* Fire broke out on parked plane at Heathrow on Friday
* Another Dreamliner turned back during flight
* Incidents a blow to Boeing after earlier grounding
* Share price closed down 4.7 percent after news
By Estelle Shirbon
LONDON, July 13 (Reuters) - Investigators started work on Saturday to establish the cause of a fire on a Boeing Dreamliner at London's Heathrow airport, a new setback for the high-tech model after it was grounded at the start of the year over battery problems.
The fire broke out on the plane, operated by Ethiopian Airlines, on Friday afternoon, when it was parked at a remote stand with no passengers on board, eight hours after arriving from Addis Ababa. No one was injured.
External scorching could be seen close to the plane's tail, in a different area from the bays containing batteries. There was no official indication of what could have caused the fire.
"The aircraft has been moved to a secure hangar at Heathrow and the investigation has begun," said a spokesman for Britain's Air Accidents Investigation Branch (AAIB).
The AAIB will lead the investigation, he said, working alongside the U.S. Federal Aviation Administration and National Transportation Safety Board (NTSB), Ethiopian Airlines and Boeing.
Analysts say Boeing will be keen to reassure airlines, travelers and investors over the cause of the fire as quickly as possible but it will be up to investigators to decide how much information to release and when. Under aviation rules there are restrictions on how much companies can say about the details of an ongoing accident investigation.
Meanwhile Ethiopian Airlines, one of Africa's top five carriers, said it would continue to fly its Dreamliner fleet.
"We have not grounded any of our aircraft," the carrier said in an emailed statement on Saturday. "The incident at Heathrow happened while the plane was on the ground and had been for more than eight hours and was not related to flight safety."
Separately, engineers from Britain's Thomson Airways were inspecting their own Boeing Dreamliner after it had to turn back during a flight on Friday from Manchester in England to Sanford in Florida because of an unspecified technical issue. Thomson Airways is one of six European airlines owned by TUI Travel , the world's largest tour operator.
The two incidents are a blow for Boeing particularly as the entire global fleet of Dreamliners had to be grounded for three months, ending in April, after one high-tech battery caught fire and another overheated.
Boeing shares closed down 4.7 percent at $101.87 on Friday, knocking $3.8 billion off the company's market capitalisation.
Quoting Mark Mangooni, Ethiopian Airlines' senior manager in Britain, the Financial Times reported that airline staff had discovered a problem with the aircraft's air conditioning system during a routine inspection and had seen sparks but no flames.
The report did not make clear when this had happened. Reuters could not reach Mangooni for comment.
Heathrow briefly closed both its runways to deal with the fire, and a spokeswoman said on Saturday that the airport was back to normal operations, although it was still dealing with a backlog of delays and cancellations due to Friday's incident.
The Dreamliner's two batteries are in electrical compartments located low down and near the front and middle of the plane. Damage to the Ethiopian plane appears to be on top of the fuselage, close to the tail, according to video from the scene on Friday.
Former NTSB Chairman Mark Rosenker said the Heathrow incident was extraordinary, coming so soon after the fleet had returned to service, but warned against jumping to conclusions.
"It's very early. No one knows where the fire started at this point," Rosenker told Reuters, adding it could be something as simple as a coffee pot left on in a galley.
A spokesman for GS Yuasa, which makes the Dreamliner batteries, said he had not received any information on the London fire.
The 787 is Boeing's biggest bet on new technology in nearly 20 years. It cost an estimated $32 billion to develop and Boeing plans to use hundreds of innovations such as its carbon-fibre composite skin and electrical system to enhance other jets.
The 787 Dreamliner fleet was grounded by regulators at the start of the year after batteries overheated on two of the jets within two weeks, including a fire in a parked Japan Airlines plane in Boston.
Boeing was forced to halt deliveries of the jet while it was grounded and airlines stopped ordering the plane at that time. Orders have since resumed and Boeing has logged 83 Dreamliner orders this year, bringing its current order book to 930 planes.
Boeing never disclosed the cost of the three-month grounding but said it absorbed most of the expense in the first quarter while still posting a 20 percent rise in profit. Its shares are up 35 percent this year, even after Friday's loss.
The plane which caught fire in London was the first of the 787 fleet to resume flight after the battery-related grounding.
A person familiar with the aircraft's configuration said the damaged area appears close to galleys and environmental control systems, but added that it was too early to link the fire to any specific equipment.
Several airlines said they were continuing to operate their 787s, including United Continental, Polish airline LOT, Japan Airlines and ANA, the world's biggest operator of the Dreamliner.
Ethiopian Airlines Dreamliners are powered by General Electric GEnx engines.