Even as accident investigators pushed to find the cause of a January fire in the battery system of a Japan Airlines Boeing 787, the entire fleet of 787s — equipped with new lithium-ion battery systems — began returning to service after a three-month grounding.
The modified battery systems were required under an airworthiness directive (AD) issued by the U.S. Federal Aviation Administration (FAA) in late April. The AD mandates the replacement of the main battery, the auxiliary power unit (APU) battery and their chargers, and the installation of main and APU battery enclosures (see “787 Batteries”). The FAA directive applies only to U.S. operators of 787s, but other civil aviation authorities worldwide have issued similar orders to operators of 787s under their jurisdictions.
The original main battery in the Boeing 787 and the auxiliary power unit battery are unique to the airplane.
Both are lithium-ion batteries consisting of eight battery cells, connected in series and assembled in two rows, each made up of four cells. Insulation sheets separate the cells from each other and from the electrically grounded aluminum battery case.1
The U.S. National Transportation Safety Board (NTSB) said, in its interim factual report on the Jan. 7 battery fire in Boston, that the cells have a lithium cobalt oxide compound chemistry, that they contain a flammable electrolyte liquid and that they have “nominal voltage” of 3.7 volts.
Each cell contains a vent disc, defined by the NTSB as “a plate that ruptures when the internal pressure in a cell reaches a predetermined level,” and the vent discs are oriented toward the battery’s exterior.
Each cell also has three internal electrode winding assemblies, described this way by the NTSB:
Each winding assembly is about 33 ft [10 m] in length and is configured with an electrode, then a separator, then another electrode and then another separator. One electrode (the anode) is a copper foil coated in carbon; the other electrode (the cathode) is an aluminum foil coated in a lithium cobalt compound. Lithium-ion batteries have primarily nonflammable components, but the electrolyte is flammable, and active material coatings on the negative (anode) and positive (cathode) electrodes contain chemically reactive components.
The battery case also contains the battery monitoring unit, which monitors for overcharging, over-discharging, overheating, imbalance and high current, and signals the battery charger unit to stop charging if any battery-monitoring thresholds are exceeded.
- NTSB. Interim Factual Report, Accident Number DCA13IA037. March 7, 2013.
The FAA said its action was intended “to minimize the occurrence of battery cell failures and propagation of such failures to other cells and to contain any flammable electrolytes, heat and smoke released during a battery thermal event in order to prevent damage to critical systems and structures and the potential for fire in the electronics equipment bays.”
The worldwide fleet of 50 787s was grounded in January after two events that occurred within days of each other involving lithium-ion batteries on the new airplanes.
The first event was a Jan. 7 fire aboard the Japan Airlines 787 about 15 minutes after it had been parked at a gate at Boston Logan International Airport after a flight from Narita International Airport in Japan. Maintenance and cleaning personnel saw smoke coming from the aft electronics bay and summoned aircraft rescue and fire fighting personnel, who extinguished a fire in the APU battery. No passengers or crewmembers were in the airplane at the time, but one firefighter received minor injuries during the event.
The second event, on Jan. 15, involved an odor and a battery-overheat indication on the flight deck of an All Nippon Airways 787 during a domestic flight in Japan. The flight crew conducted an emergency descent and diverted to land at Takamatsu Airport, where the crew ordered an emergency evacuation after they saw smoke coming from the forward fuselage. Four of the 137 people in the airplane received minor injuries during the evacuation. Published reports quoted investigators from the Japan Transport Safety Board (JTSB) as saying that the airplane’s main battery had overheated and that they found a slight swelling in the APU battery.1
Civil aviation authorities worldwide cited both events in their orders grounding all 787s.
Finding a Solution
The events prompted an intensive effort by Boeing — aided by more than a dozen battery experts from industry, government, academia and the consumer safety field — to develop a solution.
The result of their work was a collection of modifications to the original lithium-ion battery system, including “design changes to both prevent and isolate a fault, should it occur,” Boeing said, adding that production, operating and testing processes also were improved.
The modifications include redesigned internal battery components to minimize chances of a short circuit, better insulation of battery cells and installation of a new containment and venting system for both main batteries and APU batteries.
Boeing said that the new steel-enclosure containment system was designed to “keep any level of battery overheating from affecting the airplane, or even being noticed by passengers.”
Ray Conner, president and CEO of Boeing Commercial Airplanes, added, “This is a comprehensive and permanent solution with multiple layers of protection. The ultimate layer of protection is the new enclosure, which will ensure that even if a battery fails, there is no impact to the airplane and no possibility of fire.”
The modifications — developed through more than 100,000 hours of preparing and performing tests and analyzing their results — were approved by the FAA after the agency’s review of certification tests that Boeing said were designed to “validate that individual components of the battery, as well as its integration with the charging system and a new enclosure, all performed as expected during normal operation and under failure conditions.”
As the FAA approved Boeing’s solution, the company deployed teams around the world to install the replacement battery systems. The first of the 787s to be equipped with the replacement systems were back in the air in late April, with installations expected to continue throughout May for all 787s in the order that the airplanes originally were delivered.
The modified battery systems also were being installed on new airplanes at Boeing assembly plants, and Boeing said that, despite the interruption of deliveries of new airplanes, all planned 2013 deliveries of 787s should be completed by the end of the year.
Looking for Answers
The modifications were ordered as the U.S. National Transportation Safety Board (NTSB) and the JTSB continued their investigations of the two battery-related events.
As part of its investigation of the Boston incident, the NTSB held a public hearing that focused on the battery’s original design and the certification process.
“We are here to understand why the 787 experienced unexpected battery failures following a design program led by one of the world’s leading manufacturers and a certification process that is well-respected throughout the international aviation community,” NTSB Chairman Deborah Hersman said at the start of the two-day hearing.
Hersman said that although the NTSB has not determined the cause of the fire, its investigators have pinpointed the event’s origin — “short circuits in [APU battery] cell no. 6 that cascaded, in a thermal runaway, to the other cells. The temperature inside the battery case exceeded 500 degrees F [260 degrees C].” The NTSB expects to issue its final report on the fire, including the probable cause, before the one-year anniversary of the event, Hersman said.
While seeking the cause of the fire, the NTSB also is reviewing the certification and testing of the 787 and its lithium-ion battery system — a feature that, in regulatory terminology, incorporated “novel or unusual design features” that were subject to nine special conditions imposed by the FAA. Those conditions were intended to “ensure that this new technology would not pose a greater safety risk than other technologies addressed in existing airworthiness regulations,” the NTSB said in its interim report.2
In 2007, when the FAA’s notice of proposed special conditions was published in the Federal Register, the agency explained its reasons for the proposal, noting that the aviation industry had limited experience with lithium-ion batteries and that other users of the batteries, in cell phones and electric vehicles, had experienced “safety problems, including overcharging, over-discharging and flammability of cell components.”
In addition, the FAA said at the time that lithium-ion batteries were “significantly more susceptible to internal failures that can result in self-sustaining increases in temperature and pressure” than other types of batteries that had been used in transport-category airplanes.
The special conditions emphasized FAA requirements that the battery installation must maintain safe temperatures and pressures, and that it must include a method to “control the charging rate of the battery automatically, so as to prevent battery overheating or overcharging.” The FAA also required an over-temperature warning system, including a method of automatically disconnecting the battery in case of an over-temperature event or a battery failure.
During the certification process, Boeing conducted a safety assessment to identify “potential hazards that various failure conditions of electrical system components could introduce to the airplane and its occupants,” the NTSB report said. Among the potential failure conditions were those identified as “battery vents smoke/fire,” which was classified as “catastrophic” and “battery vent and/or smoke (without fire),” classified as “hazardous.”3
Tests determined that the probability was one in 10 million flight hours that one of a 787’s batteries could “vent” — a process in which a build-up of pressure inside a battery cell causes the rupture of a plate on the cell known as a “vent disc.” When the 787s were grounded on Jan. 16, however, two smoke events had occurred in a fleet that had accumulated fewer than 52,000 flight hours.
Examination of the APU battery on the Japan Airlines 787 showed that, of the eight battery cells, vent discs on three were slightly open, vent discs on four “had opened more completely, leaving a ruptured appearance,” and the disc on one cell remained intact, the NTSB report said. The area where the APU battery was installed had been damaged in ways that the report said were consistent with heat generated by the battery, and with smoke, hot gas and electrolyte discharged from the battery.
Procedures, Past and Future
During the NTSB’s public hearing, Mike Sinnett, 787 vice president and chief project engineer at Boeing Commercial Airplanes, described the certification process for the 787 and its battery system as the most extensive in Boeing history.
Boeing initially had chosen lithium-ion batteries rather than other battery types for the 787 because of their low weight, charging characteristics, power capability and safety, among other characteristics, Sinnett said, noting that lithium-ion technology was considered “a better technology” for the 787.
When tests were conducted as part of the certification process, he added, it was believed that even if one battery cell overheated and vented, other cells would be unaffected.
Boeing — as well as the FAA — has continued to review circumstances surrounding the two January events to fully understand what happened and why, Sinnett said.
Throughout the certification process, he added, “Boeing worked closely with its suppliers and the FAA to demonstrate that the battery complied with all applicable Boeing and regulatory requirements.”
The FAA provided “rigorous oversight” of the development process, he said.
Hersman said, however, that the FAA would need a considerably larger staff if it were expected to oversee every detail of the operation, and she noted the difficulty in determining precisely how detail-oriented the agency should be.
Dorenda Baker, director of the FAA Aircraft Certification Service, characterized the FAA’s oversight process as a “robust” procedure that has functioned well for 50 years.
Part of the process allows for the development of project-specific special conditions — such as those approved for the 787’s lithium batteries — when current standards are not appropriate for specific new technologies, Baker said. The intention of special conditions, she added, is to “allow innovation while maintaining safety.”
In the case of the 787, she said, “We identified a number of design features … including the lithium-ion main and APU batteries, where the current safety standards were not adequate or appropriate to address the novel or unusual design features. At the time, there was no aviation industry standard for rechargeable lithium-ion batteries.”
The special conditions were developed in reference to existing knowledge of lithium-ion batteries and knowledge of other types of batteries that traditionally have been used in large aircraft, with the goal of ensuring a level of safety for lithium-ion batteries that was equivalent to that required for other battery types.
Hersman said the NTSB’s goals in the investigation are to understand not only why the events occurred but also how the design and certification process functioned.
“We are looking … for knowledge that can be applied to emerging technologies going forward,” she said.
“The U.S. aviation community is using the same approach to certification that was created to certify our grandparents’ aircraft, and by most accounts, it has served us very well. But perhaps it is time to ask if any changes are needed to update the system that will be used to oversee the development of new and beneficial technologies on our children’s and our grandchildren’s aircraft.”
- Karp, Aaron. “Swelling Found in ANA 787’s APU Battery.” ATWOnline, Feb. 19, 2013. A subsequent report (Karp, Aaron. “JTSB: Incorrect Wiring Found in ANA 787.” ATWOnline, Feb. 21, 2013) said that JTSB investigators had found “incorrect” wiring in the airplane but that the wiring — which involved the connection between the main battery and the APU battery — likely was not related to the battery problem.
- NTSB. Interim Factual Report, Accident Number DCA13IA037. March 7, 2013.
- The Interim Factual Report cited the FAA’s definitions of a catastrophic event as one typically involving a hull loss with multiple fatalities. A hazardous event is defined as one typically involving “a large reduction in functional capability or safety margins of the airplane with serious or fatal injury to a small number of passengers or cabin crew along with physical distress or excessive workload impairing the ability of the flight crew.”