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April 15, 2024
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CFM is developing its next generation engine, known as LEAP-X.   This engine is due to power the COMAC C919 and the Airbus A320neofamily and is almost certainly going to power either a re-engined Boeing 737 or its replacement.  Understanding this new engine benefits from understanding its heritage.

CFM International, a joint venture between GE and France’s Snecma, developed its first engine, the CFM56 in 1982.  Initially sold as the CFM56-2C the engine was used to re-engine DC8s and used extensively by the USAF to re-engine KC-135s.  Indeed, the USAF is the largest CFM customer even today.  (There are 470 KC-135R and RC-135R aircraft in service, accounting for 1,966 CFM56-2 engines.)

In 1984 CFM offered a more advanced version, the -3, that exclusively powered Boeing’s 737-300.  The story behind this engine choice is legendary – a case study for any business.  Pratt & Whitney had previously offered its JT8s on the 737, and when asked to develop a new engine for the 737 update, demurred.  PW believed the industry was moving toward larger aircraft and place its bet with the Boeing 757s PW2000 engine. Looking back at the number of lost sales since then is one Pratt’s greatest regrets.

CFM offered the -3 from 1984 to 1999, and the engine changed considerably over the period.  This continuing tweaking by CFM matched Boeing’s tweaking on the 737.  The combination of the two working almost in tandem is that the 737, despite its 1960s origins, remained remarkably efficient and could compete against the much newer A320.

By 1997 CFM had refined the CFM56 into a newer variant, known as the -7B (-5B on Airbus) (The CFM56-5B entered service in 1996). This variant saw a 5% fuel burn improvement, resulting in increased range, as well as 15% lower maintenance costs  The incessant tweaking was showing benefits. Ten years later, in 2007 CFM offered a “Tech Insertion” to the -7B, which offered an additional 1% fuel burn improvement of the life of the product.  Durability was improved, too.  True to form, Boeing also kept tweaking the 737.  Currently the latest version of the venerable 737 is sporting the -7BE (“E” for Evolution) version of the CFM56.  The combination should offer an additional 2% better fuel burn, 4% lower maintenance costs and even better durability. We understand the 1% better fuel burn goal CFM set out to achieve might turn out to be on the order of 1.5%. (CFM are waiting for the results of the flight test program to confirm these numbers)

To get an idea of what an improvement in maintenance on this engine means, consider that of the CFM56s, some 70% of the -5B and -7B versions delivered still have not yet had their first shop visit.  Some engines have spent as much as nine years on wing.  This is remarkable – and yet CFM sees even improved performance on this record. It is for this reason that CFM says it offers the lowest cost of ownership.  The CFM56 boasts amazing numbers: 16,500 engines, the engine takes to the sky every 2.5 seconds somewhere in the world, with typical customers doing 8 segments per day – and dispatch reliability is 99.98%.  CFM has delivered more than 7,000 CFM56-7B engines to Boeing for the 737NG and they have all been on time and it’s the same story with Airbus.

Given this background, one can understand that a new CFM56 engine faces an awfully high, and rising, benchmark for a successor or competitive engine to meet.  But fortunately for CFM, it is able to cherry pick the best R&D its parents GE and Snecma have to offer.  For example, from the 1998 studies called TECH56, improvements developed ranged from 3-D aerodynamic design technologies, better cooling, improved compression and the TAPS combustor. Indeed, CFM benefits from GE pioneering some of this technology in the GE90 and GEnx.

Aero engines are developed around a series of tradeoffs.  This is illustrated by the following chart CFM provided. As it illustrates, there is a “sweet spot” (see chart)  that designers try to achieve, where the tradeoffs present the greatest utility and least compromised efficiencies.  Bigger is better when it comes to engine fuel burn, but you achieve diminishing returns when installing an engine that is too large for an airplane because of weight and drag.

Airlines were accustomed to operating in a world that saw fuel costs between 20-25% of total costs.  Today, the airline business has now evolved into an environment in which fuel costs range between 40-50% of costs.  Clearly this is a sea-change that cannot be ignored.  The curves in the chart have moved, and there is every expectation that the move is permanent.  CFM and its competitors have to adjust their engines because OEMs and airlines are demanding solutions to higher fuel costs.

Hence CFM’s response came to be known as the LEAP-X, building on the LEAP56 technology R&D initiatives. While airlines operate in an intensely difficult business, where exogenous impacts from volcanoes and weather and even politics disrupt operations, anything that provides stability is cherished.  CFM’s amazing numbers for the CFM56 engine obviously speak for themselves in this regard.  Fuel prices may spike, but operators want an engine that runs day in and day out, in any weather, hour after hour.

CFM is aiming its new engine to deliver the CFM56’s reliability and low maintenance costs, but also offer 15% lower fuel burn, 50% lower NOx versus the current ICAO CAPE/6 requirements and up to 15 dB better than Chapter 4 noise requirements.  It is attempting something of a tough assignment – a quantum leap in improvement over the -7 variant.   How can CFM hope to get there?

There are benefits from experience with new technologies on larger GE engines.  Using lessons learned from the GE90 and GEnx, there will be increased use of composites.  For example, a composite fan and fan case should see a 1,000lb weight reduction per ship set.  The bypass ratio, closely associated with fuel efficiency, will rise from today’s 5:1  to about 10:1.  The engine will see a hotter core without the metal temperature rising above those current experienced on the CFM56.  In addition the LEAP-X core will use ten stages rather than nine.  All told, the LEAP-X will be heavier than the CFM56, but using the above chart as a guide, CFM believes the tradeoff of higher weight is more than offset by substantially greater fuel burn efficiencies and lower noise – a greener engine all around.

The LEAP-X has so far been adopted by COMAC for its C919 and by Airbus for its A320neo program.  The new engine will have a fan diameter of about 75 inches (NOTE:  actual diameter depends on aircraft requirements).  This is not much larger than the current 72 inches on the CFM56.  The LEAP-X will have 18 fan blades compared to 24 on the CFM56-7B.  The CFM 56-5C on the A340 has 36 blades – so here is another example of incessant tweaking to improve the engine.  The overall pressure of the LEAP-X will be about 20:1, which is in the same class as the GE90.  That means an engine with about 30,000lbs of thrust will have efficiencies similar to an engine with over 110,000lbs of thrust.

CFM advises that it is currently building what it classes the “eCore Demo 2” and plans this engine to be in test by mid-year.  Design freeze is expected by the end of this year. This particular core is supposedly very close to the final LEAP-X configuration (10-stage compressor, two-stage turbine).  Full tests are expected in 2013 and first flight on a test bed 747 in 2014.

CFM seems confident they can meet Airbus’ A320neo EIS for 2016. This is a tight schedule and given the gestation of aerospace programs, timelines are best looked at warily.  Analysts like to poke holes in schedules because of the myriad things that can go wrong—which clearly apply to engine as well as airframe development programs.  But then take a look at what CFM has achieved to date. The company has developed a remarkable engine in the CFM56.  It might not be a wise idea to bet against them, because their continuous product improvement and lessons learned from GE should considerably reduce both timeline and performance risks. CFM also has something it can point to – CFM has never missed a certification date in its 36-year history.

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5 thoughts on “The foundation behind the LEAP-X

  1. Superb article! Thanks for posting. This is the most comprehensive layman’s summary I’ve seen yet.

  2. Good story – but the overall pressure ratio will be nuch higher than 20, the core alon will have a pressure ratio of about 22, so OPR will be more i the 45-50 ballpark – just like GEnx or GE90.

  3. I’ll give you kudos for a good article as well.

    Just one nit-pick though; the fan diameter of the CFM engines on the 737NG and A320 is smaller than 72 inches (61″ and 68.3″ respectively). 🙂

    Fan diameter CFM56-7B (737NG): 61.0 inches


    Fan diameter CFM56-5B (A32X): 68.3 inches


    Fan Diameter CFM56-5C (A340-200, A340-300): 72.3 inches


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