Toughened resins and simpler composite damage-detection technology certainly exist, but they have scant applicability to most current aircraft. The aerospace industry will have to wait for the and other high-percentage composite aircraft to mature before the repair arena can accommodate new material and methods. And it will have to wait for built-in barriers to fall.
As the emerging fleet flies with ever-increasing percentages of composite structure, preventing and detecting delamination are becoming essential issues. Delamination's most typical trigger is denting, not environmental wear and tear. “It's impact,” says Michael J. Hoke, president of Abaris Training Resources Inc.
Abu Dhabi Aircraft Technologies' manager for aircraft structures, Shevantha Weerasekera, agrees: “Almost all the delamination that we see can be attributed to impact damage.” Contamination from hydraulic fuel spills can also play a role. But bumps by ground service vehicles, rock strikes on propellers or flaps, even tools dropped onto a composite wing structure can initiate the delamination sequence and open avenues of opportunity for moisture ingress.
Once water is inside the structure, be it salt or fresh, it freezes and expands as the aircraft ascends. “It cracks the resin and enlarges the delamination,” says Hoke. Chances are virtually “100 percent . . . that once you've created some sort of imperfection that's exposed to the environment . . . water ingression will start,” says Applied Composites Engineering President Leigh R. Sargent.
The prevalence of delamination remains unknown. “If we knew of all the delamination flying around, we would be backlogged for the next several decades,” says Sargent.
“Preventing impacts is a wonderful goal,” says Hoke. “But we know how difficult that is.” Training helps, but it can only go so far. Educating maintenance and especially ramp personnel about locations of key composite structures on specific aircraft is critical. Hoke says, “They generally can't tell [just] by looking” because the parts are painted over.
Impact mitigation is one thing, impact reporting another. Ground crew must report if they hit a structure, whether they think they damaged it or not, Hoke says. “[For] somebody trying to protect their job, that's a tough sell,” he concedes.
A similarly tough sell is manufacturing parts composed of toughened resin systems. The more flexible and robust resins tend to be limited to select aircraft structures, such as thehorizontal and vertical stabilizers and significantly larger swaths of the 787.
Money is the problem inherent in retrofitting control surfaces, fairings, flaps and such with parts fabricated from these formula resins. “It would be a very, very expensive proposition,” says Hoke.
Sargent goes further. Fashioning toughened resin parts to replace the current crop of composites “is not financially viable,” he says. In part, it's a matter of uniform standards—or the lack thereof. Aside from their physical properties, there is a big difference between metal structures, such as aluminum, and composite structures. Metallic aircraft alloys adhere to ASTM standards that, the testing body says, “facilitate market access and trade and build consumer confidence.”
Among composite manufacturers, there is no uniform standard as such, according to Sargent. “Each manufacturer has their own concoction of resins,” he says. And it is that particular “recipe” that is qualified on a given aircraft. When it comes to contemporary composites, commonalty is given short shrift.
Combine this absence of commonalty with the manner in which composites are fashioned, and the repair and retrofit window narrows even further. “The retrofitting of older aircraft with more contemporary composites is not going to happen, because of the cost,” contends Sargent. There are myriad ways to fabricate a composite part. Technicians can machine, form or cast it. On some aircraft, they pre-preg a part in an autoclave; on others they infuse it. For example, while the 787 wingbox is a pre-preg operation, the same structure “on the new[is] done with infusion,” Sargent says.
In manufacturing an aluminum part, you buy the metal and then you make the part. With composites, “you're making the material whilst you make the part,” says Sargent. “It's a very different way to look at it.”
Examining the composite structure on at least one aircraft is a more straightforward affair now that Olympus andare building ramp damage checkers (RDC). “At a high level,” the ultrasonic device functions “as a go/no-go indicator,” says 's 787 Chief Mechanic Ron Murray.
“It's designed for line maintenance people who aren't trained in composites,” says Hoke. He contrasts the detection device with other ultrasonic and thermal inspection gear that “requires a fair amount of training for the operator to get good at it.”
With the RDC, the technician dials the device into a comparable structure adjacent to the one he wants to check. A green light means the area in question is fine. A red light means a full, non-destructive inspection is in order to measure the depth and breadth of the damage. Data gathered are then compared with allowable damage limits in the 787's structural repair manual.
The RDC is not a definitive inspection tool. It is an expediter, one whose “intent is comparable to a [coin] tap test on thin-skin sandwich structure,” says Murray. The application is singular, designed to find subsurface defects triggered by impact damage on solid laminate composite structures. Right now, Murray says the gear is approved solely for the 787.
RDC, like toughened resins, is still sequestered technology. The machine is approved for use with just one aircraft type; resin formulas are not standardized. Barriers that effectively prevent repair or retrofit of older aircraft with new, toughened parts will have to be overcome before go/no-go detection technology and new resin recipes are widely applicable. Or new high-composite aircraft will have to compose a far larger slice of the fleet. Either could take some time to happen.