But as the day wore on, federal and industry officials said, Mr. Huerta concluded that the planes had to be grounded and that it was too difficult to create an “exit strategy” before he knew what had caused the problem.
Now, as Boeing prepares to present a series of possible fixes to him on Friday, Mr. Huerta finds himself almost back where he began. Battery and aviation safety experts say they think that Boeing’s proposals are on the right track, but that too little is still known about the problems to feel certain that the measures will solve them.
“They are steps in the right direction, but they may not be enough,” said Donald R. Sadoway, a professor of materials chemistry at M.I.T.
The stakes are high both for the company, which loses money every day that the planes are not flying, and for Mr. Huerta, who has to balance safety concerns with the airline industry’s interest in flying the planes again. Despite more than five weeks of intensive work, investigators have not determined the precise cause of the problems, which also included a battery fire on a 787 parked at a Boston airport.
Boeing officials say they have identified the most likely ways in which the new lithium-ion batteries could have failed. They are asking Mr. Huerta to sign off on changes meant to practically eliminate the odds of future incidents and to protect the plane and its passengers if a problem did arise.
Boeing has support for its plans from F.A.A. engineers and outside battery consultants, who have helped the company identify defects in its original design. But without proof of what caused the hazards, other experts remain uneasy about an approach that, by necessity, is still somewhat theoretical.
“Uncovering all the possible failure modes is not necessarily cut-and-dried work,” said John Goglia, a former member of the National Transportation Safety Board.
For instance, Mr. Goglia said, in investigating the deadly crash of a Boeing 737 near Pittsburgh in 1994, the safety board found that Boeing had protected against many possible failures, but missed the problem with the rudder that caused the crash.
“I think it’s a good idea to do what they’re doing,” he said of the company’s proposals to improve the 787 batteries. “My advice to the F.A.A. is to make sure it will deliver what they think it can deliver.”
Professor Sadoway said the F.A.A. would need to make sure that Boeing had added systems to cool the eight individual cells in the batteries to keep them from overheating.
Ralph J. Brodd, a battery industry consultant in Henderson, Nev., said that if the F.A.A. approved the proposals, it should also require airlines to check the batteries after each flight.
“They’re going to have to be particularly vigilant for some time,” Dr. Brodd said.
Mr. Huerta is not likely to make any decisions at Friday’s meeting, and he will consult with the transportation secretary, Ray LaHood, on what to do, federal officials said. The agency is likely to demand flight tests and more assurances from Boeing.
If everything works out, the jets could start flying again in April. But airlines are also being cautious, with United Airlines saying on Thursday that it would not schedule any 787 flights before May 12, and possibly not until early June.
Boeing contends that adding insulation between the cells of the batteries, among other changes, would provide enough assurance that the batteries were safe to use.
Its aim is to minimize the risk of a short-circuit or fire in one of the cells spreading to the others, as investigators say occurred in the battery that caught fire in Boston.
The company would also add systems to monitor the temperature and activity inside each cell, instead of just in the battery as a whole. And it would enclose the batteries in fireproof metal containers and create tubes to vent any hazardous materials outside the plane.
Dr. Brodd, the battery consultant, said Boeing was likely to wrap each of the cells in porous, ceramic-coated fabrics or glass fibers that might be about one-eighth of an inch thick. That insulation would separate the cells enough to keep them from touching during normal plane vibrations and to prevent heat from being transferred between cells.
The porous nature of the material would allow air to pass and keep the cells cool. The bulk of the insulation might also require Boeing to make the battery case slightly larger.
Dr. Brodd said that another possible cause of the battery problems was contamination from errant metallic shavings, which could have been left in one of the cells during manufacturing. Federal officials said Boeing had recently insisted that GS Yuasa, the Japanese company that makes the batteries, strengthen its quality control.
The emergency landing by the 787 in mid-January occurred in Japan. Japan’s Transport Safety Board said this week that temperatures in the battery on that flight surged over 1,221 degrees, melting the aluminum in six of its eight cells.
Hirohiko Kawakatsu, an official in Japan’s Ministry of Transport, said on Thursday that Tokyo would need to consider any possible fixes independently before it allowed 787s to operate in Japan.
“We will consider such fixes based on our own considerations,” Mr. Kawakatsu said.
Hiroko Tabuchi contributed reporting.
Article source: http://www.nytimes.com/2013/02/22/business/faa-weighs-dreamliner-fixes-as-battery-flaw-remains-an-unknown.html?partner=rss&emc=rss