Ease of repair is reportedly one of the main reasons the Canadian airplane maker Bombardier Aerospace has chosen not to use carbon- or glass-fiber composites in the main fuselage of its composite-heavy CSeries aircraft. The high-cycle aircraft, due to enter service in 2013, is expected to sustain a lot of impact damage from ground support equipment. Since repair and maintenance standards are often based on the performance of metal-based planes, damage is easier to identify and repair in metal portions of aircraft using current techniques.
Unlike the Boeing 787 Dreamliner and the Airbus A350 XWB, the fuselage of the two CSeries models is constructed primarily of aluminum-lithium. Composites are used in the empennage, rear fuselage, nacelle, and wings. Altogether, the advanced materials give the plane a 2,000-pound weight savings.
Problems surrounding the repair and maintenance of composite aircraft have become much more visible since a GAO report questioned the safety of repair and maintenance procedures for Boeing 787 Dreamliner composites. That report said impact damage to composite structures may be difficult to detect, since that type of damage isn't as easy to see as impact damage to metals.
The Canadian CSeries CS100 commercial aircraft uses aluminum-lithium, not carbon-fiber composites, in its fuselage to save weight.
Photo courtesy of Bombardier Aerospace
In addition, the nondestructive inspection techniques commonly used on in-service aircraft for detecting and characterizing composite damage vary with each composite's construction methods and with the particular type of damage done. For example, the 787 uses at least two different types of carbon-fiber composites: carbon laminate and carbon sandwich. So repair and maintenance technicians aren't as likely to find damage in the first place if they don't use the correct technique for finding it in a given material.
Compounding that problem is the fact that composite repair itself deploys up to 60 unique materials, while traditional metal repairs require only a dozen or so, according to one study cited by the GAO report. In addition, technicians are less likely to repair a composite correctly, since the quality of repairs depends primarily on which process they use. The scale of knowledge and supplies that must be kept on hand is thus multiplied several times. Though there's a certain degree of standardization for the repair and maintenance of metals in aircraft, the same is not yet true for composites.
TJ, your points are well taken. The biggest problem of all in composite repair, though, compared to metal repair, is the lack of knowledge to identify damage in the first place, since it's much more difficult to detect. The next biggest problem is figuring out how to repair so many different materials with so many different uses and so many different possible procedures. And, by extension, lack of knowledge there, as well.
While repairs require more effort, composites would seem to be more damage resistant than an aluminum structure.
I looked for a video I'd seen several years ago: Boeing demonstrating a large skin section held vertically, and airline executives offered the chance to whack it with a sledge hammer.
I cannot find that video so this one will have to do:
Bombardier may go for traditional repair techniques on the areas in danger of ground support damage, but if composites shrug off the damage that would ding aluminum, then composites would seem to be the better bet.
Follow-up from last night's comment: After I mentioned the American Airlines flight that crashed in 1979 after a design flaw left it vulnerable to maintenance damage, I tried to remember where I had once read about that accident. Here's the answer: Our distinguished columnist, Henry Petroski, wrote about it in his book, "To Engineer Is Human."
Thanks, Alex. The news just keeps coming out on this subject. Regarding standards, that's a really good question. One of the key critiques in the GAO report was the point that you can't base repair standards and best practices for composites on the same ones that were created for metal. There are too many differences across the board, and making the same assumptions or using the same templates would be ineffective and dangerous. That may be another reason why we're not hearing much yet about the details of repair whens and hows. I suspect it's a WIP.
@Charles: You're absolutely right; materials selection involves many considerations besides the material's response to stress and strain -- which can be complicated enough, since the material may respond very differently at different temperatures and strain rates, and its properties may be different in different directions. But how a given material will perform in your application also depends on its location in the galvanic series, among other things. Cost and manufacturability are always major concerns, too. Then there are externalities such as recyclability and end-of-life issues, sustainability and lifecycle emissions, etc. And -- although I may be somewhat biased in this regard! -- this is why having a good materials engineer is a necessity.
This is a classic example of the need to beware that what looks good on paper may not always be so. As design engineers, we are often trained to consider matters of stress and strain -- bending, shear, torsional capacity, etc. But here we have a situation where the composite is apparently appropriate in matters of material strength, but not in matters of maintenance. Obviously, maintenance is a huge consideration for aircraft. In 1979, an American Airlines flight leaving Chicago O'Hare crashed, killing 271 people, after a design flaw left the engine pylon vulnerable to maintenance damage.
I'm impressed by the breadth of your recent coverage on composites, Ann. I'm wondering if you see new standards emerging out of the FAA as regards composites repair, or will we see industry-standard practices come into play first, which will become de facto methodologies for both repair and recycling?
Rob, that's the $64,000 question. I think the answer here is also 'both." Composites are definitely moving forward in aerospace, as shown by all the aircraft makers using them in greater amounts. And detection of at least certain types of damage is difficult, but apparently not impossible.
I think the answer is a bit of both. Beth, the industry apparently has been working on solving this problem along with the FAA. At least, that's what they all tell us. But it's quite difficult to find out any details. And that's where ScotCan's point comes in. As the report delineates, industry has been extremely secretive regarding the details about their materials--the type of details which must be well known for determining when and how to repair--in the name of trade secrets.
I'm curious as to whether this is a transition time for composite materials or whether there is something intrinsic to composites that makes detection of problems and repair more difficult for composites than it is for more conventional materials.
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