Experience and advanced tooling are required for nacelle repairs, which can be complicated by the combination of composite and metal components. AFI-KLM E&M
Experience and advanced tooling are required for nacelle repairs, which can be complicated by the combination of composite and metal components.

New Nacelles More Efficient But Harder To Maintain

Bigger engines mean bigger nacelles, which could complicate maintenance.

Under the hood, an engine family usually incorporates similar architecture, tweaked to different power settings. In the case of the CFM Leap 1C, however, the “hood” itself provides an important distinction, since the nacelle of the Comac C919 will be part of the first “integrated propulsion system” to enter commercial service.

Manufactured by Nexcelle, a joint venture of Middle River Aircraft Systems (MRAS) and Safran Nacelles, the Leap 1C’s nacelle reflects a closer integration of engine and nacelle design, a concept first tested on Nexcelle’s Panache (Pylon and Nacelle Advanced Configuration for High Efficiency) demonstrator in 2012. By integrating the thrust reverser into the engine pylon flange, Panache eliminated the beams that normally attach to the pylon, thereby reducing weight and drag.

Building on this work, the Leap 1C thrust reverser uses a one-piece composite O-duct instead of the more common two-piece D-doors. It also has an electrical actuation system, pioneered on the Airbus A380, rather than a heavier pneumatic or hydraulic system.

As well as its thrust reverser, a nacelle’s major components are its inlet, fan cowl (the doors that open, wing-like, on an engine being serviced) and exhaust system. These should remain the building blocks of any future system, though the lines between them could become blurred. MRAS has investigated integration of part of the inlet with the engine fan case. Meanwhile, UTC Aerospace Systems is testing an integrated thrust reverser on a PW1000G as part of its “Ecological Integrated Propulsion System” project, and other designers are pursuing their own research.

“It’s quite likely that there will be changes in the overall nacelle structure in 10 years, with different components having different levels of integration, but the basic functionality will stay the same,” says Chris Gear, chief technology officer for GKN Aerospace, a nacelle manufacturer.

Balancing Performance and Maintenance

More than just an aerodynamic sleeve for the engine, a nacelle’s functions also include rapid deceleration via its thrust reversers, containment of exploding debris following an engine failure, channeling air into the engine, noise reduction and deicing. All this must be achieved for a lower weight penalty and higher safety standards than ever before and despite the searing temperatures that engines generate.

Another design challenge arises from the wider chords of newer turbofans, which feature increased bypass ratios to improve fuel efficiency. Larger fan diameters mean bigger nacelles, which threaten to offset any fuel-burn advantages with increased weight and drag.  

“As a consequence, the main objective of future nacelle developments is to better integrate the nacelle with the engine and pylon to enable lower- drag solutions while further optimizing the traditional engine and pylon load paths,” says Marc Duvall, president of aerostructures at UTC Aerospace Systems.

UTC is the biggest manufacturer of commercial nacelles. For the company and its rivals, safety will always be paramount, but nacelle design must also strike a balance between performance, ease of manufacturing and maintainability. There are concerns that integrated systems are sacrificing repairability for performance.

“When they do fail, repairs likely will be more difficult and costly. Maintainability seems to be taking a back seat. That’s not necessarily surprising though, since the OEMs are positioned to sell replacement parts,” says Basil Barimo, executive vice president of Nordam’s repair divisions.

Pratt & Whitney hopes that geared turbofan production will return to schedule in 2017, after some delays last year. Credit: Pratt & Whitney

Components that are harder to repair mean higher sales of lucrative spare parts for nacelle manufacturers. Michael Lotzin, head of sales for aircraft-related components at Lufthansa Technik, notes that a Boeing 787 nacelle costs five times more than one for the Boeing 767, its predecessor. The 787 nacelles also have a support burden: “The fan cowl doors are matched as inner and outer pairs—that means more expensive stock, because operators need to have two pairs of spares,” he says.

Third-party MRO providers also fear that OEMs will restrict access to technical documentation to capture more of the aftermarket. To address this risk, some have signed cooperation agreements to ensure access to repair manuals. Lufthansa Technik, for instance, has a deal with UTC to provide rotable provisioning and MRO services on 787 and Airbus A350 nacelle components.

Nonetheless, no one can say if any of the above will increase life-cycle costs for new systems. Higher-cost shop visits may be worth paying for in exchange for longer service intervals and efficiency gains from lower weight and drag, but it will take thousands of cycles to see how those factors play out. 

Composites

Nacelles were one of the first aerostructures to exploit the properties of carbon fiber, the use of which grew as designers sought to lighten ever-larger components. A tipping point was reached with the Airbus A380’s nacelle, which contains 52% composite material. The balance shifted even further away from metal with the A320neo nacelle, but the pendulum may now swing back.

“In my opinion, we have now achieved a maximum ratio [of composites], certainly for short-range aircraft where the shift will be toward lower-cost solutions and decreased maintenance costs for the airlines,” says Duvall.

MRO providers have had decades to acquaint themselves with the composite materials used in nacelles, and employ a variety of inspection techniques, including tap testing; thermography; ultrasonic testing; borescoping; radiography, and etch and penetrant inspection.

Meanwhile, the qualities of the composites themselves have changed, with some manufacturers shifting toward epoxy IFS weaves that are more reparable than BMI-matrix composites. New fabrication techniques also allow more complex composite components with fewer material joins, although these also may complicate maintenance.

About one-fifth lighter than aluminum, carbon fiber is popular for its high strength-to-weight ratio. The next advance may be to incorporate the extreme thermal tolerances of ceramic composites into nacelle design, although that material’s expense may limit such a move. Instead, concerns about life-cycle costs in more complex, integrated systems could herald a shift back toward metal.

Additive Manufacturing

If metal is to regain some build share from composites, 3-D printing could prove useful. It is still a relatively slow process used predominantly for prototyping, but it does have two big advantages over subtractive machining. First, milling down a block of expensive material such as titanium creates costly waste. Second, by fabricating parts from the ground up, one layer at a time, additive manufacturing can create more complex components that do not need fasteners, thereby saving weight.

This year, GKN is set to open an R&D center in the U.S. for the metal powders used in additive manufacturing, and in 2016 it approved a joint venture to make the powders in Germany. GKN’s Gear believes the technology’s capabilities are “rapidly moving toward more critical components and systems,” one of which may be production versions of a thrust reverser’s titanium inner wall.

Duvall, in contrast, is more skeptical about the potential for additive manufacturing in nacelle production. While lauding the technology’s advantages in the design process, he says UTC has struggled to find applications where it can add value on the factory line.

“There is no doubt that additive will be in production in our next nacelle systems, but it may not be as transformational as in other commercial aircraft systems,” he says.

The Repair Cycle

Nacelle damage usually results from long-term thermal deformation or impacts from birds or ground equipment. Composites can disbond and delaminate, while aluminum parts corrode, and both can suffer from cracks, gouges, tears and dents.

Barring accidental damage, maintenance companies report significant variation in engine on-wing time. Lotzin and Lufthansa Technik and Barimo at Nordam are both reasonably impressed with the service life of the GE90, although the former highlights “increasing reports of delamination caused by foreign-object damage.” A different issue affected the CFM56-7 thrust reverser, which “had a design defect that resulted in most of the fleet being removed prematurely,” says Barimo.

It is still too early to compare the durability of old and new-generation nacelle components, says Stephane Louaze, head of customer support for nacelles at Air France-KLM Engineering & Maintenance. However, he does note that “some oxidation remains persistent,” even in newer equipment.

When damage does occur, airlines would like to perform repairs on-wing, a preference that will grow with the higher transportation costs of larger nacelles. But Lotzin thinks it could prove difficult to accommodate these wishes. “I doubt that more nacelle repairs on future nacelles will be done on-wing due to their complexity and the repair procedures enforced by the OEM,” he says.

For current-generation equipment, Barimo points out that it is relatively easy to source serviceable spares from retired aircraft, so parts can be economically replaced and sent to the shop for refurbishment. “That is not the case for the latest generation of nacelles, and it is reasonable to expect replacement parts for those products to be much more expensive,” he says.

Given this situation, operators will expect the next generation of nacelle components to justify their expense and prove to be even more reliable than the equipment they replace. 

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