The U.S. National Transportation Safety Board is examining the certification and testing of the lithium-ion battery system on the Boeing Co. 787, but still has not found the root cause of a January fire, the board said on Thursday.
The NTSB's 39-page "interim factual report" provides extensive detail on the tests performed on the battery that burned on a parked jet shortly after landing at Boston's Logan International Airport. But it also made clear that investigators remain a long way from understanding why it happened in the first place.
Among the reports findings: A system designed to vent smoke outside the plane during a battery fire failed to function because it lacked power after the battery caught fire. The system's auxiliary power unit (APU), a gas-driven engine in the tail of the plane, also was shut off at the time, and the battery is used to start that system.
"As a result, smoke generated by the APU battery could not be effectively redirected outside the cabin and aft (electrical equipment) bay," located in the fuselage behind the right wing of the 787.
Boeing had said that the venting system failed because the plane was on the ground and lacked cabin pressure to use in expelling fumes from the cabin.
The NTSB report marks a milestone in the agency's probe into one of two lithium-ion batteries that burned on 787 jets in January. However, the report signalled that the NTSB has significant additional work to do in its investigation. Japanese regulators are investigating the second incident.
The NTSB said a group focused on system safety and certification was going back through records and reviewing the testing analysis performed by Boeing and its suppliers, system maker Thales SA of France, and battery maker GS Yuasa Corp. of Japan.