We are seeing more and more aircraft these days with composite structures. Although this is something that the major airlines have been dealing with for quite a while, general aviation is not immune to working with composites. With newer aircraft using more composite components (even the virtually all-composite Cirrus and Lancair aircraft) it is inevitable that most of us will be working with composites in the hangar environment. Here are a few tips to help get you up to speed when working with composites.
Lack of uniformity
Unlike working with aluminum, working with composites is usually a very aircraft-specific process. Mike Hoke, president of Abaris Training, explains. "There is a tremendous lack of standardization between manufacturers on how the composite parts are built, how they are repaired, and the repair materials. The larger airlines that operated different aircraft were having a really hard time because they would have a mechanic that would do a repair out of a Boeing structural repair manual (SRM) one day and he would do that repair. And the next day he would go do a repair on an Airbus airplane, and the repair is totally different. It is totally differently executed with some things that totally contradict each other."
Hoke also addressed the lack of standardization when it came to repair materials. "There were also no standardized materials used. So airlines would have to stock up on expensive short shelf life pre-peg materials to meet Boeing specs, and they would have to stock a separate set of rolls to meet Airbus specs, and so forth. And the airlines discovered that on average that they were throwing away about 90 percent of all these composite materials they purchased without ever using them because they would exceed their shelf lives. But they had to have it on hand because it is a long lead time to order this stuff, and if you have damage you've got to go fix it. You can't wait three months to get the material."
Standardization on the way?
Because of the frustrations faced by the airlines on lack of standardization, a group was formed in the late '80s called the Commercial Aircraft Composite Repair Committee (CACRC). Based in Europe, this group's focus was to try to get some standardization of materials, equipment, repair techniques, and repair training. The intention was to simplify the airlines' logistics problems in supporting composites. It quickly migrated to the United States and is now administratively operated by the Society of Automotive Engineers (SAE). It is one of its aerospace committees. The committee meets twice a year, once in Europe and once in the United States. The committee has more than 100 members, with participation from the airlines, material manufacturers, regulatory agencies, and training companies. The CACRC is working toward more standardization in the area of aircraft composites.
For composite inspection and repair, there are a few resources available to help get you started. In terms of publications, there is a book published by the SAE called "Care and Repair of Advanced Composites." It is written for the A&P vs. some of the other books out there that are written for engineers and designers. But remember, it is a starting point. "Although a good resource, the book is still very general in nature. It doesn't give you any specific guidelines on how to repair a specific piece of damage on anything," says Hoke. "It does spend a lot of time talking about things that are important to almost all repairs like surface preparation for bonding, ply orientation, different fabric weave styles, and those kinds of concepts for example."
The new version of AC 43.13 1B has more on composites than the previous version. But keep in mind that this information could be just enough to get you in trouble if you try taking that information and using it to base a repair on. In the end, you need information specific to your aircraft. That usually comes in the structural repair manual. This is where you need to look for specific repair procedures. It will cover repair limits, materials used, and other information crucial to composite repairs.
There are also numerous web sites available that deal with composites. One of them is www. netcomposites.com. It has industry links, upcoming events, and technical articles pertaining to composite inspection and repair. Abaris is also developing a self-paced basics of composite repair course for its web site. It will be an overview of repair techniques that should be available in the near future.
But the bottom line is if you want to learn how to work with composites, you need hands-on training. It can give you the knowledge you need to make informed decisions regarding composite inspection and repair. "You really do need the hands-on practice," Hoke says. "It's just like anything else. For example, if you are teaching somebody how to buck rivets, you can teach them online all day long on how to buck rivets. But they will never really know how to do it until they get a rivet gun and bucking bar in their hand and go practice."
In the end, make sure you know what you are doing before you attempt any composite repair. Following is a list of some resources you can use in your quest to become better informed on composite repair and inspection.