Composite structures have been around for decades, but the last few years have seen explosive growth in their use and development. Forthcoming jets like the and the boast that at least half their airframes are constructed from these advanced materials, compared to with just 11% on the or, to go way back, just 1% on the early 747-100. Composites have taken a foothold in large, primary structural areas—for instance, the 85 will have both its fuselage and wing built primarily from carbon composites, the first business jet to do so.
The science is changing, too, as original equipment manufacturers (OEMs) find new ways to make their composite structures lighter and more durable.
The changes are starting to reverberate through the maintenance side of the industry. Repairs soon will cover larger areas with more complex contours and greater depth. “Everything is evolving very quickly,” says James Anning, president of Manitoba-based Advanced Composite Structures (ACS). ACS has been in business for 22 years, and Anning says the past five years have seen an unprecedented rate of change in composite technology and application.
“One of our biggest challenges is keeping up with the changes in the technology,” echoes Michael Hoke, president of Abaris Training Resources. “The technology keeps evolving and the lack of standards is a problem. Standardized materials and repair techniques is a long-term goal, but as every OEM considers their techniques to be proprietary, getting there is a complex and time-consuming process.”
A new consideration for those in the composite repair business is the thickness of the composite structures on the latest business and commercial jets. In the past, composites were primarily thin-skinned, about four to six layers thick, and used on lightly loaded structures, says Hoke. Now, he says he is seeing 75- to 100-ply thick solid carbon fiber. “A hole in that is very different than a hole in a thin skin, so we are having to learn new repair techniques,” he says. “Our trainers are learning those techniques now and then we will teach them to the students.” Abaris will add an advanced class to cover repair techniques for these thicker materials.
As materials become thicker, more complex and more load-bearing, Abaris is placing unprecedented emphasis on following documented repair procedures. “We beat it into students' heads again and again: use only the materials in the SRM [structural repair manual] for each airplane,” says Hoke. “Two things might look exactly the same, but there might be standard modulus (stiffness) fibers in one and high modulus fibers in the other, for example, so they'll bear bending loads quite differently.”
These nuances are crucial as composite use expands in key structural areas. There, the implications for repair go beyond increasing technicians' skill levels, demanding a renewed focus on safety and human factors.
“With composite material in the critical areas of primary structure, following proper processes and the skilled interpretation of approved NDT methods are essential to assuring structural integrity,” says Charles Seaton, business director at Heatcon Composite Systems. “At the same time, there is a greatly expanded need for safety emphasis in training.”
For instance, Seaton says that while it has always been important to adhere to source documentation, such as the structural repair manual, in completing repairs, technicians today need training that instills an understanding of why that is important at a gut level. How do you train that kind of understanding? That's the question Heatcon is exploring right now.
Seaton says his organization is looking at ideas such as creating lab scenarios in the lab in which students examine composites that have been improperly cured or poorly repaired and then discussing the chain of events that contributed to the errors. The evaluation is part of a broader course assessment Heatcon is conducting to determine (a) how to develop skills that match the increasing complexity of composite repairs and (b) how to incorporate safety management principles.
The safety management piece is largely being driven by CMH-17, the Composite Materials Handbook maintained by the CMH-17 Organization. Recently, the organization added a chapter on safety management, which Seaton helped write. Among other topics, the chapter addresses teamwork and workflow.
“Postmortems on failed repairs usually reveal a string of events that contributed to a failure,” says Seaton, adding that interaction of technicians, engineers and quality control personnel plays a critical role in error reduction as problems are rarely the result of a single event.
As part of its new curriculum, which Seaton hopes to roll out by mid-2012, Heatcon will train technicians to understand the bigger picture of how these three roles should interact and the problems that can occur when communication breaks down.
This kind of focus on training is just what the industry needs, according to ACS's Anning. “Most composite training is just a cursory introduction that's very general. It's not at the level it needs to be,” he says.
What are newly trained technicians lacking? He cites “a knowledge of how to disassemble and remove damage, how to prep new materials, taking it through the bonding process. The repairs being taught are simple and basic, which is fine to start, but it won't take you very far because today's real-world damage is a lot more complex.”
As organizations begin training their technicians to complete more complex repairs, GKN Aerospace is simultaneously examining ways to automate the process with laser technology. “We've done one stage of R&D to show that it's feasible. We are about to embark on the next stage, moving to the next generation of machine,” says John Cornforth, VP technology for GKN Aerospace.
The lasers, which have “several millimeters” of focal length, are designed to remove the resin binding a structure's carbon fibers. It ablates the epoxy about one-tenth of a layer at a time, leaving only the carbon fibers, which Cornforth says “fluff up” once the binding resin is removed. The equipment then brushes away the layer of exposed fibers before the laser can tackle the next layer. “All indications so far are that it's much quicker,” he says. “You can press a button and walk away. One person could look after several machines, so your efficiency goes up.”
GKN is working with SLCR Lasertechnik to develop a machine that ultimately can handle all the complexities of composite structure repair. “Today, we can do circular repairs on fairly flat surfaces,” says Cornforth. “Next, we will move on to more complicated repair shapes on non-flat surfaces. That stage will cover 90% of repairs.” He estimates it will take three years to complete this next stage and then another one to two years—roughly around 2016—to roll out and integrate the lasers into GKN's repair operations.
This kind of forthcoming technology suggests that the pace of change in composite repair is unlikely to slow any time soon.