Most of the flights taken every day around the world are on narrow-body jets, the Boeing 737 and Airbus A320 families that shuttle between cities on short and medium routes. And under the vast majority of those wings you will find an engine with the same corporate parentage: CFM International, the long-running joint venture between America's GE and France's Safran. For decades the answer was the CFM56. Today it is the LEAP. Understanding the difference is understanding the biggest efficiency jump the single-aisle world has seen in a generation.
The CFM56 is one of the best-selling jet engines in history, with well over 30,000 built since it entered service in the early 1980s. It powered the Boeing 737 Classic and the 737 Next Generation, the Airbus A320ceo family, and even military types like the KC-135 tanker. Its appeal was reliability and maturity: airlines knew exactly how it behaved, how long it lasted between shop visits and how much it cost to run. Thrust ratings ranged roughly from the high teens to the mid-30,000-pound class depending on the model.
By the 2010s, however, fuel prices and environmental pressure demanded something dramatically more efficient, and CFM answered with the LEAP, short for Leading Edge Aviation Propulsion. The LEAP is not a tweaked CFM56; it is a clean-sheet design that targets around a 15 percent improvement in fuel burn. It comes in three closely related versions: the LEAP-1B for the Boeing 737 MAX, the LEAP-1A for the Airbus A320neo family, and the LEAP-1C for China's Comac C919.
The core of the improvement is the same lever that transformed wide-body engines: a bigger, higher-bypass fan moving more air more gently. The LEAP runs a noticeably higher bypass ratio than the CFM56 and a much higher overall pressure ratio, so it does more with each unit of fuel. To get there, CFM leaned on materials the older engine never had. The LEAP fan blades are made from a 3D-woven carbon-fiber composite, light and strong, and the hot section uses ceramic matrix composites in places like the turbine shrouds, letting the engine run hotter without melting.
Those gains came with the usual growing pains. Introducing a new engine at very high production rates stretched CFM and its customers, and operators worked through early questions about durability of certain hot-section parts and time-on-wing before the LEAP settled into a mature, dependable product. That pattern, big efficiency step followed by a period of careful learning, is normal whenever an engine family turns over.
There is also an airframe wrinkle worth knowing. On the Airbus A320neo, the LEAP-1A competes directly with Pratt & Whitney's geared turbofan, so airlines actually have a choice of engine. On the Boeing 737 MAX, the LEAP-1B is the only option, which means every MAX sale is also a LEAP sale, an enviable position for CFM.
For a passenger, the practical differences are subtle but real: a LEAP-powered jet is quieter on takeoff, burns noticeably less fuel and produces fewer emissions than the CFM56-powered aircraft it replaces. The larger fan is why the engines on a 737 MAX or A320neo look visibly bigger, and sit slightly differently, than those on the older models. It is the same story that has played out across the industry, scaled down to the single-aisle jet: to fly more efficiently, build a bigger fan, move more air, and let modern materials handle the heat.
AviationShop Editorial Desk. Featured image: AI-generated by AviationShop. By Daniel Okafor.
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