Boeing executives expressed what they consider a “reasonable expectation” that the 787 Dreamliner would return to service in a matter of a few weeks at a briefing last Friday in Tokyo during which they detailed the company’s plan for certifying a solution to the “issues” surrounding the airplane’s lithium-ion batteries. However, Boeing Commercial Airplanes CEO Ray Conner and 787 chief program engineer Mike Sinnett acknowledged that the timing will depend completely on the U.S. Federal Aviation Administration’s certification schedule and a smooth execution of the testing.
“We cannot schedule the FAA; we understand that,” said Sinnett. “It is reasonable to expect that we could be back up and going in weeks, not in months. Now, if anything happens along the way, there could be a delay.”
While explaining the company’s plan to guarantee the prevention of “aircraft level” thermal runaway, Sinnett noted that Boeing has completed about a third of the needed certification testing. Because the certification plan calls for engineers to perform the vast majority of their testing on the ground, Boeing will need to perform a single flight test to demonstrate proper battery system function, said Sinnett.
The so-called fix, which the FAA approved for testing early last week, includes a modification to the charger and battery monitoring unit designed to narrow the acceptable level of charge–in essence, lowering the maximum charge allowed and raising the minimum level of discharge allowed. Engineers have also devised a way to “soften” the charging cycle in a way to put less stress on the battery.
Within the battery itself, Boeing plans to wrap an electrical insulator between each of the eight cells to isolate them from each other and prevent what Sinnett called “propagation.” The plan also calls for the installation of electrical and thermal insulation above, below and between each cell to prevent heat migration. Meanwhile, small holes drilled into the bottom of the case that encloses the cells and battery management unit would allow moisture to drain away from the battery. Larger holes in the sides of the case would allow a failed battery to more efficiently vent, thereby lessening the possibility of damage to other parts of the power pack.
Finally, the new design calls for the use of a stainless steel enclosure meant to isolate the unit from the rest of the equipment in the electronic equipment bays. According to Sinnett, fire could never erupt within the box. “It has been misreported that all we’re doing is building an enclosure around a potential fire,” he said. “This enclosure keeps us from ever having a fire to begin with.”
All the changes will add some 150 pounds to the airplane, essentially negating any weight benefits a lithium-ion battery delivers over a traditional nickel-cadmium type. Nevertheless, Sinnett insisted that Boeing made the right choice, given lithium-ion’s other advantages. “The battery provides a lot of power on the ground when we need it; it’s good from a charging perspective; it’s good from a shelf-life perspective,” he said.
Although Sinnett acknowledged that Boeing might never find the source of the battery failures that led to the 787’s grounding, he expressed complete confidence that the OEM has addressed all possibilities. “There are many, many cases when we have parts fail on an airplane and we don’t understand what the specific root cause is, and we make appropriate corrections to improve the part, whatever that part may be, and this has served us well in the past,” said Sinnett. “We could spend the next two years trying to figure out what the one specific root cause was, or we could design to make sure we’ve covered everything. And so that’s what we’ve done here.”