(Updates with number of injured, other details)

By Adrian Croft

LONDON, Jan 17 (Reuters) - A British Airways plane crash-landed at London's Heathrow airport on Thursday, slightly injuring 18 passengers and triggering an inquiry into why the Boeing 777 flying in from Beijing landed short of the runway.

Fire engines smothered the aircraft in foam after the landing at the world's busiest international airport extensively damaged its wings and ripped off its undercarriage.

Aviation commentators said the fact that the plane only just cleared the perimeter fence, hit the ground well short of the runway and then slid to a halt pointed to a massive loss of power in the final stages of landing.

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

"I win the lottery today," Fernando Prado, one of the passengers, said after being safely evacuated by emergency chute from the wreckage.

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.

"The flight crew showed great courage and professionalism in landing the aircraft safely," said British Airways chief executive Willie Walsh.

"All of the crew did a fantastic job evacuating the 136 passengers. They are all heroes," he added.

An airport spokesman said in a statement that the Air Accidents Investigation Branch was inspecting the aircraft.

"BA Flight 38 arriving from Beijing made an emergency landing at 1242," he said.

"Passengers were immediately evacuated and taken to a reception centre at the airport.

"Heathrow Airport's southern runway was closed immediately after the incident. It has now re-opened for take-offs only. The northern runway is operating for arriving aircraft."

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

It said 221 flights, from a normal flying schedule of 1,300, had been cancelled of which only eight were long haul.

However, while it had been allowed a limited number of night flights to soak up the backlog, it warned passengers planning to travel on Friday to check ahead.

Normally about 40 flights an hour touch down at Heathrow, just west of the capital, with a further 40 taking off.

A London police spokeswoman said: "There is nothing to suggest it is terror-related."

One eyewitness, Steve Bell, said the wheels were not down on landing, and he heard a grating noise.

"It turned about 90 degrees on landing. Its wheels were not down. Within minutes fire crews arrived and evacuated all the passengers," he told BBC News 24 television.

Among the planes delayed was a flight Prime Minister Gordon Brown was taking on an official trip to India and China. (Additional reporting by Avril Ormsby and Andrew Hough)

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