Large-scale repair of carbon-fiber structure is the latest challenge for Boeing on 787
As investigations continue into the transmitter fire which severely damaged an 787 at July 12, is gearing up for the most challenging aircraft repair and recovery operation it has yet faced on the new twinjet—and perhaps one of the most complex ever undertaken on a commercial composite aerostructure.
Questions remain as to how Boeing, regulators and the aircraft's owner, Ethiopian Airlines, will approach the rework. The main choices include installing a complex composite “patch” on the damaged area, replacing the entire tail section of the aircraft or even writing off the aircraft as a hull loss. According to Aviation Week's Fleets database, the damaged 787 (ET-AOP) was delivered in November 2012 and is one of four aircraft the African carrier is flying. It has six additionalon order.
External evidence of heat damage to the crown of the aft fuselage extends over at least 16 ft. from the base of the vertical fin forward to near the body join between the major Section 47 tail unit and the main center fuselage section. However, the charring and discoloration on the outer carbon laminate skin only hints at what the U.K. Aircraft Accident Investigation Board (AAIB) describes as “extensive heat damage” to the interior structure throughout the aft crown from the two aft doors, R4 and L4, forward.
In its July 18 report, the AAIB says the fire caused “significant thermal effects on aircraft insulation and structure. Surveying and detailed examinations of the damaged areas revealed that the greatest heat damage and highest temperatures were centered on the rear fuselage close to the crown and displaced to the left of the aircraft centerline.” It also confirms that the source of the fire coincides with the location of the aircraft's aftemergency locator transmitter (ELT), which is mounted internally on structure close to the skin.
It is unclear whether Boeing will attempt to install an external patch in order to obtain a ferry permit and fly the aircraft to Seattle or some other location with the types of equipment needed for a final “engineered” repair, whatever that may be. Despite concerns that the fire could have led to the aircraft becoming an insurance write-off, company officials indicate that plans are underway to recover the 787. If sanctioned, these repairs will involve replacing the damaged skin section with a large composite panel or panels, already containing the co-bonded fore-and-aft stiffeners that are built into the one-piece barrel sections used in the standard fuselage assemblies. Bonded and hybrid repair schemes, which combine bonded skins and stringers with bolted sub-structures such as fuselage frames, will also be used.
Boeing developed large-scale composite skin repair techniques as a key support element of the 787 program, though likely did not anticipate dealing with such a major repair task so early in the airliner's operational life. Repair technology was a major focus during development, as composites make up 50% of the 787's structure (including the fuselage barrel sections), compared to 11% on the 777 and 3% on the 767.
Insiders also quietly note that the composite structure held up well to the fire. Compared to conventional aircraft-thickness aluminum, for whichtests have shown burn-through times of 30-60 sec. in intense fires, Boeing flame tests exhibited longer burn-through times for sections representative of the 787's composite laminate skin. The aircraft maker also points out that although the fire was severe enough to visibly char the exterior of the skin, the fire did not penetrate the surface.
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.