* Battery certification process one-third complete-Boeing
* New battery has steel case, power pack improvements
* Boeing still faces rigorous testing before any restarts
* Japan says too early to estimate when flights can resume
TOKYO/NEW YORK, March 15 (Reuters) - Boeing Co said Friday its 787 Dreamliner jets could be airborne within weeks using a new battery system that includes safeguards against overheating, a prediction that drew scepticism from some regulators and industry experts.
Japanese regulators immediately warned that the timetable was impossible to predict, in part because investigators still do not know what had caused lithium-ion batteries to overheat on two 787s.
"At this time we are not yet in a position to say when flights will restart," said Shigeru Takano, the air transport safety director at Japan's Civil Aviation Board (CAB), which will assess and approve Boeing's proposed fix.
Boeing's confident assertion marked a shift from the May or June dates expected by airlines, and appeared to pressure regulators to quickly approve the new battery safeguards. Industry sources said U.S. regulators have signalled they expect a lengthy testing schedule sufficient to ensure the refitted, carbon-composite plane is safe.
Boeing's timetable also put the company at risk of missing a deadline if approval takes longer than expected, adding to a history of missed deadlines that has bedevilled the Dreamliner.
Regulators grounded Dreamliners worldwide in January after a battery caught fire on a Japan Airlines Co 787 jet at Boston's Logan airport and a battery melted on an All Nippon Airways Co flight in Japan.
On Tuesday, the Federal Aviation Administration (FAA) approved Boeing's plan to test its new battery system for certification. The agency had no comment on Boeing's new timetable on Friday.
In explaining its optimism, Boeing said it had finished three tests of the new battery system and was performing three more this week, all in cooperation with the FAA, allowing it to predict the plane would be back in the air in weeks, not months.
HURDLES TO CLEAR
"We should be able to finish those tests in the next week or two," Ron Hinderberger, vice president of 787-8 engineering for Boeing Commercial Airplanes, told a U.S. press conference on Friday. Once the tests are finished, it will be up to the FAA to approve the process, Hinderberger added.
To meet regulators' standards and ensure safety, Boeing said it will encase the redesigned battery in a steel box, pack it with added insulation, heat-resistant material and spacers, and provide drainage holes to remove moisture and vent any gases from overheating directly outside the aircraft.
"If we look at the normal process and the way in which we work with the FAA, and we look at the testing that's ahead of us, it is reasonable to expect we could be back up and going in weeks, not months," the 787's chief engineer, Mike Sinnett, said at an earlier briefing in Tokyo.
But the CAB, the FAA's counterpart in Japan, dismissed Sinnett's prediction, saying it was too early to predict when 787 operations could resume, since regulators in the United States and Japan are still investigating. Takano, the air transport safety director at the CAB, said Sinnett's comment on the battery probe was "inappropriate."
A transport ministry source, who declined to be identified because he was not authorized to talk to the media, told Reuters it was possible the 787 will fly again in "several weeks" but cautioned that regulators will take as much time as they need to assess the battery fix.
Once regulators approve the work, installing the new power packs and adding a vent will take about a week per plane, Boeing's vice president in charge of 787 services, Mike Fleming, said after the briefing in Tokyo.
Beyond the testing regimen, Boeing also faces public hearings in April on the safety of its batteries, called by the U.S. National Transportation Safety Board.
The agency is investigating the cause of the battery failures and the original process used to certify the battery as safe, and is expected to make recommendations. It is unclear how long the NTSB investigation will last.
And some investigation details remain in dispute. Boeing said Friday, for example, that the NTSB had ruled out fire erupting inside the metal battery container in the Boston incident, and that flames only occurred outside the box. NTSB spokesman Peter Knudson said it was too early in the investigation to rule out fire inside the box.
"It will be fascinating to see if they dial back a few of their recent comments," said Richard Aboulafia, an industry analyst at Teal Group in Virginia. "(FAA Administrator Michael) Huerta has been implying a long road with a lot of testing before approval."
Some experts said the NTSB may be surprised by Boeing's aggressive timetable. "I don't think they're going to be overjoyed," said John Goglia, a former NTSB board member.
Testing problems could delay the 787's return to the skies, but barring that or other problems, flights could resume in April, he said. "The only risk I can see is if they have another event while testing or early on in the return to flight."
Though investigators may never uncover the root cause of those failures, that has not stopped Boeing from addressing possible causes, Sinnett said.
"We looked at everything that could impact a battery and set a broad set of solutions," Sinnett said, suggesting this approach was more rigorous than focusing on a single cause.
The aircraft maker will also bolster quality control at battery component makers GS Yuasa and Thales Sa and install a new charger that would keep voltage within a tighter range to guard against possible overheating.
"I would gladly have my family, my wife and my children, fly on this airplane," Sinnett said.
The refitting work will be done on-site, rather than at Boeing's assembly plants in the United States. The aircraft maker does not have the capacity to work on all 50 Dreamliners at the same time and will fix them in the order they were delivered, Boeing vice president Fleming said.
Japan is Boeing's biggest customer for the fuel-efficient aircraft, which has a list price of $207 million. JAL and ANA combined account for almost half the global Dreamliner fleet.
Boeing shares rose $1.47 to $86.09 in mid-day trading on Friday.
Investors expect little impact on operations as the carriers use other aircraft to limit cancellations, with Boeing also likely seen compensating the carriers for losses.