The industry is witnessing (albeit from afar) a watershed repair just now, one scrutinized and debated as few in the annals of modern MRO. The subject is an Ethiopian Airlines Boeing 787, the victim of a crown fire in the rear of the craft’s composite fuselage as it sat empty and idle out on the ramp at London Heathrow on July 12, 2013.
The locus of the fire looks to be the 787’s lithium battery-powered Emergency Locator Transmitter. Contrary to first impression, that fire did not burn a hole through the structure. “It only looks like it in the pictures,” says Michael J. Hoke, president and owner of Abaris Training Resources, the Reno-based composites training firm. He says, “It did not burn through the laminate, but it was hot enough to burn the paint off the outside,” rendering “the illusion of a hole.”
Still, the result was significant structure damage. Hoke estimates the affected area on the inside “on the order to 10 feet long.”
Although “not privy to the exact details” of the resultant repair, he offers a possible architecture for the fix. Boeing spokesman Jim Condelles declines comment, saying only “Boeing and Ethiopian Airlines are working in partnership to complete the repair. Work has started in London and is progressing well.” The repair’s particulars probably won’t be disclosed in any real depth until early 2014.
The fix may be an adhesively bonded patch. That’s what The Seattle Times reported an unnamed Boeing engineer as saying.
When word of the extent of the damage first surfaced speculation was Boeing might have to replace a complete composite fuselage barrel. That’s apparently not the tack the OEM took.
Remember, the 787’s fuselage is composed of separate “barrels,” each crafted via fiber placement. The result is integral and one-piece, replete with stiffeners. By contrast, the 787’s arch-competitor, the A350, wraps composite skin pieces around a substructure.
Repairing primary structure
Although Hoke hasn’t physically seen the damaged Ethiopian twinjet, one approach envisions Boeing performing a double-sided scarf repair in which technicians sand layers of laminates at a very flat angle. For illustrative purposes, Hoke uses a simpler example than the 787’s fuselage, a flat piece of composite laminate with a hole punched in it. Before the tapering begins, he says, “The first thing you’d want to do is cut away the damaged material and get a nice, clean oval-shaped, or perhaps circular hole.” That leaves technicians with good material to work with.
Gradated, flat sanding produces repair plies that are larger than the hole. That’s what begets good load transfer, the essential element of any composite structural repair, particularly primary structure. “The idea is to get a long adhesive bond joint through the original structure to the repair plies,” says the Abaris executive.
Just how robust those load-bearing properties are depends on scarf angle and ply count. “They can get back to [the] full strength of the original undamaged structure — or more than full strength — if they use extra plies,” says Hoke. For a 75-ply original skin, you might want to use 85 or 90 plies in the repair. Stronger is better; but not too strong. “There’s no point in going way stronger than the original structure,” he says. Make that mistake and “you create load transitions at the edges of the repair, which can be harmful.” The risk lies in building up a “hard spot” in the composite laminate.
Next comes adhesion. A good adhesive bond means meticulous preparation of the bond line itself, that crucial part of the structure that will shoulder the load. Go back to Composites 101 and you’ll see that dirt, oil, grease, and even fingerprints can contaminate the works.
As for curing the fix, Hoke says a repair of the size of the Ethiopian craft demands either multiple heat blankets, infrared lamps, or perhaps even erecting a “temporary oven” around the structure. Technicians could employ a combination of techniques to effect a cure. He says, “There are lots of ways to do it.”
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...
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