Four Boeing 787 Dr
eamliners ar
e shown in the final assembly facility in Ever
ett, W
ash. Advanced Composites T
At present, there is no industry-wide standards process in place for testing and certifying aviation maintenance technicians in terms of their composites repair skills. Meanwhile, although some MROs are actually doing composite repairs today, their existing knowledge, machinery and tools aren’t sufficient to handle the requirements of 787s and A350 XWBs, once these aircraft start requiring regular repairs.
Why Composites Are So Different Than Metal Composite materials are not new to aviation. “In fact, the industry has been dealing with composites since the 1960s, most notably in military aircraft,” says Michael Hoke. Hoke is president and owner of Abaris Training (
www.abaris.com). Headquartered in Reno, Nev., with branch offices in Griffin, Georgia, England and Brazil, Abaris Training provides advanced composite training for technicians across a range of industries, from aerospace to automotive.
raining, a London, Ontario firm, shows a casting mold. “In the commercial sector, aircraft such as the 757, 767 and 777
rely on composites in their control surfaces, ailerons, flaps, elevators, and rudders, and in their wing/body fairings and engine nacelles,” Hoke tells Aviation Maintenance magazine. “In the Airbus family, and the Boeing 777, the vertical and horizontal stabilizers are also of carbon fiber construction.” To attend to the needs of these aircraft, many MROs have installed the necessary composite casting molds, cutting tools and autoclaves (heating ovens). For instance, Delta Airlines has had a “pretty extensive composite repair shops in Minneapolis and Atlanta for years,” says Todd Herrington, operations support manager of Delta’s Interior Engineering Group. “We do deep level repairs to engine nacelles and control surfaces to interior components,” Herrington says. “Our largest autoclave is 10’ and 20’ in diameter, so we can work with some pretty big parts.” However, it is the training of their mechanics where the biggest challenges come into play for MROs. This is because the difference between repairing a metal and a composite aircraft is akin to the difference between highly skilled butchers and talented bakers. Here’s why. Butchers don’t have to create the meat they work with. The raw material comes into the shop fully formed. The butcher’s job is simply to carve it up into the required cuts of meat. The same is true for mechanics working on a metal aircraft. Whether they are installing a new part, reshaping and repairing an old one or — in extreme cases — custom-measuring, cutting, shaping and refitting a new metal part from scratch, mechanics don’t have to smelt and cast the metal they are working with. Instead, the necessary steel and aluminum comes into the shop in workable shapes, sheets and thicknesses; no fundamental manufacturing is required. It’s not quite as convenient as dealing with meat, but it is close. In contrast, bakers have to start with raw materials — flour, eggs, and milk — and combine these into specific mixtures first. They then have to transform these mixtures into cakes, cookies and breads using the right levels of heat for the right amounts of time. In the same vein, unless the composite part can simply be switched out with a new premade one, technicians doing composite repairs have to combine and mix ingredients to achieve a final result. The techniques they use include wet lay-up and prepreg repair. In a wet lay-up repair, dry cloth is cut to shape and saturated with mixed wet epoxy resin, generally in several plies. The wet repair plies are vacuum-bagged to extract air and compact the plies together. They are then heated to the prescribed temperature; generally between 180 and 250 °F. Next, the composite is cured for the proper time, usually for an hour or two, depending on the resin system used. In a prepreg repair, plies of cloth or unidirectional tape that are pre-
impregnated with uncured resin from the manufacturer are laid up, vacuum bagged, and cured at high temperature in a similar manner. In some cases, autoclaves can be used on individual parts to increase the compaction pressure during the curing process. But most repair
28 Aviation Maintenance |
avm-mag.com | August / September 2012
Advanced Composites Training
Boeing
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