Investigators have been working very closely with the FAA on a review the agency has under way of its sanctioning of the 787's certification for flight, Hersman said. The FAA awarded the certification in August 2011.
"We are evaluating assessments that were made, whether or not those assessments were accurate, whether they were complied with and whether more needs to be done," she said. "I think that is important before this airplane is back in the air, to really understand what the risks are and that they're mitigated effectively."
Nine days after the battery fire in Boston, another battery overheated on an All Nippon Airways 787, leading to an emergency landing in Japan. The same day, FAA officials ordered U.S. carriers with 787s — there's only one, United Airlines, with six planes — to ground the planes. Aviation authorities in other countries swiftly followed suit. In all, 50 planes operated by seven airlines in six countries are grounded.
The 787 is Boeing's newest and most technologically advanced plane. The groundings have become a nightmare for the company, which has about 800 Dreamliner orders from airlines around the globe.
Boeing was already spending more money on each 787 it built than it collected from airlines who bought it. On Wednesday, UBS analyst David Strauss estimated that Boeing will spend some $6 billion in cash this year on the plane, while an "extended 787 grounding would result in an even bigger cash burn," he wrote.
"As long as (the) 787 remains grounded, Boeing is faced with the choice of either slowing production or building physical inventory. It will build inventory for now," he wrote. Boeing already has about 46 787s that have been built but not yet delivered. Many of those were built early on and require more work before they can be handed over to customers.
Boeing currently builds five 787s per month. After the groundings it reiterated its plans to boost production to 10 per month by the end of the year, and said it planned to deliver at least 60 of the jets this year.
AP Airlines Writer Joshua Freed in Minneapolis contributed to this report.
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