Boeing(BA.N) faces a public and revealing test of the carbon-composite technology used in the 787 Dreamliner following a fire that broke out aboard one of its planes at London's Heathrow airport.

British investigators say that the Ethiopian Airline's (EAD.PA) lithium-ion batteries likely did not cause Friday's fire, allaying fears about a return of the problem that grounded the Dreamliner for more than three months earlier this year, when one battery caught fire and another overheated.

Wall Street, and passengers, so far appear little concerned: the stock is expected to stabilize on Monday after slipping 4.7 percent on Friday from a near all-time high. Airlines are keeping their 787s in the air and passengers are not canceling trips in Japan, the 787's biggest market.

But the visible scorching on the top rear of the fuselage of the 250-seat plane puts a major innovation of the 787 - its lightweight, carbon-plastic composite construction - under a spotlight with a fresh set of questions around the plane that Boeing and investors had hoped were behind it.

The key question for both: can the burned plane be fixed easily and at a reasonable cost?

While composites have been used in aerospace for decades, the 787 is the first commercial jetliner built mainly from carbon-plastic materials, whose weight savings, combined with new engines, are supposed to slash fuel costs 20 percent and operating costs by 10 percent compared with traditional aluminum alloy.

In designing the Dreamliner, Boeing engineers also added a weight-saving electrical system that was sorely tested when its lithium-ion batteries overheated on two 787s in January. The system also suffered a fire in 2010 during the plane's test phase, and could come under scrutiny again if the Ethiopian Airlines blaze is traced to an electrical fault.

The two systems are supposed to put Boeing at least a decade ahead of its rivals in the way aircraft is designed, built and operated. Boeing wants the 787 to become its most profitable passenger plane - and a fountain of innovation to feed designs of other future planes.

Now they are both being tested again at a time when the company is designing new planes and building up its factory production to fill a record book of orders.

Boeing declined to comment other than to say it is cooperating with the investigation of the fire.

CARBON TEST

Such extensive composite repairs have not previously been performed on an operating commercial plane. So the Ethiopian Airline fire is the first chance airlines, financiers and competitors will have to see a real example of how and at what cost the repair can be done.

"Everyone in the industry is going to follow this closely," said Hans Weber, president of TECOP International and an aviation consultant who has worked on composite testing technology. "It's the ultimate test."

Carbon-composite technology and repair have been in use much longer than lithium-ion batteries. Boeing and others have had carbon fiber in military planes, such as the B2 Stealth Bomber, for more than 25 years.

The 787's composite skin can be patched by grinding out the damaged section, applying fresh layers of fiber and resin and then curing with heat under vacuum pressure, according to a Boeing engineer with knowledge of the process. The work can be done on site, and repair stations have been learning to make repairs to service the plane around the world.

But the true cost and complexity of repair remains a key question for industry, airlines and competitors. In developing its rival A350 plane, Airbus(EAD.PA) used composite panels that are bolted to a framework, much like aluminum planes are made, a technology it saw as less risky to build and service.

Boeing chose to build one-piece, barrel-shaped fuselage sections that are bolted together to form a fuselage that it says is more aerodynamic and cheaper to maintain.

Boeing could make a new piece of fuselage and attach it if the damaged area was not too large, said the Boeing engineer. In a worst case, the entire rear section of the fuselage could be replaced, Weber said, an expensive fix that might cost more than the plane is worth.

PASSENGERS UNFAZED

The Ethiopian Airlines fire was noticed eight hours after the plane had been parked at a remote stand, the airline said, adding it was not a safety issue because the plane was not in flight and no passengers were aboard.

Fires break out on parked planes about 60 times a year, and most are from "human error" such as leaving a circuit on or cigarette butt, Weber said. A fire on a different type of plane might have gone unreported. Britain's Air Accident Investigations Branch termed the 787 fire a "serious incident" and said the initial investigation was likely to take several days.

Passengers appear to be sticking with the Dreamliner for now. Over the weekend, major travel agents in Japan, where most 787s are operating, said they had not seen reduction in bookings for 787 flights, and the plane remains in flight on the 13 airlines that currently operate it.

"We've received no such inquiries," said an official at JTB's Yurakucho branch in Tokyo. The company typically sells package tours, "and if there's a trouble, we change aircraft or routes for our customers."

In the near term, many stock analysts say they expect the stock to rise following the Friday decline. "I think investors will largely look past the incident, absent more info that suggests ongoing problem," said Carter Copeland, an analyst at Barclays in New York.

Boeing's stock has climbed more than 40 percent this year as investors focused on the company's record pace of jet production, which is generating cash.

Some are more cautious. Jeff Straebler, managing director and investment analyst at John Hancock Financial Services, said: "Until there is more information available on the cause, I don't think any judgments should be made."