Sunday, May 1, 2005
Composites in the Sky with Dreamliner
Boeing's next-generation, mid-range workhorse airliner--the 787--will feature more non-metals than ever before in the primary structure of a 200-plus passenger airplane, 50 percent by weight, in fact, but the deputy chief mechanic's mantra to maintenance metalheads is "have no fear."
In 2008, Boeing's new 787 Dreamliner commercial airliner will begin to fly passengers into an age of materials technology that's been a long time coming. With advanced composites comprising half the airplane's construction materials--especially solid carbon fiber laminate in the wing and fuselage--reinforced polymerics can finally claim the skies as never before. All Nippon Airways is Boeing's launch customer, offering the largest launch order in history for a new Boeing airplane, at 50 initial units valued at $6 billion.
Bringing big jet range (4,030 to 9,734 miles) to mid-range configuration in three 787 models, Boeing expects the carbon fiber composite structure of these airplanes to help achieve a 20 percent reduction in maintenance cost, 20 percent increase in fuel efficiency, and a 20 percent decrease in emissions as compared to competing aircraft. By engineering multiple components into integrated single parts (at a typical ratio of 1 to 20 in reduced parts count compared to aluminum), composites will also reduce the weight of the 787-8 baseline model by as much as 40,000 pounds compared to an equivalent all-metal, mid-range airplane.
Finally putting this amount and complexity of composites into the design of a 197-foot wing span (on the 787-8) and in the primary structure of the 19-foot wide by 182-foot long fuselage may give some wrench-benders pause. Comfort levels among mechanics and technicians may start to jangle and questions arise in alarm. What's this going to mean in terms of additional composites training needed, variance in maintenance time and necessary equipment, and inspection standards? Will larger single parts push the envelope in sanctioned repair limits, and if my expertise is primarily in metal, what do I know from carbon fiber solid laminate?
To which 787 chief mechanic Chris Tasche answered, "with appropriate training, I've found that mechanics have readily adapted to the repair technology required for composites. And the airlines have been dealing with these materials for over 20 years now. I don't believe the 787 will pose a greater challenge for them." Tasche was also the chief mechanic on the 777-300 and spent five years with a major U.S. carrier, developing that airline's overnight and first heavy maintenance checks.
Deputy chief mechanic for the 787 program Justin Hale added that when it comes to repairing the solid laminate composites on the 787, "A&Ps can think bolted repairs." For primary structure, through-the-thickness bolted repairs of solid carbon fiber laminate in the 787 fuselage and wing are now possible. The solid laminate avoids many of the challenges associated with more traditional honeycomb laminate structures. Further, Hale observed that "for the most part, many repairs for the 787 will look very familiar to a mechanic used to working on metal aircraft." That's not to imply that the composites training A&Ps have acquired won't be useful, because the Dreamliner will utilize limited amounts of honeycomb structure with carbon fiber skins, as well as fiberglass and aramid fiber composites in certain components, such as the engine nacelles and wing tips. Repair of these will follow conventional composite protocols of the wet layup, vacuum-bagged, and heat blanket-cured type.
Lessons learned from the 777
Hale added that Boeing has repeated a process determined to be crucial during the development of the 777 (which carries 10 percent composites by weight, mainly in the horizontal and vertical stabilizers): talking to operators to get their input regarding common repair and maintenance issues and hopefully addressing those issues as much as possible concurrent with design. "We've been talking to mechanics about our new plastic airplane," Hale recounted. "And demonstrating to them some composite repair techniques that are faster and easier than with aluminum parts, but also showing the similarity to metal repair. The point we want to make is, `listen, this doesn't have to rock your world.' We're seeing a reassuring level of confidence about the 787 after such demos."
Some of the suggestions 777 customers had for Boeing that would reduce maintenance requirements and that are being answered in the 787 include:
- Adding to the laminate thickness around the doors.
- Solid carbon fiber laminate leading edge high-lift devices that eliminate all metal-bond in the slat wedges.
- Solid carbon fiber laminate trailing edge high-lift surfaces and ailerons with no honeycomb core.
Another carryover from the 777 horizontal stabilizer and floor beams is Toray's Torayca carbon fiber prepreg, with T800/3900-2 carbon fiber, to be used in the 787's solid carbon fiber laminate components. Gary Oakes, associate technical fellow, Commercial Airplane Services/Structures for Boeing Commercial Airplanes, touted the material's toughness and strength both in terms of inflight performance and resisting impact damage, as well as its manufacturability. "From the 777 program," he said, "we know a lot about this material, which reduces risks in using it on a new aircraft. And because Toray is currently the only qualified supplier of this prepreg for these components, this answers one complaint from operators, mainly too much inventory required for repair materials."
He added that "we're clear that this `black' airplane has to operate in a predominantly `grey' airplane world. So we're basing our composite repairs on the skill sets we anticipate already exist in that world."
Oakes emphasized that inspection for the new airplane "will not involve a raft of new techniques beyond what's
now standard for the 777 or even other Boeing models with composites onboard [such as the 737 and 767]." This includes the new carbon fiber, one-piece barrel fuselage design, which he identified as the largest and most unique area for inspection. "Modified non-destructive inspection [NDI] that takes the new plane's size, geometric configurations, and assembly into effect will readily accommodate inspecting the fuselage in-service, when non-destructive inspection is specified," Oakes concluded.
Sooner rather than later
Because composites are driven by ultimate strength performance rather than fatigue, which is the case with metals, "you have to design the repairability into the structure, you can't do it later," said Oakes. He's referring to analytical provisions considered in the design stage that will ensure restoration of ultimate strength capability as reflected in composite component design and composite repair protocols. He added that "we aren't integrating multiple components by way of composites unless there's a real structural benefit to doing so. Structural composites do what they are designed to do: carry load, while meeting design requirements for damage tolerance, durability, and repairability."
To which Hale offered that "the 787 has a more monolithic structure, such as hat stiffeners integrally bonded to skins rather than mechanically attached stringers, or one-piece fuselage sections built without lap splices in the skin. This will change the way some repairs are designed."
Ultimately this change offers what Oakes calls "an inherent robustness in the structure that aluminum has a hard time matching." Speaking of robustness, Boeing expects that initial C-checks for the 787 may well be extended to 36 months, and a first full structural inspection to 12 years.
The 787 follows the "no growth" design philosophy of the 777 in terms of the solid carbon fiber laminates. This means structures with damage that is not likely to be visually detected are designed to carry residual ultimate load for the operational life of the airplane. "Our method of complying with the FARs relies on two levels of damage for aluminum structure," Hale explained, "but manifests different-ly for composites since we won't have fatigue cracks that grow." These levels include damage not likely to be detected during visual inspection under normal lighting from a distance of five feet, and damage that is likely to be detected visually under these same conditions.
Does Boeing ex-pect to sanction larger wet layup repairs with alternate materials to the Toray prepreg for the larger composite components on the 787? Oakes acknowledged that size limitations for repairs outside those specified in structural repair manuals represent a common problem for operators, and Boeing is taking that into account in determining 787 repair protocols. "We're looking at very large area repairs from a major incident in an upfront study," Hale said, "such as a tail strike or collapsed nose gear, which can do substantial damage to the fuselage. Most damage beyond SRM limits would likely involve bolted repair," he added, "and there will be size limitations for those due to their effect upon structural performance. But we're aiming toward being as practical as possible in our repair allowables."
Composites power up
To travel at Mach 0.85, the 787 will use either Rolls-Royce Trent 1000 engines or the GE Aircraft Engines (GEAE) GENX powerplant, with a common attachment interface for both. A fourth iteration of GEAE's successful GE90 engine baseline architecture, the GENX features 1,500 pounds of composites. A new braided carbon fiber forward fan case--fabricated through carbon fiber resin film infusion (RFI) and autoclave cure--replaces aluminum and offers a 350-pound engine weight reduction. The engine will also use the tape-layed, carbon fiber fan blades (with titanium leading edge and polyurethane erosion coating) already proven over the past decade on the GE90.
The composite fan case, which is 111 inches in diameter, acts as a structural containment housing should one of the blades accidentally be released. The 23-stage GENX powerplant (10 stages of compressor, four stages of booster, two stages of high pressure turbine and seven stages of low pressure turbine) operates at a maximum of 75,000 pounds of thrust, so a blade-out could be catastrophic. "We expect lower maintenance costs than with a traditional aluminum fan case," said David Crall, GENX module manager, cold section, "based on the generous defect limits we've seen in half-scale blade-out rig tests. Our testing of these parts with sizable flaws and repairs in them is helping build the experience necessary to substantiate generous inspection limits during production and subsequent operation by our customers."
Doug Ward, manager of composite technology for GE Transportation, told Aviation Maintenance that it is unlikely any wet layup repairs will be allowed on the fan case because of its critical structural requirements. He added that there is abradable and acoustic material inside the fan casing that may require repair. These materials can be patched on-wing using a room-temperature curing paste. In the event of severe damage, the acoustic panels and other composite flowpath components can be rebonded to the fan case at the depot level.
When consulted about the GENX design, customers made it clear they wanted composite fan blades because they reduce maintenance costs through reduced dovetail wear, one-time lubrication, retention of balance between shop visits, reduced inspection requirements compared to hollow titanium blades, and resistance to most bird strikes via the titanium edging.
GEAE has reduced the number of blades to 18 from 22, and Ward said that if a fan blade should require replacement, this can be accomplished on-wing in about two hours.
Intelligent Engine technology is built in to the GENX to maximize component lives, via sensors that will be monitored by a separate onboard computer. This system will improve the accuracy of deterioration trending and identify the sources of deterioration. "When a GENX engine comes in for overhaul," said Crall, "we'll be able to accurately determine which modules need maintenance action and which can be returned to service without maintenance." And speaking of rapid return, the ability to remove the composite blades out the front of the engine and to separate the fan case from the high-temperature propulsor will allow these components to be readily mated with a new propulsor. "This will reduce the number of spares an operator has to keep on hand," added Crall.
Reduced overall weight, improved fuel efficiency, longer lifetime relative to fatigue performance, corrosion resistance, reduced part count, even containment capacity for a loose fan blade rotating at 2,600 rpm--composites clearly offer significant performance benefits. But why has Boeing embraced these materials at this record level for its commercial aircraft, and why now? "The technical level of aluminum can't go much farther in terms of pushing airplane performance and gaining cost advantages in maintenance and repair," said Oakes. "And Boeing has developed a level of expertise in analytical and design tools and in manufacturing with composites even since before the 777, to create opportunity with composites rather than risk, and at a reasonable cost. In a way, we think the stars are aligned for the 787."
"Designers would have liked to use more composites sooner," Hale added, "but a sufficient degree of proven manufacturability and safety had to be in place first. We feel those elements have reached critical mass for this aircraft."
"Safety is inherent in the quality of an airframe's design," added Tasche, "regardless of the materials used, and also in the quality of the maintenance performed once the aircraft is in service. Gaining the benefits of light weight, resistance to corrosion, fatigue, and impact damage from composites can only improve safety over the life of this airplane."
The timing also seems to be right for applying innovations in robotization, sensors, assembly, and tooling technology. Advanced tape lamination, fiber-placement, and several resin-infusion processes will be used to build the 787 composite components. Surface-mounted sensors will potentially offer real-time data for flight condition load monitoring that Oakes said "could allow for more focused inspections of composite structure, especially in the case of hard landings." Also, Boeing has quick composite repair processes in development for small-area damage that is based on an epoxy-bonded outside patch that can be applied in less than an hour.
As for additional training to perform maintenance and meet maintenance requirements on the 787, Hale said that the production team is creating a curriculum for engineers, technicians, and inspectors to reflect the structural differences of this new airplane. "While a slightly different skill set will be required," he added, "overall training for maintenance and repair of the 787 will mainly expand upon what is already known regarding both metal and composites to reflect this aircraft's uniqueness."
In his view, Tasche believes "the widespread application of composites can be seen in the same light as transitioning from analog electronics and cable-operated flight/engine controls to digital and fly-by-wire technology. Despite the conservative nature of the aviation industry, the last 25-plus years have shown that we went down the right path with these systems technology advances. I believe history will also show that we are making the right decision in electing to use more composites on the 787. I'll also say personally that airline engineers and mechanics are a relatively skeptical bunch, not easily convinced until they can gain firsthand knowledge. I know they're wondering if the 787 will be as low-maintenance and trouble-free as we say it is, and we're ready to show them the durability, maintainability, and incorporation of our in-service experience in development and design of this new aircraft."
The Airbus Answer To Composite Structure
Airbus has announced it will launch the A350, a 245-passenger mid-range passenger jet to compete with Boeing's new 787. Air Europa is the first airline to commit to the new airplane, with expected service to begin in 2010, and using GE Aircraft's GENX engines. Dr. Roland Th�venin, senior expert in composites for Airbus, said that the A350 will feature 33 percent composite structure, up from the 25 percent demonstrated aboard the superjumbo A380, which was unveiled in January. He characterized the Airbus approach to composites as a step-by-step evolution of the ratio of metal to composite "based on the result of dedicated tradeoffs, which always compare available technologies." Some 60 million flight hours with composite structural components on Airbus airplanes also factor into the OEM's materials-ratio decisions.
Resin transfer molding and resin film infusion fabrication techniques previously used in limited composite components are now being used for structural elements such as the rear fuselage frames and rear pressure bulkhead. Fiber placement (automatic tape layup) is being used for the rear fuselage and auxiliary power unit mounts.
The A350 wing, which will be composed of carbon fiber reinforced plastics in spars, upper and lower skins, and with ribs remaining metallic, will deliver exceptional low and high-speed efficiency due to the integration of the "droop nose" design from the A380. Airbus expects to spend $5.3 billion to develop the A350. Th�venin said primary carbon prepreg suppliers include Hexcel and Cytec, the same as on the A380, and that maintenance protocols will also be the same for both aircraft. Wet layup techniques have been directed by techniques developed through the Commercial Aircraft Composite Repair Committee (CACRC). Customer input as to common maintenance/repair issues has also been solicited and incorporated.
The main applications of composites on the A350 are monolithic, laminate structures, such as the vertical and horizontal tailplanes, flaps, center wing box, keel beam, and rear pressure bulkhead. Only bolted repair is suitable for these monolithic laminates, Th�venin said, which is similar too Boeing's repair scheme for the 787. Key secondary carbon fiber reinforced plastics components include the pylon fairings, nacelle cowlings, various doors, belly fairing skins, and overwing panels. Airbus is also working within the framework of the CACRC to qualify materials and processes for repair of these sandwich composite structures by means of bonded repairs.
Within its materials palette, Airbus uses GLARE--a hybrid sheet made of alternating layers of aluminum alloy and glass fiber--to improve fatigue behavior in the upper fuselage shell. Th�venin noted that, from a maintenance standpoint, this material can be treated the same as typical metal aluminum sheet.
For large-area structural damage, he added that Airbus is expanding its strategic, global locations for making both A380 and A350 spares available to customers.
Finally, Airbus is working a step process to provide customers economical benefits from sensor health monitoring systems, and has targeted 2008 for the first generation technology readiness milestone of using SHM to provide maintenance-related benefits.
-- By Vicki P. McConnell