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Saturday, August 1, 2009

Passion for Precision: Composite Repair Training

By Vicki P. McConnell, Technology Editor

The historic fly-in of more composite intensive aircraft has already begun, from Bombardier’s Learjet 85 to Cessna’s Corvalis 350/400TT models to the Boeing 787 and Airbus A380, in fifth-generation F-22 and F-35 fighters and multiple unmanned aerial vehicles. Most helicopter manufacturers have switched to composite rotor blades, and new engines from both GE and CFM feature composite turbofan blades.

This dramatic change in primary materials is on par with the switch from wood and fabric to aluminum. Composites have a language and a look all their own, and mechanics who repair them need a deft set of hands and an equally deft mindset to understand the materials and manufacturing basics of fiber-reinforced laminates. Today’s challenging economy is affecting whether companies offer composite repair training in-house or outsource to private companies, or even formally train beyond on-the-job supervision. AM asked representatives from the diverse resources for composite repair training what has changed most in their composite repair curricula and in the student profile to reflect the advent of more composites-intensive aircraft.

Put Tools in Hand

"More pervasive use of composites means more adhesively bonded repairs and more attention to damage detection through non-destructive inspection (NDI)," states Mike Hoke, president of Abaris Training Resources. Over the past 26 years, his company has trained nearly 15,000 technicians, engineers, mechanics, inspectors and managers by continually honing the balance of hands-on practical projects with theoretical data in 17 different types of courses. The result cultivates what Hoke describes as a form of "kinesthetic" learning that engages both brain and body. "Sometimes this means helping students overcome repair habits learned on the job that have limited their perspective on materials, equipment or technique options until they are exposed to a broader scope of possibilities." The Abaris answer is to offer three "Advanced Composite Structures: Fabrication and Damage Repair" courses in sequence and "Adhesive Bonding of Composites and Metals."

It’s been said that "Divinity is in the details," and Abaris instructors wholeheartedly agree. "There’s nothing inherently difficult in composite repair, but it does require careful attention to detail, and in many techniques such as scarf repairs, considerable precision," Hoke says. Multiple factors may be at play in repair on a component with lightning strike protection, for example, involving restoring not only the original strength and stiffness of the composite but also the electrical conductivity of the protection material.

"Damage detection in composites is very important, since a very small ding on the surface may be barely visible, yet belie more severe, larger-area damage deeper in the laminate." Addressing such issues is part and parcel of the company’s course, "NDI For Technicians and Inspectors."

Putting theory into practice with every course, students conduct wet layup repair and vacuum bagged patch cures. Here resin and fiber variations and different core materials demand recognition.

"The way woven carbon fiber cloth works in repairs is so different from unidirectional tape, and entirely different from handling aramid and glass-reinforced materials," Hoke relates. "Students get a chance to test out the way that carbon fiber satin weave compares to plain weave cloth in tight radii, and to experience the tack levels of different resins under temperature, and then determine for themselves what works best."

Abaris expects to train 168 FAA aviation safety inspectors this year in an advanced composite maintenance and repair course tailored for them. At the core of this course, and all those offered by the company: giving students the tools to distinguish both "why" and "how" repair design and processes must meet the same airworthiness requirements as the original structure.

Laminate Basics

Wilson Boynton, president of Renaissance Aeronautics Associates and Advanced Composites Training (RAA/ACT), believes mechanics should have more training in the building blocks of composite material science. "Such knowledge is fundamental in composites technology and should be grasped before repair is ever begun."

ACT offers 14 training courses in composite manufacture, maintenance, structural repair, non-destructive testing and repair design engineering; all have Transport Canada and Canadian Aviation Maintenance Council (CAMC) accreditation, and 12 have been accepted by the FAA for Inspection Authorization renewal.

"For anyone performing composite repair, we teach five essential aspects of laminate design: isotropy, symmetry and balance under applied load, thermal balance related to coefficients of thermal expansion (CTE), and ply nesting. This theory is applied to practical hands-on repair exercises, with an emphasis on making repairs under less than ideal flight-line conditions. This helps students understand the consequences of repair that doesn’t correctly consider laminate design." Original equipment manufacturers are mandated to supply structural repair manuals (SRMs) for each aircraft that generally outline the repair limitations of location, size, technique, original material system used and suitable repair materials. Familiarity with the organization and presentation of existing SRMs is always desired in repair training, but OEMs can be reticent to share them.

Cindy Foreman, owner and senior instructor of Composite Educational Services (CES Composites) and author of the textbook, Advanced Composites, stresses that SRMs can vary significantly, and even leave a mechanic hanging for lack of specifics related to composite repair. In her 20 years of conducting composite repairs and training repair techniques, primarily on commuter aircraft, she has encountered the equivalent of "the check is in the mail" in SRMs.

Namely, SRM pages that indicate "a composite repair update is coming soon," yet no original pages listing composite components, material systems, or repair allowables. Foreman says some updates have arrived as much as two years later. Some students trained in her course have had to work with OEM technical reps to jointly develop industry-standard composite repairs that may subsequently be added to SRMs (hopefully in much more timely manner).

RAA/ACT offers a five-day course, "Advanced Structural Repair Methods: Beyond the SRM."

The Society of Automotive Engineers-Aerospace Division/Commercial Aircraft Composite Repair Committee (SAE/CACRC) has created a teaching repair manual that includes a non-proprietary SRM that follows the general organization of SRMs from major OEMs. It will be available from the FAA Technical Center later this year. Also helpful in this regard from Abaris, "Advanced Composite Drawing Interpretation."

The downturn in the economy has not altered United Airlines’ commitment to continuous learning and educational development for its composite repair technicians, both in-house and off-site, according to Gary Derheim, composite maintenance instructor at the San Francisco maintenance center. Four in-house courses offer over 200 hours of content in composite repair processes (shop, airframe maintenance overhaul, line, and a refresher course). UAL instructors have been trained at Abaris, and Derheim praises the "broad spectrum of composite knowledge that we gain in those courses as the ‘backbone’ of understanding composite material repair processes. Our instructors help technicians learn to interpret SRM and internal documentation, focus on improved safety in handling repair materials, and boost confidence for conducting repairs." New composite techs are paired with more experienced techs to gain practical on-wing experience. The experienced techs attend a three-day refresher course at least every two years, including hands-on shop specific repairs. Ultimately, this training "gives our technicians a chance to study challenging but infrequent repairs and to hone their skillset in performing regular repairs at highest level," Derheim says.

Engineers Get Onboard

Aviation design engineers and structural analysts can benefit equally from composite repair training — especially if they find themselves tasked with helping write an SRM for a new composite aircraft or design a composite repair. Consultant Scott Beckwith of BTG Composites teaches "Composite Materials Design, Testing and Fabrication" and "Composite Materials Failure Analysis and Repair" for Seminars for Engineers, a Sensor Products company. He believes that it’s important to convey "a firm understanding of what damage has been inflicted in a composite component and under what accept/reject criteria a repair should or should not be initiated." This, he says, combined with sufficient materials science to consider the way a repair affects material behavior under load, will reinforce that a truly successful repair restores design allowables "as if there had never been damage and repair." Beckwith has been developing and presenting composites training internationally for nearly 30 years. The next session of Composites Failure Analysis and Repair is Nov. 4 – 5 in Miami. "All of our engineering courses include hands-on fabrication of test specimens that are tested to failure and compared to design predictions," says Hoke of the Abaris lineup: "Repair Analysis and Substantiation," "Composite Laminate Design and Analysis," "Design of Composite Structures" and "Design and Analysis of Composite Structural Joints." In 2010, an "Advanced Composite Design Analysis" course will be added.

Standardizing Certification

The input of industry experts to composite repair training has been at the heart of dedicated efforts by the CACRC for the past 20 years. Through workshops and industry working groups, CACRC has sought to adopt standard repair practices, offered input to the section on damage tolerance in the Composite Materials Handbook (www.CMH17.org) and, through the training task group, identified specific skills for engineers, technicians, and inspectors involved in composites maintenance.

CACRC has worked with the FAA to create an awareness course with a focus on safety issues for those involved in composite repair; one outcome of this effort has been a checklist of teaching points that SAE plans to publish this year as Aerospace Information Report (AIR) 5719.

Charles Seaton of Edmonds Community College (Lynnwood, Wash.), which offers a Composite Technician Certificate program, is also a CACRC training task force member and a principal investigator for the FAA on training course development. He describes the awareness course supported by AIR 5719 as "three years in development in order to achieve industry consensus, which distinguishes it from other course offerings, and the first that identifies the specific learning outcomes for anyone taking an overview course in composite materials maintenance."

Thick-Section Repair

Perhaps the most anticipated change in composite repair in aircraft such as the 787 is the potential to apply bolted repair techniques common in metalwork to thick-section solid laminate. Boeing reports that 787 structural laminates range in thickness from less than 10 plies to more than 100 plies, and the company’s SRM for the aircraft contains methodology developed for making such repairs with both scarf and bolted technologies utilizing a time-efficient single cure.

Trainers interviewed here report that thick-section repair is the latest addition to existing curricula, although Boeing’s 777, the Airbus A320 and the F-18 fighter already utilize bolted repairs on solid carbon fiber laminate. CES Composites’ Foreman comments that even with a detailed SRM from a manufacturer, composite repair training should emphasize that bolted repair on composites will not result in reinforcement to a damaged area in exactly the same way that sheetmetal does.

Lou Dorworth, division manager for Direct Services at Abaris, comments that "while a tapered scarf probably provides a better repair than drilling holes and putting bolts through solid laminate, the down side is that an incredible amount of good material is removed and replaced with non-autoclave repaired structure." He also notes that any new holes drilled in the structure could create the potential for fasteners to loosen under vibration and cause damage, so bolt tightness and hole precision and sealing will be critical. Also, patch material will require a CTE match with carbon fiber composite.

Bolted repair could create a repair ‘knock down’ factor, Dorworth suggests, though commercial aircraft tend to be designed at 150 percent of allowables. Boeing says repair margin for bolted repairs has been considered from the outset with the 787 so as to permit bolted repairs that qualify as Category A, damage-tolerance permanent repairs. Eric Chesmar, UAL’s Principal Engineer for Composite Structures in San Francisco and chair of the CACRC design task group, believes that for bolted repairs to solid laminate, the special fasteners needed for these thicker repairs, along with the doubler materials, may present more of a maintainability and logistics challenge, which are not new.

His view on making repairs to any composite component is simple: "always follow the book (SRM) and if in doubt, contact the OEM’s engineers."

When the Light Bulb Goes On

The trainers interviewed here see it often: a student’s relief when the light bulb goes on during a composites training course, when the language of these unique materials and look of laminate damage makes more sense. Repeatedly, students report post-training that their composite repair courses have resulted in more consistent, reliable composite repairs, safer materials handling and reduced material waste. Sometimes the savings can be measured in the millions of dollars when a repair avoids the cost of an expensive replacement part. Sometimes the measurement is more personal: in confidence, and in a stronger individual skillset that boosts productivity, professionalism, and safety.

Gaining this "educated eye" often coincides with the awakening of a special passion for the challenges of working with composites. If this passion can be taught (encouraged, enlightened) along with the technical procedures of composite repair, then the value of the training is priceless — even in today’s economy.

For More Information

Abaris Training Resources, Inc., Reno, NV, www.abaris.com

Renaissance Aeronautics/ACT, London, Ontario, www.raacomposites.com

BTG Composites LLC, Taylorsville, UT, swbeckwith@aol.com

Seminars for Engineers, Madison, NJ, www.seminarsforengineers.com

CES Composites, Monument, CO, www.cescomposites.com

FAA Technical Center, www.actlibrary.tc.faa.gov

Edmonds Community College, Lynwood, WA, www.edcc.edu

SAE/CACRC Training Task Force, http://www.sae.org/servlets/works/committeeHome.do?comtID=TEAAMSCACRC

Aviation Maintenance podcast, "Why the Increasing Use of Composites Makes Training More Vital," April 27, 2009, http://www.aviationtoday.com/podcasts/?54c7f74f5cd58823337aed.

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