No trace has been found of the Boeing 777 aircraft, which disappeared a year ago this week carrying 239 passengers and crew, in what has become one of the greatest mysteries in aviation history.

MH370 vanished from radar screens shortly after taking off from Kuala Lumpur, bound for Beijing, early on March 8. Investigators believe it was flown thousands of miles off course before eventually crashing into the Indian Ocean.

The search of a rugged 60,000 sq km (23,000 sq mile) patch of sea floor some 1,600 km (1,000 miles) west of the Australian city of Perth, which experts believe is the plane's most likely resting place, will likely be finished by May.

Australian Deputy Prime Minister Warren Truss told Reuters that a decision would have to be taken well before then as to whether to continue into the vast 1.1 million sq km area around the primary search zone if nothing has been found.

Discussions had already begun about what to do in that event, including the possibility that the search might be called off, said Truss, who is also transport minister.

"For many of the families onboard, they won't have closure unless they have certain knowledge that the aircraft has been located and perhaps their loved ones' remains have been recovered," Truss said in an interview.

"We clearly cannot keep searching forever, but we want to do everything that's reasonably possible to locate the aircraft."

Truss compared the search, already the most expensive of its kind, with another great mystery from an earlier era, the hunt for missing aviation pioneer Amelia Earhart, who disappeared in 1937 during an early attempt to circumnavigate the globe.

Four vessels owned by Dutch engineering firm Fugro, equipped with sophisticated underwater drones, have searched about 40 percent of the previously unmapped expanse of sea floor that has been designated the highest priority.

DECISION TIME

Australia and Malaysia contributed to evenly split the costs, estimated at up to AUSD 52 million (USD 40.5 million), but Truss warned that continuing the search beyond that area would be impossible without more international help.

"We put in the amount of money that we believed was necessary to do this job well and thoroughly with the best available equipment," he said. "We have to make other decisions, then, about how long the search should continue."

Military radar showed the plane turned back across Peninsular Malaysia after contact with it was lost. A handful of faint "pings" picked up by a commercial satellite for around another six hours helped narrow down its likely final location.

Martin Dolan, Chief Commissioner of the Australian Transport Safety Bureau which is leading the search, said he remained confident that the plane would be found in the remainder of the so-called "priority search area".

If, however, the search has to be expanded into the much larger surrounding area, the costs could prove prohibitive.

"It's almost impossible to get your head around the scale of what's involved here," he told Reuters.

"If you take the theoretical maximum of the possible area for the aircraft – 1.1 (or) 1.2 million sq km – you're talking about orders or magnitude in terms of cost and time above what we're currently doing, and that's something that governments will obviously have to bear in mind."

Most of those on board the lost flight were Chinese or Malaysian.

Chinese Foreign Ministry spokesman Hong Lei said Australia, China and Malaysia had cooperated closely on the search.

"The Australian side has put in a large amount of personnel and material resources and we are deeply grateful for their help," Hong said. "The search effort is still ongoing and we hope the relevant work will produce progress."

The Malaysian government did not immediately respond to requests for comment.

Loss-making Malaysia Airlines, whose fortunes worsened when another of its Boeing 777's was shot down over Ukraine on July 17, killing all 298 people on board, was delisted at the end of 2014 as part of a USD 1.8 billion government-led restructuring.

Rounding out a bad year for Malaysian-affiliated carriers, an Indonesia AirAsia flight from Surabaya to Singapore crashed on Dec. 28, killing all 162 people on board.

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