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 over three months earlier this year, when one battery caught fire and another overheated.

The visible scorching on the top rear of the fuselage of the 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 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 jet built mainly from carbon-plastic materials, whose weight savings, along 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 could again face scrutiny 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 boosting production to fill a record book of orders.