Returning a damaged 787 to service was a key early test for Boeing's composite repair strategy
On the morning of July 12, 2013, the quiet atmosphere of 's 787 Operations Control Center (OCC) in Everett, Wash., was broken by a phone call from at . The caller asked if Boeing knew there was smoke coming from a 787 parked at the airport.
The monitoring team instantly switched the big display screen in the OCC to the BBC and saw TV images of an787 emitting smoke and surrounded by fire fighters. The disturbing scene prompted an inevitable feeling of deja vu among Boeing engineers who, just months earlier, had battled through the redesign of the aircraft's overheating battery system. Ironically, the smoking Ethiopian aircraft, Line No. 44, had been the first 787 to return to service following the three-month battery-related grounding of the fleet.
By the time an investigation had pinpointed a malfunctioning emergency locator transmitter (ELT) as the culprit, Boeing was already focusing on how to return the 787 to service. Heat damage was particularly severe to the crown of the composite fuselage close to the intersection of the aft Section 47 and the Section 48 tail unit and, at a stroke, caused the largest structural damage yet sustained by any 787. With a fix requiring the replacement of a skin section measuring approximately 25 ft. in length and 18 ft. in arc length, the operation was instantly viewed by the wider industry as a classic test for the repairability of primary structural composites—which are now entering commercial service in an unprecedented volume with the advent of the 787.
Although airline confidence in the day-to-day resilience of composites has come a long way since 2004, when Boeing's sales pitch for what was then called theoften involved inviting skeptics to bash a specimen with a hammer, the Heathrow incident presented Boeing with a chance to prove that the 787 is just as reparable as a conventionally built airliner.
Despite the scale of the damage, the company was well-prepared for the repair work, says 787 vice president for services and support, Mike Fleming. “We knew we had to be ready because, on the support side, you knew something could happen. Someone could drive a truck into the side of an aircraft. We did work on the repair of large sections of the fuselage barrel sections for events such as a nose-gear collapse, or a tail drag or for damage around a door, so we pre-engineered parts and repairs for instances like that. However, the ELT was on the top of the fuselage where we don't typically see damage, but we used the same methodology and analysis.”
A replacement crown section was removed from a one-piece fuselage barrel section on the production line and transported to London. The damaged 787 had meanwhile been towed to an area of the cargo ramp to the south of Heathrow, where the scorched crown section was cut out under cover of a temporary protective structure. As the new upper panel intersected with the area around the leading edge root of the vertical stabilizer, this was also removed in preparation for installation of the replacement section. The new panel—complete with co-molded, longitudinal stringers and attached frames—was secured in place with a bonded splice plate as well as mechanically fastened to the surrounding frames and stringers.
“From a structural standpoint, it was a fairly straightforward combination of bonded and mechanical repairs,” says Fleming. “It went pretty well as expected, but to see what our [aircraft-on-ground] team did was fantastic, and the operation was impressive,” he adds. While Boeing is unwilling to discuss the size of the aircraft-on-ground (AOG) team involved, Fleming says it was “consistent with us having to do a large-area repair on a metallic aircraft.”
The job was also handled in a routine and confident way thanks to almost a decade of development, test and manufacturing experience on the 787, in addition to earlier structural composite work on the 777 and other smaller components, he says.
“We've been working with a much bigger scale of composites in the factory and in flight test. It's analogous to when we went from analog to digital aircraft,” Fleming says. “There was a skill set of mechanics around analog, and that changed with theand 777. The skill set changed as you went to fly-by-wire, and you see a similar evolution taking place on the materials side.”
The repaired 787 flew on a test flight from Heathrow on Dec. 21, and returned to service shortly after. Aside from this major event, Fleming says “the real success has been composites.” He notes that “there was a lot of anxiety in the industry about lightning strikes and 'ramp rash,'” the day-to-day damage caused by impact with ground vehicles and airport stands and ramps. “There were concerns about lightning strikes particularly from Japanese customers, but I quit counting after 20 lightning strikes,” Fleming says. Composites have proved very durable, he points out.
“We took our first [lightning strike] just before entry into service [October 2011], and it was a really big strike. It was estimated it would have punched a really big hole, but we covered it with sealant tape to prevent moisture getting in, and then later went back and did a bonded repair with a 4-inch-by-7-inch patch,” Fleming says. “Since then, most of the strikes have caused no damage or just some discoloration. But as far as damage due to strikes, we don't have any evidence the 787 has been hit any more than any other aircraft.”
As with every other aircraft in regular service, the 787 has endured its fair share of “ramp rash,” according to Fleming. “We have had a lot of things run into the aircraft, but in general it has been fine, and where there has been an impact, it has frequently been within allowable damage limits. In those cases, we just protect the area from moisture intrusion with tape, which prevents freezing and expansion [that could lead to porosity and delamination],” he says. “It all depends on the size of the damage. Most of the ramp rash does not require any repair at all.”
To assist ground engineers in deciding whether to dispatch an aircraft with any form of suspected “ramp-rash” damage, Boeing provides a handheld ultrasound tool that Fleming says “checks for problems.” Most often, no delamination is detected and “the only reason we find out about an incident is when we take a delay,” he adds.
However, he acknowledges that Boeing has had to tackle “other large-area repairs and has had to do 'scarf' repairs.” The bonded-scarf repair is one of three main composite repair methods specified for the 787, along with a quick “patch” repair method and a more conventional bolted repair. The bonded-scarf method, which is a conventional vacuum-debulked bonded process familiar to most involved in composite layup, involves tapering down the material ply by ply around the damaged area and “sanding down to the bottom layer,” Fleming says. “Then you can rebuild it from the inside as well if you have a hole,” he adds.
The scarf method also enables a flush finish with improved aerodynamics compared to bolted repairs. Because several structural partners contribute to the 787 airframe, there are differences in the carbon fiber/epoxy specification between various sections. To mitigate this and save time in fixing the aircraft, Boeing has also qualified common repair materials throughout the 787 to make the process transparent to the repair technicians.
The quick-cure patch process involves epoxy bonding a precured composite patch over the damaged area and can be completed “in an hour,” Fleming says. The process, which involves the use of a chemical heat pack to cure adhesive at a relatively low temperature, provides a temporary method of restoring enough residual strength to allow the aircraft to continue in service.
Depending on circumstances, some airlines have preferred to make a bolted repair, which involves mechanically fastening a cover plate over a damaged spot. Similar to the repair process traditionally used on metallic aircraft, the repair can be made with titanium (preferred) with carbon fiber and aluminum as options provided additional steps are taken with the latter to protect against galvanic coupling. The choice of whether to use a bolted or bonded repair frequently depends on how much time the airline has available, Fleming says.
“On composites, you can spend more time preparing the aircraft, performing the ultrasound inspection, and so sometimes it is quicker to do a metallic repair,” he adds. However, Fleming cautions that in the longer term, the bonded repair avoids the eventual need to fix holes that have to be drilled into the solid laminate for the bolted repair.