TOKYO (AP) -- Japan's two biggest airlines and the U.S. aviation agency grounded more than half the Boeing 787s in use around the world after an emergency landing of one of the jets exposed a battery fire risk in the technologically advanced aircraft.
The All Nippon Airways 787 that landed Wednesday had been leaking electrolyte from a main battery located in an electrical room below the cockpit, according to the airline. It said burn marks were found around the battery. The domestic flight landed at Takamatsu airport in western Japan after a cockpit message showed battery problems and a burning smell was detected in the cockpit and cabin.
The 787, known as the Dreamliner, is Boeing's newest jet, and the company is counting heavily on its success. Since its launch after delays of more than three years, the plane has been plagued by a series of problems including a battery fire and fuel leaks. Japan's ANA and Japan Airlines are major customers for the jet and among the first to fly it.
Japan's transport ministry said it received notices from ANA, which operates 17 of the jets, and Japan Airlines, which has seven, that all their 787s would not be flying. The grounding was done voluntarily by the airlines. Kyodo News agency quoted transport ministry investigator Hideyo Kosugi as saying that electrolyte from the ANA plane's main battery had leaked through the electrical room floor to the exterior of the aircraft.
In Washington, the Federal Aviation Administration was temporarily requiring U.S. carriers to stop flying 787s. United Airlines has six of the jets and is the only U.S. carrier flying the model, but aviation authorities in other countries usually follow the lead of the country where the manufacturer is based.
Boeing said it was working around the clock with investigators.
"We are confident the 787 is safe, and we stand behind its overall integrity," Jim McNerney, company chairman, president and CEO said in a statement.
The 787 relies more than any other modern airliner on electrical signals to help power nearly everything the plane does. It's also the first Boeing plane to use rechargeable lithium ion batteries, which charge faster and can be molded to space-saving shapes compared to other airplane batteries. The plane is made with lightweight composite materials instead of aluminum.
Worries over potential fire risks from lithium ion batteries, with their well-known flammability, predate the launch of the 787. In a May 2011 report, the FAA outlined various improvements in containing and preventing onboard fires but also noted that electrolyte leaks could make a fire more hazardous due to the high energy density and power capacity of such batteries.
The FAA had issued special precautions for installation of such batteries on board the 787s.
Japan's transport ministry categorized Wednesday's problem as a "serious incident" that could have led to an accident. Boeing, the U.S. National Transportation Safety Board and Federal Aviation Administration were sending representatives to work with the Japanese government on the investigation, NTSB spokeswoman Kelly Nantel said.
It was unclear how long the Dreamliners would be grounded. ANA and JAL canceled some flights or switched aircraft.
The earliest manufactured jets of any new aircraft usually have problems and airlines run higher risks in flying them first, said Brendan Sobie, Singapore-based chief analyst at CAPA-Center for Aviation. Since about half the 787 fleet is in Japan, more problems are cropping up there.
"There are always teething problems with new aircraft and airlines often are reluctant to be the launch customer of any new airplanes," Sobie said. "We saw it with other airplane types, like the A380 but the issues with the A380 were different," he said.
Video shot by a Japanese TV reporter, who was on board the ANA flight, showed the interior of the plane as passengers were evacuating, sliding down chutes, and then walking away on the tarmac. The video also showed crew members evacuating the aircraft. ANA and the fire department said one man had minor hip injuries from using the emergency slide, but the other 128 passengers and eight crew members were uninjured.
Japan's transport ministry had already started a separate inspection Monday of a 787 operated by Japan Airlines that had leaked fuel in Tokyo and Boston, where the flight originated.
A fire ignited Jan. 7 in the battery pack of an auxiliary power unit of an empty Japan Airlines 787 on the tarmac in Boston. It took firefighters 40 minutes to put out the blaze.
A computer problem, a minor fuel leak and a cracked windscreen in a cockpit were also reported on a 787 in Japan this month.
Boeing has said that various technical problems are to be expected in the early days of any aircraft model.
GS Yuasa Corp., the Japanese company that supplies all the lithium ion batteries for the 787, had no comment as the investigation was still ongoing. Thales, which makes the battery charging system, had no immediate comment.
Much remains uncertain about the problems being experienced by the 787, said Masaharu Hirokane, analyst at Nomura Securities Co. in Tokyo.
"You need to ensure safety 100 percent, and then you also have to get people to feel that the jet is 100 percent safe," Hirokane said.
Associated Press writers Kelvin Chan in Hong Kong, Joan Lowy in Washington and Mari Yamaguchi in Tokyo contributed to this report.