Agencies in the United States have cleared aircraft manufacturer Boeing to restart test flights of its grounded 787 Dreamliner in order to get more data on potentially faulty batteries.
The US Federal Aviation Administration said late on Thursday it would allow test flights, under more stringent rules, to
monitor the batteries in flight.
It followed an earlier, one-time flight to move a 787 from Texas to Washington state.
Thursday's approval also came with demands for a closer look at how the batteries were approved, which may delay resuming delivery of Boeing's newest aircraft.
Earlier in the day, Deborah Hersman, head of the US National Transportation Safety Board, said regulators must review the "special conditions" used in approving lithium-ion battery technology on the Dreamliners.
"There have now been two battery events resulting in smoke, less than two weeks apart, on two different aircraft," Hersman said.
The findings are at odds with what Boeing told the FAA when that agency was working to certify the company's newest and most technologically advanced plane for flight, Hersman said.
The 50 Dreamliners in service were grounded worldwide on January 16, after a series of battery incidents, including a fire
on board a parked 787 in Boston and an in-flight problem on another plane in Japan.
The groundings have cost airlines tens of millions of dollars, with no end in sight.
Boeing investors took the news in stride, pushing shares slightly higher on the day. Analysts said the market was focusing on the wrong issue: the short-term question of fixing the battery, versus the longer-term prospect that the entire battery system might need to be approved again.
If the battery needs to be re-certified, "you're talking about changes to the 50 they have delivered, significant amount of
engineering commitment on the 787-9. I see this as still having a significant amount of question marks," said Ken Herbert, an analyst at Imperial Capital in San Francisco.
Boeing shares were three per cent higher since the 787 was grounded in January, despite the headaches it has caused the
planemaker and the demands for compensation.
Even short sellers - investors who seek out shares that are likely to fall - have largely left the stock alone.
According to Markit's Data Explorers, just 0.3 per cent of shares available for borrowing were being used for short bets as of Wednesday.
"The market is focusing on the battery short circuit, which implies a simple fix," said Carter Leake, analyst at BB&T
"But they're missing the much bigger issue, which is the questioning of the certification process. Hersman is basically saying we're questioning the original certification altogether."