Until smoldering batteries forced safety regulators to ground Boeing's new 787 Dreamliner jets last week, the aircraft manufacturer was flying high, with soaring profits and a recently regained No. 1 ranking in jet deliveries over Airbus.
But the grounding, prompted by a battery fire on one jet and the emergency landing of another, has knocked Boeing off stride. Now, investors as well as government officials are paying close attention to see how big the issue becomes for the company, which is one of the nation's biggest exporters.
Although company officials said they expected to find a solution quickly, federal regulators on Sunday ruled out one simple explanation — that the battery was overcharged. If the problems prove more complicated, they could threaten Boeing's plans to expand production of the planes, and the jobs that go with them.
"Boeing has a lot at stake, for its headlining airliner and for the company brand," said Scott Hamilton, the managing director of the Leeham Company, an aviation consulting firm in Issaquah, Wash.
Mr. Hamilton said he had no doubt that Boeing would "work its way through this." But until more is known about the batteries, he said, "it's impossible to draw conclusions about what went wrong, what the fix is, how long it will take and what the long-term damage to the 787 and to the Boeing brands will be."
In what would seem to be the worst possible outcome right now, Boeing might also have to redesign its powerful new lithium-ion battery system, or even switch back to older, safer models. Aviation experts said such changes could cost hundreds of millions of dollars and shave off some of the 20 percent savings in fuel costs that the new jets have delivered.
Analysts say Boeing, which has about $80 billion a year in sales, has the financial muscle to weather the problems and make production of the next generation of airliners succeed in an industry familiar with outsize bets.
But the recent incidents were a reminder of the manufacturing and testing mishaps that had delayed the development of the planes. And any lengthy new delay could tax the patience of airlines and investors who thought the Chicago-based company had put the problems behind it.
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Boeing's stock has dropped only 3.4 percent, to $75.04 a share, in the two weeks since the battery fire on a 787 parked at Logan International Airport in Boston. The Federal Aviation Administration grounded the jets after another 787 made an emergency landing in Japan on Wednesday because of a smoke alarm in the cockpit. The F.A.A.'s order applied to six United jets; an additional 44 around the world have also been grounded.
David E. Strauss, an analyst at UBS, said big investors were "cautiously optimistic" that the batteries just came from a bad manufacturing batch or could be fixed with minor changes.
But, he said, "if the F.A.A. came out tomorrow and said to redesign the battery, and Boeing said it would take three months, the stock is going to go down on that."
"Investors have been expecting that Boeing would finally start freeing itself of the cash drain from all the problems in developing the plane and that they would start to see more rewards now," he added.
The National Transportation Safety Board said Sunday that it had ruled out excessive voltage as the cause of the battery fire on the 787 in Boston, adding to the mystery of the cause.
Besides the hazards to passengers if fire or smoke escaped from the battery containers, the problems are important because the 787 relies more on electrical systems than previous planes. Its use of electric rather than hydraulic systems is one of the innovations, along with more efficient engines and a lightweight carbon-composite structure, enabling the plane to save on fuel.
Boeing officials have said they had not previously had any problems with the batteries during 1.3 million hours of flights by their test pilots and eight airlines. Marc R. Birtel, a Boeing spokesman, said Saturday that one lithium-ion battery caught fire in 2006 during tests that Boeing held with the F.A.A. But he said the problems stemmed from the way the test was set up, and not from the battery design.
Depending on its agreements with each airline, Boeing was probably spending several hundred thousand dollars to $1.5 million a day to compensate carriers for lost passenger traffic or the need to lease other jets to maintain service, analysts said. Those penalties might be paid as a mix of cash and discounts on future plane purchases.
Boeing also plans to keep building 787s, at its current rate of five a month, to keep its supply chain intact. Mr. Strauss, the UBS analyst, estimated that the company spent $175 million to $225 million on each plane, so that would tie up more cash if it took time to fix the problems.
And the costs would mount even more rapidly if the planes remained grounded for several months or significant design changes were needed. That could slow Boeing's plans to increase production to seven jets a month in mid-2013 and 10 a month by the end of the year.
Still, Boeing, which also has a giant military business, has about $11 billion in cash, and it cannot build some of its other planes, like the 737 and the 777, fast enough to keep up with the demand.
The 737 and 777 programs "are printing money," said Richard L. Aboulafia, an aviation analyst at the Teal Group in Fairfax, Va.
"This is why small, niche companies don't build large commercial jets," he said. "You need multiple product lines to insulate yourself from problems."
One risk for Boeing is that adjustments in the lithium-ion batteries could require changes in other electrical equipment. But the biggest risk is that the batteries could prove too volatile, and Boeing would have to redesign its systems to use heavier and less-efficient nickel-cadmium batteries.
Some technical experts doubted that such a drastic change would be needed. But smaller lithium-ion batteries have caused fires in cellphones and laptops in the past. Cessna was forced to replace lithium-ion batteries on its CJ4 business jet with nickel-cadmium after a battery fire on the plane in 2011.
Airbus is using lithium-ion batteries in its new A350 XWB jet, which is scheduled to enter service in 2014. But Airbus said the batteries in its plane, which will compete with the 787 for sales, were smaller than Boeing's and carried lower voltages, making them less vulnerable to problems.
The F.A.A. set a series of conditions in 2007 to ensure that the 787's batteries did not overheat or spew flammable materials. It was reviewing whether the company had properly taken those steps.
"Until they can replicate what happened, people won't feel 100 percent confident," said Howard A. Rubel, an analyst at Jefferies & Company. "You know they have to replicate it."