Boeing 787 Dreamliner tests 'missed fire risk'
Tests on Dreamliner batteries carried out by Boeing when they were first certificated for flight missed the high risk of fire, US regulators have said.
Boeing's tests underestimated the frequency of "smoke events" in the lithium ion batteries, the National Transportation Safety Board said.
They also failed to spot that overheating in one battery cell could cascade to the others, causing a fire.
The regulator said it still had many more leads to investigate.
On Wednesday, the NTSB said it would take weeks for it to complete its investigations. Its head, Deborah Hersman, said the regulator hoped to produce an interim factual report - but not a final report - within 30 days.
The ultimate decision to allow Boeing's 787 Dreamliners to re-enter service will be taken by the Federal Aviation Administration in the US, and by other countries' air regulators in the rest of the world.
The NTSB head said that Boeing's safety checks suggested that a smoke event would occur less than once every 10 million flight hours.
But the 787 planes have only clocked up 100,000 hours of flight-time since entering commercial services, and have experienced two battery fires.
The 50 Dreamliners in service have been grounded since 16 January, after a battery in a Japan Airlines 787 plane caught fire in Boston, and an All Nippon Airways flight was forced to make an emergency landing because of a battery malfunction.
So far, the regulator has discovered that the Boston fire was caused by a short circuit within one of the cells of the plane's multi-cellular batteries, and that a fault in the battery's design allowed the resulting overheating to spread to the other cells.
However, the NTSB has not yet found out what caused the short circuit in the first place.
Ms Hersman said they were now following up two lines of inquiry - the actual cause of the short circuit, and why the certification process failed to pick up the defect in the battery's design.