By Jeremy Lovell

LONDON, Jan 18 (Reuters) - London's Heathrow airport, the busiest international airport in the world, struggled to return to normal on Friday after a crash landing this week caused travel chaos but only minor injuries.

British Airways said it planned to operate all of its long haul flights and up to 90 percent of short haul departures, but advised passengers to call ahead for flight information. Twelve short haul flights had been cancelled on Friday morning.

All 136 passengers walked away from BA flight 83 from Beijing on Thursday after it crash landed well short of the runway following what aviation experts said appeared to be a massive power loss on its final approach.

Air crash investigators, examining the fuselage and wreckage of the Boeing 777 strewn across the end of the runway, said they would issue preliminary findings within 48 hours but that a full explanation of the accident could take months.

Prime Minister Gordon Brown, whose flight to China to promote business ties was delayed by the accident, praised the crew of the giant airliner.

"I think it's right to pay tribute to the calmness and professionalism of the BA staff and the captain, and what he achieved in landing the plane," Brown told reporters shortly after his arrival in the Chinese capital Beijing.

The wheels of the plane, which had a routine maintenance check in December, were in the field where it crashed, several hundred metres (yards) from the runway.

BA declined to comment on reports of a loss of power. But it praised the pilot and crew for the way they handled the crisis.

"All of the crew did a fantastic job evacuating the 136 passengers. They are all heroes," said British Airways chief executive Willie Walsh.

An airport spokesman said the Air Accidents Investigation Branch was inspecting the aircraft.

BAA, which runs Heathrow, said 18 passengers suffered minor injuries.

It said 221 flights, including eight long haul flights, from a normal schedule of 1,300, had been cancelled on Thursday because of the accident.

Police said there was nothing to suggest the accident was terror-related. (Editing by Stephen Weeks)

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