Regulator questions Boeing Dreamliner tests The National Transportation Safety Board (NTSB) said that its investigation of the battery that caught fire last month on a 787 operated by Japan Airlines showed that the failure of a single cell rapidly spread to the rest of the battery. Deborah Hersman, the board's head, said that by contrast, Boeing's tests had not shown any evidence that the failure of a single cell in the battery could have such an effect. "The 787 fleet has accumulated less than 100,000 flight hours, yet there have been two battery events resulting in smoke less than two weeks apart on two different aircraft," Ms Hersman said yesterday . Assumptions used to certify the battery must be reconsidered. Fires in two lithium-ion batteries on Dreamliners in the space of two weeks in January prompted the US Federal Aviation Authority to ground the 50-strong fleet of 787s. The US company has invested more than $20bn (£12.8bn) in the 787, which made its debut in late 2011 after a three-year delay. Its ground-breaking design and materials sought to create a more fuel-efficient plane. Boeing has said that it remains confident in the 787 and has more than 800 outstanding orders for the jet, including from Virgin Atlantic and British Airways. The company said last night that the "787 was certified following a rigorous Boeing test program and an extensive certification program conducted by the FAA." It added that it would "not hestitate" to improve testing processes.