TAKAMATSU, Japan (Reuters) - A team of experts from U.S. aviation authorities and Boeing Co arrived in Japan on Friday to inspect a 787 Dreamliner passenger jet that made an emergency landing on a domestic All Nippon Airways Co flight earlier this week.

U.S. Federal Aviation Administration (FAA) technical advisor Eric West (3rd L) and a member (4th L) of the Japan Transport Safety Board (JTSB) inspect All Nippon Airways' (ANA) Boeing Co's 787 Dreamliner plane, which made an emergency landing on Wednesday, at Takamatsu airport in Takamatsu, western Japan January 18, 2013. REUTERS/Issei Kato

The incident prompted regulators in the United States and around the world to ground the 50 Dreamliners already in service. Battery-related problems are being investigated after warning lights indicated a battery problem on the ANA flight on Wednesday.

The 787, a lightweight, mainly carbon-composite aircraft, has been plagued by mishaps, raising concerns over its use of lithium-ion batteries.

The five representatives from the U.S. National Transportation Safety Board (NTSB), Federal Aviation Administration (FAA) and Boeing are helping Japanese authorities in the investigation of the aircraft, which remains parked at the side of Takamatsu airport in western Japan.

The Japan Transport Safety Board (JTSB) aims to end its initial checks by around midday on Saturday, a person familiar with the matter told Reuters, and will make further decisions based on how the investigations go on Friday.

GS Yuasa Corp, the Japanese company that makes batteries for the Dreamliner, said it also sent three engineers to Takamatsu to help the investigation.

A person at the company, who asked not to be named due to the sensitivity of the issue, said: "Our company's battery has been vilified for now, but it only functions as part of a whole system. So we're trying to find out exactly where there was a problem within the system."

Shares in the Kyoto-based battery maker rose 3 percent on Friday, having dropped around 18 percent since January 7 when a battery-related problem affected a parked Japan Airlines (JAL) 787 in the United States.

BIGGEST MARKET

Regulators in Japan said it was unclear when the Dreamliner could be back in the air. Japan is the biggest market so far for the 787, with ANA and JAL operating 24 of the 290-seat wide-bodied planes, which have a list price of $207 million.

Separately, the country's transport ministry said a fuel leak on a JAL-operated 787 last week was due to a malfunction in a drive mechanism that controls a valve. It said the British company that makes the valve was investigating. The ministry declined to name the firm.

Keeping the 787s on the ground could cost ANA alone more than $1.1 million a day, Mizuho Securities calculated, noting the Dreamliner was key to the airline's growth strategy.

JAL has cancelled 8 Dreamliner flights on its Tokyo-San Diego route until January 25, affecting some 1,290 passengers, and is switching aircraft for another 70 flights scheduled to fly the 787.

The JTSB has said the battery on the ANA flight that made the emergency landing was blackened and carbonised, had a bulge in the middle and weighed 5 kg less than normal.

The use of new battery technology is among the cost-saving features of the 787, which Boeing says burns 20 percent less fuel than rival jetliners using older technology. The plane represents a leap in aircraft design, but the project has been plagued by cost overruns and years of delays. Orders for the Dreamliner last year helped Boeing overtake rival Airbus as the world's largest manufacturer of passenger jets.

"This could turn out to be a minor technical problem, but the FAA has turned it into a significant marketing challenge for Boeing," said Loren Thompson, defence consultant and chief operating officer of the Lexington Institute, a Virginia-based think tank.

(Additional reporting by Yoshiyuki Osada, James Topham, Mari Saito, Issei Kato, Herng Shinn Cheng, Ruairidh Villar and Andrea Shalal-Esa; Writing by Ian Geoghegan; Editing by Ken Wills and Alex Richardson)

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JAL 787 fuel leak caused by valve-related malfunction - Japan ministry

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