Not since the dawn of the jet age has so much anticipation surrounded the introduction of a new generation of commercial turbofans. Designed to break the paradigm for efficiency, the debutants promise double-digit reductions in fuel burn, as well as an unparalleled single-leap improvement in emissions and lower noise.
To achieve these significant jumps in thermodynamic and propulsive efficiency, the emerging generation of single- and twin-aisle-aircraft engines employ an unprecedented array of new materials and design technologies. They are also engineered to operate at pressure and temperature levels never before seen in commercial service and yet offer the same, if not better, standards of reliability and time on wing than current-generation turbofans. So how will the engine manufacturers meet the challenge of these seemingly mutually exclusive goals?
Leading the charge in the single-aisle market are CFM International’s Leap-1 family and Pratt & Whitney’s PW1000G geared turbofan; between them they are in line to power upward of an astonishing 10,000 new aircraft already on order or option. The new-engine field in the growing widebody market is dominated by General Electric’s GEnx-1B/-2B and the soon-to-follow GE9X, and by Rolls-Royce with the expanding Trent 1000, 7000 and XWB families.
CFM’s Leap family includes the -1A, currently in flight test on the Airbus A320neo, the -1B in test for the Boeing 737 MAX and the -1C in development for the Comac C919. Aimed at the demanding, high-cycle world of the narrowbody aircraft networks, the Leap engine designers worked with a “maintainability team” for the final four years of development, says Gareth Richards, Leap program manager. “As the design went through, they assessed it from a maintenance perspective. It was a very structured process and culminated in development of a three-dimensional view of the engine, which can be used to check access to every part in a virtual reality environment.”
The final placement of line-replaceable units (LRU) and features to enable easy access, removal and replacement, was determined in this virtual environment. “You see things [virtually that] are not always obvious when you look at a drawing,” says Richards. CFM targeted a “remove/replace” time of 30 min. for most of the LRUs, sensors and other replaceable parts. “The majority met that objective,” he adds.
Reliability targets for Leap are based on an enhanced level beyond that of the CFM56. “We have worked hard to improve the design, and we [acknowledge] that the CFM56 has had a lot of time for refinement. We don’t get that luxury with Leap. The CFM56 is the industry leader, and we need to match it. One way was to demand a very high reliability specification on our LRUs,” says Richards.
The combined assembly of Leap engine accessories, LRUs, full authority digital engine control (Fadec) and actuators has been put together as a full shipset for a series of systems tests in an aerospace equivalent of a torture chamber in Evendale, Ohio. The testing puts the entire system through a punishing cycle of extreme vibrations, temperatures and humidity levels to simulate continuous operations in harsh conditions. Testing will continue “well into 2016,” says Richards, who adds that although individual LRUs and components are subjected to such testing on a routine basis, “this is the first time we’ve done tests like this as a system.”
As part of testing for overall reliability prior to service entry, a complete Leap engine will also be run through 40,000 cycles before the first enters commercial service in 2016. “We are at around 11,000 cycles, so we are on our way,” he adds.
Other design innovations for improved maintenance include an advanced Fadec with enhanced sensing and diagnostic capabilities. “A typical Fadec has the ability to self-test, and we’ve extended that to all the subsystems attached to the Fadec. We’ve also replaced switches that measure ‘on-off’ signals with transducers that can measure more, provide feedback and have no moving parts,” says Richards. “These transducers can self-test the system itself so we can tell if a situation is real, rather than being a faulty signal; [we can now] diagnose the engine with a new degree of sophistication.”
CFM plans to use the vast amounts of data from these systems to interrogate the operation of the engine in “myriad ways that couldn’t be done before,” says Richards. CFM is leveraging GE’s computer analytics know-how to exploit the data, which will stream off the Leap fleet in ever-expanding volume. “The goal is to have an initial shop visit 20,000 cycles after first run. That’s a big number, so the more intelligence we can bring to keep the engine operating successfully, the more we can eliminate maintenance needs.”
A related initiative is to use the data to segment the fleet. “One size does not fit all,” says Richards, who explains that inspections required for engines operated in one region may not necessarily be required for identical engines operated in more benign environments. “We are connecting the dots on the real way the product is used—not based on the assumption of some engineer. We are putting infrastructure in place to support it with gigabytes of storage on the aircraft and Wi-Fi access at the gate to download the data. We are building an engine with so much data capability that we can’t yet even foresee all the situations where it will pay off.”
A major design decision driven by maintenance considerations was to locate the accessory gearbox and major LRUs—electronic engine control, hydraulic and fuel-pump systems—on the fan case. “It’s not the preferred choice if you are an aerodynamicist, because you’d prefer them to be on the core,” says Richards, referring to the design challenge of minimizing the relatively high drag of the newer higher-bypass engines on single-aisle aircraft. However, from an engine-designer perspective, fan case mounting is better because of lower ambient operating temperatures and easier access for maintenance. “We effectively took a penalty for the sake of the customer,” he adds.
CFM’s debris rejection system also is designed for lower maintenance and improved performance. This system is common to the Leap as well as GE’s GE90 and GEnx families, and incorporates a series of variable bleed valves (VBV), which extend into the flow entering the compressor to divert debris into the bypass duct. The fan spinner pushes heavier particles to the outside of the flow entering the core. At lower power during taxiing or reverse thrust, when debris is highest, VBVs open to reject particles to the fan stream, reducing erosion of the compressor blades over time. The VBVs are closed at high power and in cruise to avoid a performance penalty. “Because of that, we’ve been able to eliminate periodic borescope inspections of the compressor module entirely,” says Richards.
The first versions of Pratt’s PW1000G GTF are nearing entry into service on the Bombardier C Series and Airbus A320neo, with other derivatives following close behind on the Mitsubishi Regional Jet, Embraer’s E-Jet E2 series and Irkut’s MC-21. From an MRO perspective, Pratt says the fundamental geared design of the engine holds key advantages. By connecting the fan to the low-pressure spool via a gearbox, the design eliminates several life-limited part (LLP) low-pressure stages and removes 2,000 airfoils as well as offloading the engine core temperature by hundreds of degrees.
Pratt, effectively reintroducing itself to the single-aisle commercial aircraft market with the PW1000G, has taken additional steps to ensure the engine design captures maintenance-driven lessons from as broad a spectrum as possible. “From a line maintenance perspective, we’ve gathered worldwide operator input to design the GTF’s internals and externals to simplify maintenance. Using customer feedback, we incorporated several major features,” says Jill Albertelli, vice president of next-generation product family 30K (30,000-lb.-thrust) programs.
“Prior to the GTF engine launch, Pratt & Whitney did extensive MRO reviews of best practices [among] our engineering, tooling and support equipment experts,” says Albertelli. “We engaged our Columbus [Ohio] Engine Center and Pratt Canada’s West Virginia MRO facilities with focused mechanic input. We reviewed the GTF engine design concepts and architecture with each MRO facility. We also focused on engines that they overhaul and looked at what went well and what they would like to see changed in the GTF engines.”
Discussions centered on flange locations, bearing compartment layouts, modularity, part repairability, borescope port layout and overall external configurations, Albertelli says. “While mechanics were performing the work, we were allotted time to witness and assess ease of maintenance, human factor issues, and [talk to the workers]. The team visited multiple repair sites, diving into . . . issues that drive unnecessary repairability costs.” The team came away with more than 300 recommendations for the basic engine design architecture, she adds.
Key design features include borescope ports for every stage of the engine, the ability to blend all stages of the compression system on-wing to address damage from foreign objects and a special design for the high-pressure compressor top case, which provides accessibility and allows the removal of fan-exit guide vanes on wing. The company has also validated removal and installation of LRUs with operator line mechanics, and selected a core-mounted accessory gearbox to reduce external part count by 30%.
“From a shop visit perspective, the smaller part count and LLP disk reduction lessen the overall work scope,” says Albertelli. Pratt says airlines have long ago stopped questioning the expected reliability of the fan-drive gear system at the heart of the GTF. Exhaustive endurance tests on the gear mechanism and its lubrication system were run on several elaborate rigs; hours and flight loads were replicated, as well as unusual angles and attitudes. “The fan drive gear system [FDGS] is designed to run for infinite life with no life-limited parts,” she says. “Our overall objective for the GTFs is to deliver dramatic fuel and environmental savings, up to $1.5 million per year, while maintaining overall maintenance requirements consistent with today’s narrowbody engines.
“The FDGS . . . shares an oil system with the main bearing systems. Adding oil to the main oil tank addresses the entire oil servicing requirement,” Albertelli says. “Also, we’ve designed the FDGS for on-wing, simple, borescope inspection, as well as being removable on-wing.”
For GTF engines, Albertelli says, there will be a network comprising Pratt & Whitney shops along with multiple partner company setups. Some independents will offer GTF overhaul solutions—for “choice and competition within the MRO landscape.”
Pratt says it will include advanced diagnostics and analytics as part of its Big Data initiative to establish a predictive model to monitor engine-event performance. Designed to produce a proactive approach to maintenance planning and requirements, this intelligence is meant to help operators optimize fleet operations and reduce maintenance costs. The Big Data project initially focused on field operational data and system health information data from PW4000; a similar predictive analytics model is being built to support the V2500 engine fleet.
General Electric’s new large engines, the GEnx-1B/-2B and GE9X, derive many of their maintenance-oriented design features from the experience gained from the GE90. “The GEnx engine leveraged the GE90 architectural advantage of the [modular] fan and propulsor that allow for the fan to be separated and the propulsor easily shipped for maintenance for lower costs, more flexibility and lower spare engine costs,” says Brian Pfeiffer, GEnx customer technical programs and flight operations director. This architectural approach, which was also used for the Engine Alliance GP7200, will also be used for the GE9X, he adds.
After two decades of operations with the only composite fan blades in commercial service, Pfeiffer says fan-set maintenance on GE90/GEnx families remains easier than for engines with titanium fan sets, which require periodic re-lubing of the area around the blade roots, and dovetail fitting in the hub.
Also highlighted is the debris rejection system for maintenance and performance enhancements. The VBV-based system is said to be particularly appealing to operators in more challenging environments.
Special considerations for maintenance were key in designing the GE9X—the physically largest aero engine. Developed exclusively for the Boeing 777X, the 105,000-lb.-thrust turbofan generates less power than the GE90-115B but has a 134-in.-dia. fan, 6 in. wider than the current GE 777 engine. Given customer emphasis on maintainability, “GE has placed a significant focus from the very beginning of the GE9X product design process,” says program leader Chuck Jackson.
“This early focus on ‘design for maintainability’ has let us drive maintainability features into the design, as opposed to working them after most of the product design is finished,” says Jackson. “We instituted ‘keep-out’ zones around critical hardware [like LRUs] in the computer modeling to preserve easy removal. This greatly reduces the work required [from a part-removal aspect] when you need to remove a line component.” Regular tasks—fan blade lubrication and engine control system rigging, for instance—also were streamlined or eliminated by accounting for known-wear mechanism or system variations early in the design process.
The GE9X will be the first high-thrust, large commercial turbofan to make extensive use of ceramic matrix composites (CMC). Lighter than the high-temperature metallic alloys normally used in the hot section, CMCs are expected to help improve fuel consumption. “They also change the game relative to addressing key wear modes, such as oxidation and corrosion, found over time on hot section parts,” says Jackson. “As parts made from CMCs do not have the same chemical makeup as metallic components, the oxidation/corrosion processes are greatly reduced or eliminated,” thereby leading to greater reliability and durability, and ideally, fewer required inspections.
Like other new-generation engines, the GE9X will be a lot “smarter” than its forebears, thanks to additional sensors and built-in self-monitoring systems. “The key feature for the GE9X centers around the ability to predict maintenance activities—what we call analytics-based maintenance,” says Jackson. “If we know a particular part will need to be serviced, and we can identify that need well in advance, the airline can then plan for that activity during a more convenient time [e.g., during A checks] or, even better, plan an on-wing mitigation action.
“To facilitate this ability, we are including additional processing capability and sensors on the engine to collect key information on how it is performing. We will then marry this data with our Predix cloud-based computing platform built on sophisticated physics-based algorithms and predictive modeling to produce advance notice,” he adds. “These notices are generated on an engine-by-engine basis for each key service requirement.”
Rolls-Royce also sees inherent maintenance advantages in the baseline three-shaft configuration of its Trent engine family, the latest versions of which are about to be tested for the A330neo, Boeing 787 and A350-1000. Key changes in the Trent 1000 TEN, which will power all versions of the 787 including the double-stretch 787-10, include features derived from the Trent XWB engine, which began development after initial versions of the Trent 1000.
The changes include adoption of a Trent XWB-style “rising line” intermediate pressure compressor in which the aft stages rotate at higher speeds, as well as the introduction of three stages made from bladed disks, or blisks. The TEN also incorporates a modulated turbine clearance control system for better performance retention, and an adaptive high-pressure cooling system, which uses a fluidic control switch to actively match the amount of bleed air to the specific phase of flight.
The Trent 7000 for the A330neo is based on the TEN, but adapted to work with the new Airbus electrical bleed air system (EBAS) to power the more conventional air-driven cabin systems of the reengined twin. The Trent 7000 already incorporates intermediate- and high-pressure compressor bleed ports to assist with surge margin control; it will be configured with two additional bleed outlets to supply the EBAS.
The low hub-to-tip ratio and static inlet configuration of the three-shaft architecture “helps the upkeep and performance of the core,” says Trent XWB marketing head, Tim Boddy. “The arrangement centrifuges out FOD [foreign object damage] and presents less of a target area. Also, because it is a static inlet, we can recirculate core air into vanes and provide a bit of heating into the front of the engine,” he adds. The anti-icing function prevents ice formation by ducting bleed air from the intermediate compressor to the core engine section stators. “They help prevent ice breaking off and causing nicks and dings, and provide more protection for the core. This maintains higher performance for longer and reduces the maintenance requirement,” says Boddy.
The ongoing trend for larger fans and smaller cores has opened up new “real estate” on the fan case for mounting LRUs. “It moves them to a cooler area, and it is a place where mechanics can troubleshoot issues quickly. We can almost take a Formula 1 [automotive racing]-type approach to enable fast intervention and the replacement of LRUs in 25 min. If you have to change a pump, actuator or solenoid bank, you can do it without having to remove major parts,” he adds.
The TEN/7000 will be the first Trent to incorporate composite electrical harness rafts, which are designed to simplify installation and maintenance of the engine’s pipes and cables—attached to the turbofan casing. “[A product of our R&D, rafts] hold all the pipes, wiring runs and bracketry together. Using the single raft unit, people can interrogate them quickly via the engine health-monitoring (EHM) system or physically. Just take the raft off and put it back on,” says Boddy.
The XWB and latest Trent 1000 derivatives are also the most sophisticated in terms of embedded sensors and monitoring systems. “It’s all about prognostics and EHM capability and using it to the best advantage; as we get to the Trent 1000/XWB, we have tried to push the boundaries of what we can monitor,” says Boddy. The information will feed our push to utilize Big Data in the maintenance program. “We have [this ability] because we have big fleets with lots of information. [We’re looking at] measuring an engine to map normality. Through this process we will gather EHM data for FOD events like birdstrikes and volcanic ash and grit, which are simply a fact of life. They produce nicks and dings on blades that could produce a vibration, so you know something has shifted.” Operations-room personnel will then recognize when an event has happened and be able to have tools and spare parts ready when the aircraft reaches its destination, he adds.
The company is also introducing advanced technologies such as on-wing bore blending and automated laser ablation to help speed up and improve maintenance and repairs.