Safety agency deals blow to Boeing The head of the US transport safety regulator has fiercely criticised the process that approved a new battery type implicated in two safety scares on Boeing's Dreamliner aircraft last month, in the latest blow to the 787 programme. Deborah Hersman, chairman of the National Transportation Safety Board, said on Thursday both the Federal Aviation Administration, the US regulator, and Boeing, the aircraft's manufacturer, should reconsider their assumptions about the batteries" safety. The criticism underlines the scale of the task facing Boeing as it seeks to persuade the FAA the aircraft can safely return to service. Ms Hersman was speaking a month after fire broke out in the battery of a Japan Airlines 787 at Boston's Logan Airport, shortly after its arrival from Tokyo. Following the first fire, another 787 - operated by Japan's All Nippon Airlines - was forced to make an emergency landing in Takamatsu, western Japan, on January 16 after the pilot noticed problems with its battery. Aviation regulators worldwide subsequently grounded all 50 of the revolutionary aircraft that had been delivered to airlines. Ms Hersman was giving an update on the NTSB's investigation both of the Logan fire's immediate cause and how the FAA first accepted the batteries were safe. There has been criticism since the two incidents of the extent to which the hard-pressed FAA relied on tests by Boeing as the basis for its safety certification. The batteries are critical not only because of their potential to cause fire but because of their role in powering aircraft systems if the engines, which generate power for electrical systems, fail. The NTSB investigates transport accidents in the US, while the FAA sets safety rules and standards and certifies new designs as safe. Ms Hersman said Boeing had assessed during the certification process that a failure in a single battery cell was likely to result in smoke emission from the battery less than once every 10m flight hours. Boeing had also assessed that a failure in a single battery cell - which has now been identified as the Logan fire's cause - was not likely to affect other cells in the battery. "The 787 fleet has accumulated less than 100,000 flight hours," Ms Hersman said. There have now been two battery events resulting in smoke less than two weeks apart, on two different aircraft. The investigation had also shown that a problem in a single battery cell could spread to its neighbours and cause smoke and fire, Ms Hersman said. "The assumptions used to certify the battery must be reconsidered," she said. I think the FAA and Boeing have to evaluate the assumptions that were made during the certification process. The aircraft maker chose lithium-ion batteries - instead of traditional nickel-cadmium batteries - to save weight on the aircraft, which is far lighter than conventional jets.