Boeing has disclosed that the 787 battery redesign package will include an improved battery charger, as the company delivered its most detailed and forceful description of the design changes and defence of its actions.
The new battery charger unit adds to several previously-disclosed new protections to allow the 787 to return to flight, possibly within weeks, with a pair of GS Yuasa's 32V lithium-ion batteries providing start-up and back-up power onboard the aircraft.
Boeing's certification plan, which was approved two days ago by the US Federal Aviation Administration, calls for hundreds of hours of laboratory testing and a single flight test to validate the systems.
"That will be the extent of flight testing," says Mike Sinnett, Boeing 787 chief project engineer. "It's not an extensive flight testing programme."
Ray Conner, chief executive of Boeing Commercial Airplanes, flew to Tokyo to brief Japanese transport regulators, and also hosted a public webcast to describe the plan in detail.
"We're here this week to discuss our solution and take feedback from the Japanese authorities and our customers," Conner says.
The battery redesign features a new charger with reduced maximum charging levels, a higher maximum discharging level and a softened charging sequence.
The charger is being improved even as safety investigators continue to search for the root cause of the short-circuits that led to two battery failures in January. It also addresses one of the paradoxes of the investigation. Overcharging is the only possible cause of a thermal runaway event, Boeing says, but neither 787 battery incident involved an over-charged battery based on the flight data recorders.
Boeing also revealed how it has redesigned the battery enclosure to "eliminate" the possibility of a fire caused by an over-heating battery. The enclosure now allows the electrolytes within the battery to vent if they become overheated within the battery box. Any fumes emitted by the electrolyte are then vented directly offboard through a newly-installed, dedicated vent line.
"This enclosure keeps us from ever having a fire to begin with," Sinnett says.
Boeing is keeping the same GS Yuasa batteries in the 787, but redesigning the layout of the eight cells. Instead of the formerly densely-packed unit, there are now empty spaces between each of the cells. The spaces are intended to absorb heat in case any individual cell malfunctions and overheats, rather than spread the thermal runaway event in a chain reaction around the battery.
Boeing also is "enhancing" the manufacturing process for each cell and the battery as a whole. The voltage range also is being tightened.
The goal is prevent a battery malfunction from occurring in the first place. If it does happen, the empty spaces between the cells should help prevent a thermal runaway, and the dedicated vent should prevent any damage within the aircraft.
Boeing also clarifies what it called several "misconceptions" about the severity of the battery incidents, which had been described by safety investigators as involving flames and, in one case, an "explosion".
In fact, the main battery on the All Nippon Airways 787 that made an emergency landing at Takamatsu, Japan, never caught fire. The auxiliary power unit battery on the Japan Airlines 787 in Boston did catch fire, but it was limited to two, 7.62cm (3in) flames, Sinnett says.
"We can say with certainty that after the battery failed, the airplane responded exactly the way we had designed," Sinnett says.
In Boeing's view, neither incident met the company's internal definition for the condition called "thermal runaway". That is a situation in which there "is so much energy, so much heat and so much flame that it would put the airplane at risk", Sinnett says. "We know very clearly this was not the case in the Logan event and the Takamatsu event."
Boeing now has to prove that the redesign package works. The company must first test and certificate the battery and enclosure changes, then install the new systems on the existing fleet before retrofitting aircraft that are undelivered or still in the production flow.
The return to flight process is continuing despite the lack of a conclusive root cause for the battery failures. Sinnett acknowledges that such a root cause may never be found and that 787 batteries might continue to have problems.
"Parts fail. We know that some day a battery may fail. We need to make sure there is no significant impact at the airplane level when it does."