(Reuters) – Boeing 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 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.

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