That finding was a relief to Boeing and the 13 airlines that own the planes, which were grounded for four months worldwide this year after two episodes involving fire or smoke from the batteries.

But independent experts said it was hard to understand what could have caused heat intense enough to sear the carbon-composite skin on the top of the jet, leaving the possibility that Boeing could still face a setback if problems are found with another system on the plane.

Britain’s Air Accidents Investigation Branch, which is in charge of the inquiry, said in a statement Saturday that it was still trying to identify the cause of the fire, which occurred on an unoccupied Ethiopian Airlines 787 at Heathrow Airport. The statement said that the fire resulted in smoke throughout the plane and extensive heat damage in the upper part of the rear fuselage. But, the investigation branch said, the damage was not near either of the plane’s lithium-ion batteries. “At this stage,” the statement said, “there is no evidence of a direct causal relationship” between the batteries and the fire.

The regulator said its initial inquiry would most likely take several days. Other safety experts said the causes could include heated elements left in a galley just below where the fire burned the jet’s carbon-composite skin, a poorly installed part, or a short in the plane’s electrical system.

But given all the fire-retardant materials in the plane, Hans J. Weber, an aviation consultant at Tecop International in San Diego, said it was puzzling how the fire could have gotten hot enough to cause so much damage to the plane, which had been parked on the runway for eight hours.

The innovative planes were grounded in mid-January after the incidents involving fire or smoke coming from the new and more volatile type of batteries. Boeing and its suppliers have invested more than $20 billion in the Dreamliner, which use lightweight carbon materials and more efficient engines to cut operating costs by some 20 percent. Boeing expects to sell thousands of planes over the next two decades. The first 50 planes delivered began flying again between late April to early June after regulators approved a series of fixes, including adding insulation between the battery cells and encasing the batteries in a steel box.

Ethiopian Airlines said on Saturday that it was continuing to fly its other 787s because the fire at Heathrow occurred after the jet had been on the ground for eight hours and “was not related to flight safety.” The airline did not speculate on the cause of the fire.

United Airlines and 11 foreign carriers also have the planes, and several said on Saturday that they continued to fly them as the fire at Heathrow is investigated.

The fire caused no injuries, but it disrupted travel. Boeing’s shares had rallied in recent weeks, but investors reacted nervously on Friday, sending Boeing’s shares down 4.7 percent.

Smoke came from the plane, named the Queen of Sheba, about four and a half hours before it was scheduled to depart for Ethiopia. The plane was connected to an external ground power source, according to people briefed on the episode.

It was also not clear if any maintenance was under way or how long the fire had been burning, though it was intense enough to burn the top of the fuselage near the tail. That area is in a complex section where large parts of the plane are joined together.

The two lithium-ion batteries, which are lighter and generate more energy than conventional batteries, are under the cockpit and just behind the wings toward the bottom of the plane.

Safety experts said that some of the wiring in the plane’s new electrical system, which is more extensive than in other jets, would have passed through the damaged area, which was above the rear galley. It was also possible the fire migrated from another system or part of the plane.

The Financial Times quoted an Ethiopian manager in Britain as saying that maintenance workers had discovered a problem with the plane’s air-conditioning system during a routine inspection and had seen sparks but no flames. The report did not say when the inspection occurred, and aviation-safety officials in the United States were not sure what to make of it.

Thomson Airways, a charter airline in England, said on Saturday that it had replaced and tested several parts on a 787 that had cut short a flight on Friday. It said the plane would fly again on Sunday.

A team from Boeing was at the Heathrow site along with representatives from the airline and from two American government agencies, the Federal Aviation Administration and the National Transportation Safety Board.

Boeing and the two agencies had no comment Saturday on the possible cause of the fire, deferring to British investigators.

The 787 has had a history of mishaps since entering service in late 2011. Several airlines, including United, Qatar Airways and All Nippon Airlines, have been forced to divert flights because of electrical problems or other reasons. The airlines worry that all the problems could require more fixes or make passengers reluctant to fly on the plane.

Boeing has delivered 68 787s so far, with orders totaling 930 planes.

Article source: http://www.nytimes.com/2013/07/14/business/fire-on-boeing-787-dreamliner-at-heathrow-in-london.html?partner=rss&emc=rss