A lithium-ion battery that caught fire aboard a parked Boeing 787 in 2013 in Boston had design flaws and should not have been certified by the FAA, the NTSB said on Monday.
The National Transportation Safety Board said the battery, manufactured by GS Yuasa, experienced an internal short circuit that led to thermal runaway of the cell. This condition caused flammable materials to be ejected outside the battery's case and resulted in a small fire, the NTSB said in its report on the incident.
The agency said its investigators found a number of design and manufacturing concerns that could have led to the short circuiting, including the presence of foreign debris and an inspection process that could not reliably detect defects.
No one was hurt in the January 2013 incident aboard a Japan Airlines plane. The fire broke out while the 787 Dreamliner was parked at Boston's Logan Airport after passengers and crew had departed.
Another battery overheated on an All Nippon Airways plane later the same month, prompting regulators to ground the global fleet until April that year.
Boeing redesigned the battery and charger and designed a steel box to contain fires and vent hot gasses outside the plane.
Boeing said it agreed with the NTSB's conclusion that a short circuit led to the fire.
"We remain confident in the comprehensive improvements made to the 787 battery system following this event, and in the overall performance of the battery system and the safety of the airplane," the company said in a statement.
Lithium-ion batteries are widely used in cars, laptops and smartphones and have a tendency to overheat through processes that are not well understood by scientists.
Because the battery was a new technology, the FAA had required Boeing to demonstrate its safety in aircraft.
The NTSB faulted Boeing for ruling out the possibility of thermal runaway in its safety assessment of the battery, and it criticised the FAA for certifying the battery without thoroughly scrutinising the potential danger.
The NTSB said it is recommending that the FAA improve the guidance it provides to the aircraft industry and to FAA engineers on safety assessments involving new technology.