The National Transportation Safety Board (NTSB) has ‘narrowed the focus’ of its investigation into a 787 battery which caught fire in January. It determined that both thermal and mechanical damage to the batteries originated in a single cell, which had short-circuited. “That cell showed multiple signs of short circuiting, leading to a thermal runaway condition, which then cascaded to other cells. Charred battery components indicated that the temperature inside the battery case exceeded 500 degrees Fahrenheit,” the NTSB said in a statement. Meanwhile, the FAA said it would allow Boeing to conduct 787 test flights, with the goal of collecting “data about the battery and electrical system performance while the aircraft is airborne," according to a joint statement by US transportation secretary Ray LaHood and the FAA’s Michael Huerta. A pre-flight inspection will be performed, with the status of the batteries monitored during take-off and during flight. Should an in-flight status message about the battery be received, the aircraft must land immediately, the FAA said.