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CFM’s LEAP into the future

CFM International says that LEAP - the newest engine from a nearly 40-year partnership between GE and Snecma - is on track for the first full engine test later in 2013 and certification in 2014. The engine offers a 15 per cent improvement in fuel efficiency and lower noise and emissions, while holding the line on maintenance cost and reliability.


CFM International says that LEAP - the newest engine from a nearly 40-year partnership between GE and Snecma - is on track for the first full engine test later in 2013 and certification in 2014. The engine offers a 15 per cent improvement in fuel efficiency and lower noise and emissions, while holding the line on maintenance cost and reliability.

To trace the roots of CFM’s next-generation LEAP engine, you have to go back many years. From a technology perspective,
the engine’s legacy reaches back some 20 years to the beginnings of GE90 development. And as far back as eight years ago, CFM worked with customers to gain their perspective on what they wanted in the next generation of powerplants for the single-aisle workhorses of tomorrow.

The payoff of that long perspective, CFM asserts, is an engine that will offer best-in-class fuel efficiency and emissions, while preserving the reliability and maintenance costs that have made the CFM56 product line legendary. 

When CFM executives talk about the LEAP programme, it’s with the distinct air of confidence that comes from treading on familiar ground. While the combination of technologies represented in LEAP are new to the CFM line, development, testing and planning for entry-into-service (EIS) is familiar territory, with CFM having been through 21 EIS and six major engine certifications on the CFM56 family over the last 30 years.

The LEAP engine was officially launched in July 2008. To date, it has been selected as the sole powerplant for the 737 MAX and COMAC's C919, China’s new 150-passenger single-aisle aircraft, and it is one of two engine options on the A320neo. The engine variants are the LEAP-1B, LEAP-1C, and LEAP-1A, respectively.   These aircraft are scheduled to enter commercial service in the 2016/2017 timeframe. To date, CFM has garnered orders for more than 4,600 LEAP engines across these three platforms.

The LEAP development programme has four guiding principles with ambitious goals for each. LEAP is designed to provide: 15 per cent better fuel efficiency; reliability and maintenance costs equivalent to the current CFM56 family, which are agreed to be the best in the industry; NOx emissions that are 50 per cent lower than CAEP 6 protocols; and noise levels that are 10-15dB lower than Stage 4 requirements.

This approach enabled CFM to successfully implement the strategy the company set in 2008: to be the sole powerplant on the new C919, to maintain sole-source status on the new 737 MAX, and to be one of the engine suppliers for the A320neo.  

There’s no question that the competition for the next generation of single-aisle airliners is hot, and given the prospects and history of the segment, it’s not hard to see why.  While the large high-bypass ratio engines for widebodies garner a lot of attention, narrowbody orders account for the overwhelming majority of transactions each year, and the cumulative numbers are staggering.

For instance, the workhorse CFM56 has been in service for more than 30 years, with more than 25,000 engines delivered and more than 630 million flight hours accumulated — the equivalent of 60,000 years of continuous operation.

Demand for single-aisle aircraft that offer mission flexibility is expected to remain strong. Current forecasts call for roughly 20,000 aircraft — 40,000 engines — in this market segment over the next twenty years.

Legacy of technology

The CFM 50:50 partnership with General Electric (GE) dates back nearly 40 years, and has been extended to at least 2040. The partnership unites two business cultures, which leverages the inherent strengths of both and ultimately means better decision-making.

LEAP engines incorporate revolutionary technologies never before seen in the single-aisle aircraft segment. The new engine combines advanced aerodynamic design techniques, lighter, more durable materials, and leading-edge environmental technologies, making it a major breakthrough in engine technology.  

The 15 per cent better engine fuel efficiency compared to today’s best CFM56 engine, at current fuel prices, translates to as much as $1.6m in fuel cost savings alone for customers per aircraft, per year. LEAP technology will also achieve double-digit improvements in CO2 emissions and noise levels, all while providing the industry’s best reliability and lowest maintenance costs.

One of the most aggressive technologies going into the engine is an all-new wide-chord composite fan, a first for CFM.  For LEAP, the fan will have just 18 blades, half the number on the CFM56-5C, and 25 per cent fewer than the CFM56-7B.

Building the fan required the development of new resin transfer moulding production processes, a development that has been underway at Snecma for nearly 20 years. The fan has been undergoing ground tests since early 2009, including a 5,000 cycle reliability test, blade-out tests, bird strike testing, and acoustics analysis - validating the design.

The composite fan and containment case pay off in terms of weight savings. The LEAP engine will be 1,000 pounds lighter per ship set than if the fan and case were made of metal. And because of GE’s experience with wide-chord composites on the GE90 and GEnx, they are confident about durability as well: to date, there have been no airworthiness directives (ADs) on the GE90 fan blades, and in the course of some 35 million flight hours in more than 18 years, only a few blades have been taken out of service.

The engine core draws heavily on GE's expertise which was developed for the GE90 and GEnx programmes, with compressor, combustor and coatings technology all being pulled forward into LEAP to improve performance while maintaining reliability.

CFM has completed testing on three builds of an advanced core, logging more than 550 hours of testing and validating CFM’s performance and operability predictions. The company has also installed LEAP hardware, scaled to size, in four builds of GEnx engines to gain even more test data prior to the first full engine test in the autumn of 2013. 

Some of the weight savings from the composite fan are absorbed by a stiff, double-wall compressor case, which is designed to prevent the core from flexing due to torque induced at rotation by the large fan, thereby reducing risk of blade rub and incumbent performance degradation.

The turbine blades themselves are designed using advanced fourth-generation 3-D aerodynamics to optimise performance. The first five compressor stages are blisks, which minimise air leaks by eliminating dovetail joints between blades and disks. In total, the 10 stages of compression create a 22:1 pressure ratio, which CFM claims is the best in the industry.

The fuel nozzles and combustion chamber are optimised for low emissions. Twin Annular Pre-Mixing Swirler (TAPS) fuel nozzles, first developed as part of CFM Project TECH56 in the late 1990s and now in commercial service on the GEnx, premix air and fuel and enable the engine to run at lower peak temperatures with longer residence time, key factors in reducing NOx emissions. The ‘T’ in TAPS refers to a nested pilot nozzle that runs rich at low engine speeds. However, as power is increased, fuel flow is directed to the lean-running cyclone nozzle that premixes air and fuel.

TAPS also makes for a more compact combustion chamber and eliminates the need for dilution holes, reducing stress on the chamber and diminishing cracking of the combustion chamber liner. Due to the precise control of fuel and air and the solid, double wall liner, exit temperature variation is reduced, improving durability of the high-pressure turbine (HPT) components, which are in the most brutal temperature environment in an engine and are major drivers of maintenance and overhaul costs.

CFM is also using advanced additive manufacturing to build the state-of-the-art fuel nozzle. The use of the technique opens the design space for engineers to design the part the way it needs to be, rather than in a way that accommodates tooling and other subtractive manufacturing requirements. For example, the nozzle produced traditionally would have required more than 25 individual pieces to be brazed into the part; a time-consuming process that did not provide an optimised design. With additive, that number has been reduced to less than five parts.

The two-stage high-pressure turbine incorporates 3-D aerodynamic design, advanced coatings, and GE-developed casting technology to improve cooling, the key to maximising the life of the blades. The LEAP HPT has undergone thousands of hours of component tests, giving CFM assurance that the core can run with higher thermal efficiency than the CFM56 core, but at equal blade metal temperatures — a key driver in hitting the goal of having LEAP maintenance costs equal to those of the CFM56.

Another key feature in the HPT is the first commercial introduction of ceramic matrix composites (CMCs) in the stage 1 HPT shroud. This material has been in development for more than 30 years. At one-third the weight of a comparable metal part, CMCs couple the thermal capability of ceramics with the durability that the matrix design provides. Using the very light material with outstanding thermal capability allows CFM to use less cooling air, which will provide fuel efficiency.

Another advanced material, titanium aluminide, is being used in the front stages of the low-pressure turbine. The material provides great thermal capability and significant weight savings.

Maintainability/reliability

Maintenance costs are a key component of the LEAP programme from a variety of perspectives. First and foremost, customer exercises indicated that maintenance and reliability were a foremost concern of airlines and other stakeholders. And with the increasing prevalence of fixed-cost-per-hour operating agreements, CFM’s economic case for LEAP is dependent on creating a reliable, durable engine with predictable costs right from the start.

An extensive test programme leading up to EIS in 2016 is key to validating those costs. The LEAP programme calls for running a total of 28 different engine builds that will accumulate more than 40,000 cycles prior to EIS, so that launch customers receive a totally mature product.

In addition to the coatings and combustion technology, CFM is employing other designs and lessons learned from the GE90 and GEnx programmes to meet its reliability targets — and to enable the engine to retain performance over its service life.

For example, the core is designed to be “FOD-free” (foreign object damage-free) with several techniques employed to keep particulate matter out of the core, reducing blade erosion so that performance is maintained over the life of the engine. The wide-chord fan blades centrifuge a lot of particles out of the core flow, expelling them with the bypass air.

The spinner is also designed to deflect particles, and the booster inlet is moved aft and has a low profile, which also ensures that fewer particles get into the core. Finally, variable bleed vanes — the LEAP debris rejection system first developed for the GE90 — open inward. At low power settings, the doors open to bleed off some of the air, providing an additional path to steer particles away from the turbo machinery. No one else offers this type of FOD rejection feature.

The approach has been validated on the GE90, which has extensive operating experience in the Middle East, where particle contamination can be particularly vexing.

Another technology feature of the engine is active clearance control at the HP turbine case. Cooling flow at the HP turbine can be periodically programmed to increase over the service life of the engine, with increased cooling restoring tip clearance and maintaining engine efficiency.

But not all maintenance and reliability measures trace back to exotic technology. CFM’s experience in managing its suppliers also plays a key role. Dispatch reliability can be dramatically impacted by engine accessories, and CFM is employing aggressive strategies to make vendors more accountable.

LRU (line replaceable unit) supplier product support agreements will be integral to vendor selection, and commitments to repair turn times (TAT), reliability, repair effectiveness and response time will be required with business reviews to ensure all milestones are being met.

Line maintenance issues also play a role in the engine design. For example, the accessory gear boxes are mounted at the eight o’clock position on the fan case for quick access. This location allows one person to quickly access the LRUs, and because the AGB is on the fan, no cool-down time is required prior to access — an important consideration given the quick turns common to narrowbody operations.

CFM believes it has a historic advantage over the competitors in maintenance cost over a range of aircraft applications where competing engines are offered to airlines, and is committed to keeping LEAP maintenance costs similar to existing CFM costs, which are the lowest for single-aisle engines.

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