(Reuters) - Boeing Co rolled out the Dreamliner's chief engineer to try to quell concerns about the new jet following three mishaps in as many days, including an electrical fire that caused severe damage to a plane.

At a news conference on Wednesday, the engineer, Mike Sinnett, defended the 787, the world's first plastic plane, and said its problem rates are at about the same level as Boeing's successful 777 jet.

Relatively few technical problems prevent 787s from leaving a gate within 15 minutes of scheduled departure time, he said. "We're in the high 90 percents," he said. "We're right where the 777 program was" at this stage.

The prevalence of more significant issues, such as a battery fire, is in the same order of magnitude as previous programs, he added. "There's no metrics that are screaming at me that we've got a problem."

Sinnett explained in detail how the lithium ion battery system that burned on Monday was designed by his team to be safe and prevent smoke getting into the cabin in the event of a fire during a flight. "I am 100 percent convinced that the airplane is safe to fly," he said.

Asked why smoke entered the cabin on Monday, Sinnett said the plane lacked cabin pressure to expel smoke because it was on the ground. In that scenario, "We expect that there would be sufficient time to evacuate the plane safely," Sinnett said.

The battery fire, on a 787 jet operated by Japan Airlines, occurred in Boston on Monday while the empty plane was parked at a gate after passengers had deplaned. That was followed by a fuel leak on another JAL 787 on Tuesday, and by brake problems on an All Nippon Airways 787 that forced the airline to cancel the flight on Wednesday.

These mishaps represent the most serious test of confidence in the Dreamliner since it began flying customers just over a year ago, following more than three years of delivery delays.

The U.S. Federal Aviation Administration and the National Transportation Safety Board are looking into what caused the fire, which came just weeks after Boeing endured a string of other electrical problems that briefly grounded three of the planes. The new jet also has suffered an engine failure and fuel leaks in the 14 months it has been in service.

Sinnett said the electrical faults that occurred in rapid succession in December were traced to a single lot of circuit boards manufactured at one time. He didn't name the supplier.

Analysts said they did not think regulators would ground the 49 Dreamliner jets currently in service due to this week's incidents, but some expected days or weeks to pass before firm details about the mishaps emerge - making it difficult to assess the severity of the problem, and the cost to fix them.

"It's clear through the conversation (from Sinnett) that it appeared to be manufacturing as opposed to design issues," said Jason Gursky, an analyst at Citigroup in San Francisco. "The fact that we've seen a multitude of small issues crop up and are not seeing the same issue time and time again would support that view."

Further detail from regulators are likely to take more time. In July, regulators took three days to decide whether to launch an investigation of a General Electric engine that failed on a 787, and another week passed before they provided details.

"We'd expect a similar timeline here," said Deutsche Bank analysts Myles Walton and Amit Mehrotra, in a note to clients Wednesday.

Boeing declined to discuss any aspect of the investigation into the battery fire. Analysts said the company still faces an image problem over the build quality of its marquee plane.

"There's no doubt in my mind that on the engineering side they are doing the right thing as far as dealing with these issues," said John Goglia, a former National Transportation Safety Board member and mechanic.

"They need to really reach out strongly with information to the press corps to make sure they understand exactly what happened and exactly what they are doing about it."

Boeing shares closed up 3.5 percent Wednesday, after losing more than 5 percent earlier this week.

"TEETHING PROBLEMS"

Of this week's incidents, the battery fire is of most concern. Lithium-ion batteries are heavily scrutinized by those who use them - not just airlines, but increasingly automakers as well.

"We cool our batteries. We put them through tests like you wouldn't believe," General Motors Chief Executive Dan Akerson said during a roundtable event Wednesday.

Shares of Japan's GS Yuasa Corp, which makes batteries for the 787, fell sharply for a second day on Wednesday.

Before Wednesday, Boeing had said little about the problems, though some of its most critical customers, like the CEO of Qatar Airways, have come to its defense.

Qatar Airways, the largest customer of the Dreamliner in the Middle East with an order for up to 60 of the aircraft, currently has five 787 jets. CEO Akbar al-Baker said the airline had no other issues since noting an electrical problem on one of its jets in December.

"Of course there will be teething problems from time to time, but this is foreseen with any new aircraft program," Al-Baker told reporters at an event in Doha on Wednesday.

Baker said he had no plans at the moment to cancel any plane orders with Boeing. "When we have to start grounding planes, then it becomes an issue and then they (Boeing) have to get their check book out," he said.