“When I look at these issues they're the same kind we've had on other aircraft. We didn't want those, and we worked just as hard to make those problems go away. We're not complacent and we're not sitting back.” None of the problems have “raised significant concerns beyond our experience base,” says Sinnett.
However Sinnett acknowledges that the spate of electrical system issues last month “was a surprise. Certainly the event on United Airlines was a surprise to us—we'd only seen one thing that looked like that before (a previously undisclosed event in June), then we saw three more. We were in the middle of corrective action. We had a one-off manufacturing flaw that led to a generator channel (one of six) becoming inoperative. We'd only seen it once in 100,000 hr. on the system. Then we had the similar event on the United airplane and two days later a similar one on Qatar. We realized all three of those boards came from the same 16 boards in one manufacturing lot.”
Sinnett says the Boston event has no connection to other events and is not a sign of widespread design issues. “We are certain that whatever took place at Logan is not related to any previous power-system event. It was a different part of the system with different results and different manifestation. There have been no other endemic issues. We replaced and did some minor redesign of generators but we haven't seen anything that would lead us to question the overall safety or design of that system,” he adds.
The APU battery is one of two primary batteries in the 787; the other is the main battery in the forward E/E bay. Both are provided by Japan-based battery manufacturer GS Yuasa as part of the Thales-supplied electrical-power conversion system. The 787 contract, first announced in 2005, marked the first commercial aviation application of lithium-ion technology and was selected over contemporary nickel-cadmium because it provided 100% greater energy storage capacity and double the energy from the same-size unit.
However, in April 2007 the FAA issued a notice of proposed special conditions concerning the use of lithium-ion batteries on the 787 in which it noted that these types of batteries “are significantly more susceptible to internal failures that can result in self-sustaining increases in temperature and pressure (thermal runaway) than their nickel-cadmium or lead-acid counterparts.” The agency said that overcharging, in particular, could result in a “self-sustaining fire or explosion.”
Sinnett says, “Because they're lithium ion, the batteries contain a lot of energy and can release it quickly. Unless you design it appropriately that can be a problem. When it is over-charged it can carry more than it is designed for. It is designed so you can never over-charge it. Multiple redundancies are built into the system. Two [safeguards] are built directly in the battery and two are located outside and are independent of the battery. So it is protected with multiple layers. We demonstrated by test and analysis that we are sufficiently safe.”
Presenting other scenarios, Sinnett explains “there are a number of things that can cause a single cell to overheat, to discharge and then to vent smoke and—if it gets hot enough—to burn. One is over-discharge in which you let the battery go down too low over successive periods. That can cause damage and lead to a short circuit. So we protect against it by putting in a circuit which protects it from over-discharge or over-charging.
“The other is over-heating of cells, [which could] cause them to vent, or a manufacturing defect could cause a short circuit. But we've had 1.3 million hr. of operation in flight and are pretty confident in the overall design. However we can't assume anything, so if there was a manufacturing defect that would lead to a discharge of potential energy we'd expect the battery cell to vent—which looks like smoke. If the system detects smoke it configures the airflow so it goes through the E/E bay and overboard. If there are failures we know of, that is how it would work.”