Boeing is confident it can complete its test work plan in short order, mainly because of the engineering development work already completed behind the scenes, says 787-8 Engineering Vice President Ron Hinderberger. Following the FAA's March 12 decision to accept Boeing's redesign proposal, these tests have begun accumulating credit toward certification. Hinderberger says Boeing anticipates “completion should be done within the next week or two.”
New details of the redesigned battery system also reveal why Boeing has remained confident of FAA approval for the fix. The battery enclosure, which is designed to prevent a fire erupting rather than simply containing it, is made of 0.125-in.-thick stainless steel. A 1-in.-dia. titanium vent pipe connects the back of the enclosure to the outer skin of the aircraft where new exit holes—one for each battery—will be cut through the composite skin. The vent pipe is designed to evacuate vaporized electrolytes from the battery should any, or all, of the eight cells in the unit fail. In the event of a cell failure, a small pressure port in the rear of the enclosure is designed to rupture under pressure from the building vapor. The vapor will then exit the aircraft via the vent pipe.
The ability of the design to accommodate the simultaneous venting of all eight cells was one of the key additions to the revised certification requirements proposed by the RTCA, adds Hinderberger.
Boeing says the resulting pressure gradient is sufficient to evacuate the pipe and the enclosure without the need for additional forcing, both in the air and on the ground. Testing at Boeing indicates that even if vaporized electrolytes continue to accumulate inside the enclosure, it cannot be ignited. Only briefly did the mixture deflagrate for milliseconds when fed directly with oxygen. The vent pipe “does not create any new certification requirements as such,” says Hinderberger, who adds that both outlets for the forward and aft electrical and electronic equipment bays are positioned so they do not lead to re-ingestion of the vapor.
The redesign of the battery itself includes the addition of higher-temperature-resistant phenolic glass insulation between individual cells as well as between cells and the battery walls. “We are removing material inside that was only good to 150C [degrees centigrade] and replacing it with phenolic glass laminate, which can go to far in excess of 500C. We've put that between the cells and sidewalls and also installed them on bottom and top, so no matter which way [the battery cells] would move it will come into contact with the phenolic glass, eliminating a possible path to short circuit,” he says.
The battery will also sit on a redesigned frame containing drain holes to allow moisture to escape. In its testing, Vice President and 787 Chief Project Engineer Mike Sinnett says, Boeing found that moisture paths can lead to short circuits in cells which could lead to “stress in the cell,” or the buildup of heat and venting of vaporized electrolytes.
The redesign also addresses the risk of overcharging, which Boeing's original design work indicated was the only probable cause of a serious battery fire. “At an airplane level, what we are concerned about is a whole battery event that is so energetic it puts the aircraft at risk through heat and flame. The only known ability to create what I just described is overcharging,” says Sinnett. A revised charger will reduce the maximum charging levels to prevent overcharging and increase the minimum charging level to prevent “deep” discharging. The revised system will also “soften” the wave form in the charging sequence.
Sinnett says that during the two test flights in February that followed the FAA's initial clearance to fly ZA005 for data collection, “we saw a charging signature which made us ask questions regarding whether the battery could be seeing stress as a result of that signature. We decided to change the circuitry in the charger so we didn't see that signature anymore, in case it was a contributor. We don't know if it was a contributor, but we addressed it anyway.”
Boeing also detailed the beefed-up new inspection procedure that all batteries will undergo before installation. “We put the battery through tests of individual cells, then—once we have an assembled battery—we test it as a complete battery. At a cell level, we do more extensive voltage checks over several days and chart its voltage over time. We look for the cell to discharge or create a fingerprint, so to speak, that's 'out of family.' We are going to only allow [the batteries] to discharge at a certain rate that is less than we did before, so we expect to reject a higher number of parts than we did before,” Hinderberger says.