio dropped from 24,000 to 20,000 pounds of thrust as a result of this move, which also produced a flat-bottom nacelle known as the hamster pouch and a smaller fan. The MAX initiative was started in 2011 as the need for fuel efficiency increased and the design's shortcomings became apparent. When airlines demanded efficient, wider turbofan engines in the 1980s, Boeing encountered the exact same obstacle they would face decades later: the 1960s fuselage sat too close to the ground. By the time the 2010s arrived, the required engines had grown so colossally large that flattening the bottom of the casing was no longer enough. Airbus gave CFM a massive 78-inch (198 cm) canvas for the A320neo because the A320 airframe sat high off the ground. Boeing, demanding the exact same thermodynamic efficiency, forced CFM to scale the LEAP engine down to a highly constrained fan for the MAX just so it wouldn't scrape the runway. To gain clearance, Boeing had to physically relocate the entire engine forward and upward, completely corrupting the airplane's aerodynamics and causing the nose to pitch up. Having run out of room for physical hardware workarounds like the hamster pouch engine housing shape, Boeing turned to a digital workaround instead. The severe, forward-cantilevered placement introduced intense physical loads on the wing structure and created the exact nose-up pitch characteristics that Boeing's MCAS software was secretly built to mask. How CFM Shrank The LEAP: Aerodynamic Side Effects Credit: Alexandr Zimovskoy | Shutterstock The A320neo's LEAP-1A engine, which has a bigger fan, taller landing gear, and a greater bypass ratio than the 737's rejected LEAP 1B, is an example of what CFM might have created with more space. Because it requires less thrust, the LEAP 1B has a redesigned low-pressure turbine construction, a lower bypass ratio, and less weight while maintaining the sophisticated core of its predecessor. After much discussion, Boeing decided to go with a bigger fan in order to increase efficiency. However, this choice created problems for the airframe, requiring the nacelles to be moved, which altered flight dynamics and potentially conflicted with federal stall regulation standards. CFM International upgraded the low-pressure turbine of the LEAP-1B engine from seven stages to five in order to lower the weight of the turbofan core in response to the strict design requirements of the MAX. In spite of the heavier engine housing of the CFM56-7B, developers used lightweight titanium alloys and ceramic matrix composites to improve thermodynamic efficiency. The prototype 737-8, which first flew on January 29, 2016, is powered by two LEAP-1B28 engines and demonstrated a significant reduction in fuel burn on medium-range flights. However, the reliance on software for engine integration linked to legacy flight controls laid the groundwork for future operational challenges and disruptions in commercial aviation. Where Boeing Found The Room: An Inflexible Airframe Credit: Coby Wayne | Shutterstock Three structural changes were made to the 737 MAX airframe by Boeing. The first modification was to lengthen the nose landing gear, which raises the engines and lifts the front of the aircraft, giving it a considerably higher nose than the NG model. Maintaining a certain minimum distance between the engine and the ground required this adjustment. There were fewer major changes to the wing box construction because the engines were positioned higher and farther forward on the wing. The engine housing's bottom was flattened to resemble the nacelle design from 1984, allowing for increased ground clearance. These changes have practical implications for ground personnel, as the hazard zone in front of each engine has expanded, affecting the safety of workers around the aircraft. Extensive flight testing was required to evaluate how moving the massive engines forward and upward altered the airflow dynamics surrounding the wings and nacelles. This physical displacement fundamentally disrupted the aircraft's aerodynamic stability, creating a distinct nose-up tendency that directly impacted how the jet handled and forced engineers to modify the flight controls. To address these handling deviations without embarking on a costly structural redesign, Boeing chose to fix the physical problem digitally. By embedding the Maneuvering Characteristics Augmentation System (MCAS) into the software, Boeing successfully satisfied regulatory handling criteria while preserving the single pilot type rating and avoiding catastrophic financial penalties tied to airline contracts. Software Compensation: The Maneuvering Characteristics Augmentation System Credit: fikrizzudinoor | Shutterstock During flight tests, the 737 MAX engine configuration caused a slight nose-up tendency in various manual flying conditions. To address this, Boeing developed the MCAS to lower the nose when approaching such situations. Although the FAA and EASA later determined the airframe itself was inherently stable without it, Boeing chose to manage this hardware characteristic entirely through software, critically designing it to rely on a single, non-redundant Angle of Attack (AoA) sensor. This fragile design choice proved catastrophic. On October 29, 2018, ​​​​​​​a Lion Air 737 MAX 8 crashed into the Java Sea shortly after taking off from Jakarta, Indonesia, resulting in the deaths of all 189 passengers. Investigations revealed that a false high angle of attack reading activated the MCAS, causing the aircraft's nose to pitch down. Following this, Ethiopian Airlines Flight 302 also crashed in March 2019, leading to a total of 346 fatalities from both incidents, and the subsequent grounding of the MAX by regulators. Due to its technical issues, ​​​​​​​the 737 MAX was grounded for 20 months worldwide. While investigators picked apart the deadly relationship between the automated trim software and the plane's aerodynamics, Boeing's production lines ground to a halt. More than 400 undeliverable, newly built airframes sat stranded in desert storage lots, choking the supply chain and ultimately forcing the manufacturer to pay out over $8.3 billion in airline customer compensation. The crisis finally ebbed on November 18, 2020, when the FAA cleared the MAX to resume commercial service. The re-certification followed a complete overhaul of the flight control logic. MCAS now requires concurrent data from both AoA sensors, activates only once per aerodynamic event, and can be easily overridden by the flight crew.