Behind scaffolding erected more than a week ago at London's Heathrow Airport, where the Ethiopian Airlines plane has been idled since summer, a repair team will glue a giant composite plastic skin patch into the burned crown of the fuselage, said two people with knowledge of the details. Boeing has scheduled five weeks for the repair, one said.
It's a very complicated procedure, but less so than the alternative option that was suggested by some observers: pulling out and replacing the entire aft fuselage section, which Boeing fabricates as a single-piece barrel.
Company spokesman Doug Alder confirmed that repair work has started in London but declined to provide details beyond saying the work "is progressing well."
Speaking Tuesday at the Cargo Facts aircraft symposium in Seattle, Asrat Begashaw, acting managing director of Ethiopian Cargo, said management expects to have its damaged 787 back in service in about two months.
Because the 787 is the first large commercial jet made from carbon-fiber-reinforced plastic composites, there's no precedent for the substantial damage to the Ethiopian jet's hull.
Fixing it is the first major test of the repairability of Boeing's Dreamliner, so it's vital to giving airlines confidence the jet will prove robust in service.
The aviation industry is watching "with very high interest" because the damaged piece is a load-bearing structure that's critical to safe flight, said Eric Casterline, president of Heatcon Composite Systems, a leading maker of composites-repair equipment in Seattle.
"Boeing's repair is going to be pushing the limits of what's been done in the past," said Casterline. "This needs to be done right. The aerospace community needs the assurance that this can be done."
If the jet's fuselage were a traditional aluminum one with aluminum skin panels riveted together, this repair could be done relatively easily by replacing any damaged panels.
Boeing's one-piece barrel sections for the 787 are also harder to fix than the structure Airbus has designed for its composite plastic A350.
Airbus fastens together composite panels to make the A350 fuselage, closer to the traditional structural design of a metal airplane and similarly replaceable in case of damage.
Boeing's design is lighter in weight and enables a streamlined manufacturing process, but it must prove to be repairable, too.
The fire on the Ethiopian Airlines 787 broke out July 12 when the airplane was parked and empty.
Investigators believe it was likely caused by the incorrect installation of a small lithium battery inside an emergency locator transmitter, an electronic device found on many planes that transmits location data to satellites in the event of a crash.
The fire caused intense heat damage above the ceiling at the back of the passenger cabin. It scorched a large area of the carbon-fiber skin on the crown of the fuselage in front of the vertical tail fin.
While some speculated that Boeing might have to replace the entire fuselage barrel, company engineers decided against that, as first reported last week on aviation blog Airchive.com.
To begin the repair, the jet maker several weeks ago fabricated a full rear fuselage barrel in its North Charleston, S.C., factory.
It then cut out the crown section to supply a skin patch for the repair of the jet in London, said a Boeing engineer, one of the two people with knowledge of the details.
Several other pieces of composite skin were cut from the same barrel section to provide an inventory of repair patches for future use.
A different Boeing engineer, who is an expert in composite repairs, said putting in the patch will be complex and difficult.
But he said it would have been even more trouble to take the other approach - disconnecting all the wiring, air and fuel systems and then inserting a full fuselage section that would mesh exactly with the section in front of it and the tail behind it.
On condition of anonymity, this engineer explained the likely repair process.
Boeing will cut out the skin damaged by the fire probably in a rectangular cut with rounded edges, he said.
It will cut the patch to the same size and shape and drop it into the space as a plug. The tiny gap around the patch will be filled with paintable sealant that will stretch and compress as the fuselage is pressurized and unpressurized.
Then mechanics will work on the inside, gluing a splice plate to the original skin and to the patch, overlapping both by about 4 inches.
The glue is a superstrong adhesive that is cured using heat blankets, held under pressure by vacuum bags applied to the area.
Heatcon makes the electronic boxes used to monitor the temperature and pressure, which must be controlled carefully for hours as the adhesive hardens.
In addition to the skin patch, Boeing's mechanics must repair the stringers - the stiffening rods that run longitudinally along the fuselage - which will be cut when the damaged section is removed.
They'll have to lay wet composite tape precisely in the shape of the stringer at the point where it was cut and cure it to hardness, again using heat blankets and vacuum bags maintaining the precise temperature and pressure.
All this work will be intricate and must be done precisely to ensure the glue sets as expected and the repair is safe.
University of Washington professor Mark Tuttle, director of UW's Center of Excellence for Advanced Materials in Transport Aircraft Structures, said proving a bonded repair is sound can be difficult.
"One issue with adhesive bonding is you can inspect the bond with ultrasonic inspection and find no gaps or voids, but nevertheless the strength of the bond is lower than anticipated," he said.
He said Boeing will stress-test the result after the repair is finished, including flight tests with instruments attached to measure the strain on the patch.
The Federal Aviation Administration will likely have to sign off on the airworthiness of the jet after repair, Tuttle said. The FAA did not respond to an emailed query for this report.
The Boeing engineer with knowledge of the repair preparations said the company will gain valuable experience from this repair of a major composite skin section.
"Chances are there'll be issues along the way as they figure out their expertise," he said. "It's all brand-new ground."