Boeing is poised to start data-gathering flight tests of the 787 for the battery failure investigation, but certification and safety officials are indicating what the manufacturer and its customers have feared the most: It could be months and not weeks before the aircraft returns to service.
The gloomy prognosis is confirmed by U.S. National Transportation Safety Board Deborah Hersman, who says the board has “a lot of work to do” and is still “weeks away” from even pinning down the cause of the first battery fire, on a Japan Airlines aircraft in Boston on Jan. 7. The NTSB investigation has pinpointed Cell 6 as the origin of the fire that broke out in the aircraft's eight-cell lithium-ion auxiliary power unit (APU) battery. Just prior to the event, the voltage “unexpectedly dropped from its full charge of 32 volts to about 28 volts,” says the board, which is consistent with the charge voltage of a single cell, Hersman notes.
“We know Cell 6 had multiple short circuits and got into a runaway condition and it propagated into other cells,” she says Hersman. The NTSB is “looking at the state of the cell charge, manufacturing processes and design of the battery. There are a lot of things we are looking at,” she adds. Next steps specifically outlined by the board include an evaluation of the validation methods originally used for certification of the battery as well as more testing of field replacement batteries.
“This investigation has demonstrated that a short circuit in a single cell can propagate to adjacent cells, resulting in smoke and fire,” says Hersman. As a result, “the assumptions used in the certification of the battery must be reconsidered.”
The parallel Japan Transport Safety Board (JTSB) investigation into the Jan. 16 mid-air main battery failure on an All Nippon Airways 787 is moving on to the contactor and battery diode module maker in France, after in-depth analysis of the battery, the monitoring unit and charger were conducted in Japan and the U.S. The JTSB conducted CT scans of the main and APU batteries from the ANA aircraft at a Japan Aerospace Exploration Agency facility as well as a scan of the main cells at the battery maker GS Yuasa in Kyoto.
Like the NTSB, the Japanese investigators report that a thermal runaway condition in which the cells are destroyed in a chain reaction was observed. It also says that adjacent Cells 3 and 6 were particularly damaged while all eight cells showed internal fusing. It also reports that the ground wire of the battery chassis was broken, though there is speculation this may have been caused by the action of the fire fighters responding to the incident.
The only glimmer of good news for Boeing is the FAA's decision to approve the start of test flights to verify the vibration and temperature environment experienced by the batteries in the forward and aft electronic/electrical equipment bays. Clearance to take this first step toward ultimately testing and certifying a modified battery and monitoring system was signaled by the FAA on Feb. 7, the same day as an agency-approved one-off ferry flight of a newly painted 787 from Texas back to Everett.