According to airline sources, Boeing is saying that the JAL event, for instance, does not in fact show evidence of a classic “thermal runaway,” as described by the NTSB on Jan. 24, but was a more limited event in which only three of the battery's eight cells were involved in a chain reaction. Although the battery was completely gutted in the ensuing blaze, Boeing investigators believe the total damage resulted from long-term burning caused by the initial failure, not a full-scale thermal runaway.
Similarly, 787 operators have been briefed on the sequence of events in the ANA incident which, according to the aircraft maker, showed that the built-in safety systems worked as intended to warn the crew of the battery malfunction and vent smoke from the electronic/electrical equipment bay. The focus in that case is why the precautionary system that is designed to prevent the battery from over-discharging evidently failed.
But with the fleet grounded, and 787s now coming off the production line at a rate of around six per month, Boeing's reassurances could be seen as mere semantics by airlines that have already endured 3-4 years of delays. The two serious failures and the unexpectedly high number of battery changeouts throughout the fleet—more than 100—prior to the groundings reveal a system that was less than robust, at best, and unstable, at worst.
Boeing has no plans to interrupt or slow the 787 production rate, which McNerney confirms is on track to reach seven per month by mid-year and 10 per month by year-end.
“I don't expect to hear anything in the next 3-6 months,“ says the CEO of one major Boeing supplier. In his view, it would be extremely challenging to ramp down production now only to increase rates again within a short period of time. The supply chain would have a hard time handling such fluctuations, he argues. And for Boeing it is probably easier to keep production on pace and put aircraft in storage than to perform constant change management.
Despite the fact that engineers have been pulled off their assignments to deal with the battery situation and consider possible implications for the electrical system, McNerney says this is “not a significant” drain on resources. Because the efforts are focused on the battery, he adds that there has been no chain reaction or knock-on effect to the build-up for the 787-9, final assembly of which is due to start in coming months.
“I can assure that there is a comprehensive root-cause analysis and related series of technical efforts that I am confident will identify the root cause of these incidents. And so confidence in the process, confidence in the right resources, confidence that it's not distracting to the balance of Boeing, and when we know the answer, we'll know the answer, and we'll act on it,” McNerney says.
There are mixed signals as to whether ancillary charging control devices remain prime suspects along with the battery itself. Investigators are trying to determine if the short circuit, in the case of the JAL fire in particular, was caused by an undetected manufacturing flaw or something else, such as a foreign object.
Battery experts confirm to Aviation Week that NTSB testing at the Carderock Div. of the Naval Surface Warfare Center laboratories includes examination for signs of short-circuiting caused by the build-up of a structure inside the battery called a dendrite. These accumulate in lithium batteries, usually through uneven absorption and desorption of lithium ions, and they can penetrate the inner membranes that divide the anode and cathode. The dendrite formations, which can also be triggered by foreign-object particles on the electrode surface, introduce a physical contact between the positive and negative electrodes, thereby generating a short circuit.