The composite repair technique, although expected to be complex and costly, is also likely to be relatively easier that a similarly scaled repair to an aluminum-skinned airframe because no testing or checking will be required to match the ductility of the replaced skin with the surrounding structure. All aluminum aircraft skins are subjected to annealing, a heat treatment which makes them more workable and reduces internal stresses, and skins adjacent to those which have been affected by fire must be checked to evaluate their condition.
Nonetheless, the repair task is expected to be the most severe composite field repair challenge faced by Boeing. In addition to the installation of the new crown section, it will also include fitting of the internal fuselage frames, as well as replacing the insulation and cabin paneling, sidewalls and fittings. Systems and secondary support structures in the aft section will also be replaced.
“This is the largest incident that I have seen as far as damage from heat and fire,” says Paul Jonas, director of environmental test labs and special programs at the National Institute for Aviation Research at Wichita State University. “This is a significant event. It does look like a lot of heating.” He estimates that fire temperatures may have been in the 1,000-1,200F range to cause the type of paint damage seen in photographs of the scene. “Composites are pretty much self-extinguishing. If you put flame on and take it off, it doesn't propagate.”
Jonas says Boeing's first step before designing and certifying a one-off “patch” for the airframe will likely be to scope out the damage using A-scan and C-scan non-destructive ultrasonic tests with handheld instruments. The equipment sends sound waves at different frequencies into the composite structure and measures the reflected energy to find discontinuities in the signal that, when compared to a reference or undamaged material, can indicate damage or voids.
The process of patching a damaged area is similar for composite and metal aircraft: Cut out the damaged panel, install a replacement, splice the ribs and flange, and feather the outside surface, says Jonas. The patch itself will tend to be overly conservative, sacrificing weight for the added safety margins. “You don't try to optimize a whole lot for weight—it doesn't pay off,” he says, adding that an airframer will typically use analysis to show the structural integrity of the fix as part of the repair certification process. “The patch comes like a new part number in the Boeing systems,” says Jonas.