Investigators have not found enough evidence in the charred remains of two Boeing 787 batteries to determine why they overheated, and they are expanding their examination of other electrical components, regulators in Washington and Tokyo said on Friday.
Adding to concerns about the batteries, industry officials said that United Airlines had replaced three batteries on the six 787s it received from September through December because they were wearing out sooner than expected.
All Nippon Airways, the first airline to get the 787 and its biggest operator, said this week that it had replaced 10 batteries on its 17 jets from May to December. Investigators are now looking for these batteries and others from different operators to see if they can provide any clues to the problems that led to the grounding of all 50 787s delivered so far.
Investigators also are delving into Boeing's supply chain. The National Transportation Safety Board said Friday that it would send an investigator to France to test a part that connects the battery to the plane's wiring.
Besides the battery, this connector is the fourth component to come under scrutiny in the jet's innovative electrical system. Investigators have also inspected the plants that made the battery charger and a controller unit, which are both in Arizona, as well as a facility that makes the battery's monitoring unit in Japan. The battery itself, using a volatile lithium-cobalt chemistry, is made by GS Yuasa in Japan.
In releasing an update on Friday, the board said it was still testing the battery that ignited on a Japan Airlines 787 while parked at Boston's Logan International Airport on January 7.
Another 787, owned by All Nippon, made an emergency landing in Japan on January 16 after the pilots received warnings of a battery problem and smelled smoke.
Kelly Nantel, the safety board's spokeswoman, said the board was still performing laboratory tests on the battery that was badly damaged in the Boston fire.
Other government officials said there was still no leading theory to explain why the two batteries emitted fire or smoke.
The 787 is the first commercial plane to use large lithium-ion batteries. The battery for the auxiliary power unit is in the plane's midsection. A second, under the cockpit, is the main battery and provides emergency power.
To assist in the investigation, the safety board has sought out the Naval Surface Warfare Center, which has worked with lithium-ion technology since at least the 1970s. Earlier this week, the board shipped the undamaged battery from the Japan Airlines plane for testing at the group's Carderock Division, in West Bethesda, Md. Those tests, which included electrical measurements and infrared thermal imaging of each of the battery's eight cells, found no anomalies, the safety board said on Friday.
Next week, the board plans to short all of that battery's cells. The test could highlight any problems in the cells.
Investigators hope that the examination of other batteries that were replaced in the months before the Boston fire will yield some clues. All Nippon said five of the 10 batteries it replaced had lost most of their charge. The three United batteries that were replaced also showed low power levels.
Despite the two incidents with the batteries and the questions about their reliability, Boeing's chief executive, W James McNerney Jr, said Wednesday that the company saw no reason so far to switch back to older but less volatile types of batteries.
McNerney acknowledged that airlines had needed to replace the new batteries at a "slightly higher" rate than Boeing had expected.
"What we know is that the replacement cycle that we've been experiencing there has been for maintenance reasons," McNerney said. "There is no incident where we're aware of where a battery has been replaced due to any kind of safety concerns."
But in an interview on Friday, John Goglia, a former member of the National Transportation Safety Board, took exception to McNerney's statement. Goglia said X-rays of the batteries that had been replaced could show if there were any changes in their makeup and might provide clues.
Investigators have said there could still turn out to be minuscule defects in the batteries. But Goglia, who is now an aviation safety consultant, said the loss of charges in a number of batteries suggested that there also could have been a problem with the battery charger or the circuits controlling the battery.
"There could be something in the system that tells the battery charger what to do that is giving bad information, or something could be diverting the charge from being received at the battery," he said.
As regulators and industry officials settle in for what could be a prolonged search for the cause of the safety incidents, Boeing's engineers have been studying ways to better contain or vent any smoke or excess heat if a battery malfunctions.
Officials at the Federal Aviation Administration have said they will not allow the planes to fly again until the cause of the safety problems is clear and fixes have been identified. Besides stopping all passenger flights, the agency directive that grounded the planes has also prevented Boeing's test pilots from making flights.
Boeing has stopped delivering planes to customers, but it has not slowed production.
Industry officials said Boeing hoped to come up with interim safety measures, including more frequent inspections of the batteries, that might persuade regulators to allow it to resume at least the test flights. But the executives said Boeing, which floated a similar idea earlier, had not yet presented a new proposal to regulators.