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In this April 1, 2014 file photo, provided by the U.S. Navy, the Bluefin 21 autonomous sub is hoisted back on board the Australian Defense Vessel Ocean Shield after successful buoyancy testing in the Indian Ocean, as search efforts continue for missing Malaysia Airlines Flight 370. (MC1 Peter D. Blair/AP)
In this April 1, 2014 file photo, provided by the U.S. Navy, the Bluefin 21 autonomous sub is hoisted back on board the Australian Defense Vessel Ocean Shield after successful buoyancy testing in the Indian Ocean, as search efforts continue for missing Malaysia Airlines Flight 370. (MC1 Peter D. Blair/AP)

Search for Flight 370 hangs on unmanned, torpedo-like craft Add to ...

Finding the wreck of Malaysia Airlines’ lost Boeing 777 in the Indian Ocean’s inky blackness 4-1/2-kilometres down now depends on an unmanned submersible that will slowly scan the sea bottom before surfacing daily so investigators can pore over its digital images.

Bluefin-21, a yellow, torpedo-like submersible on loan from the U.S. Navy, was launched Monday from the Australian naval vessel Ocean Shield, on the first day of what could be weeks or even months of painstaking missions, slowly building a detailed picture of the ocean floor where “pings” were heard, suggesting the missing aircraft crashed in one of the world’s most remote oceans – more than 1,500 kilometres northwest of Perth.

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After six hours in its initial dive, Bluefin exceeded its design depth limit of 4,500 metres Tuesday morning as it followed the ocean floor, triggering an automatic abort of the mission. The submersible's protective software returned it to the surface where it was safely recovered and its truncated mapping data downloaded and studied, the Australian search agency said in a statement.

But with no firm evidence as to whether the Beijing-bound jet actually changed course and flew, with no distress calls, for more than six hours and thousands of kilometres in the opposition direction before its fuel ran out about dawn on March 8, the search hangs on slender acoustic threads.

“Don’t be over optimistic,” warned Angus Houston, the former air-force chief now heading the Australian-led search-and-recovery effort. “Let’s hope that the very strong signal that we were receiving [was] actually coming from the black box … but we can’t confirm that until we lay our eyes on” the Boeing 777.

More than a month has elapsed since Malaysia Airlines Flight 370 with 239 on board vanished, barely an hour after leaving Kuala Lumpur. Investigators believe someone in the cockpit deliberately switched off the aircraft’s transponder – which identifies it to air-traffic control – and then programmed several course changes.

Searchers, guided by careful analysis of hourly digital “handshakes” from the Boeing’s satellite communications dome to an Inmarsat satellite, focused on a long arc in the Indian Ocean. Sophisticated listening devices, towed by Ocean Shield, detected “pings” believed to be from the locator beacons attached to the the jet’s flight data and cockpit voice recorder. The battery-operated beacons, activated on impact or immersion, would last only 30 days and have now faded away.

“There have been no confirmed signal detections since last Tuesday,” Mr. Houston said in explaining why the acoustic search has been ended.

He warned that mapping the ocean floor to find wreckage could take a long time, noting that when Air France’s Flight 447 crashed in mid-Atlantic after its pilots lost control of the aircraft, the plane’s last known location was less than seven minutes before impact but it still took two years of bottom-mapping to find the flight data and cockpit voice recorders.

“We have to wait to see if the Bluefin-21 finds wreckage on the bottom of the ocean,” he said. The autonomous submersible needs two hours of diving just to reach the ocean floor, then will spend 16 hours slowly scanning along predetermined tracks with its sonar – using reflected sound waves to build a detailed three-dimensional image of the bottom. It needs another two hours to return to the surface where the Bluefin will be hauled on board Ocean Shield to have its data downloaded and be refuelled for another mission. A 24-hour cycle is planned.

Mr. Houston also warned bottom-mapping effort might fail. “I would caution you against raising hopes [that it] will result in the detection of the aircraft wreckage. It may not.”

Bluefin will be operating at the edge of its design limits to withstand the crushing pressures 4,500 metres below the surface. Searchers hope the “pings” reduced the most likely zone for finding wreckage to hundreds of square kilometres – down from thousands.

With Bluefin only able to scan 40 square kilometres daily, the search could drag on.

Any imagery that seems worth additional investigation will require Bluefin’s sonar to be swapped for a camera and lighting in an attempt to make a firm identification. If the Boeing 777’s wreckage is located, it will take additional and painstaking searches to find the armoured, orange flight recorders. To recover them will require a deep-diving submersible with claw-like appendages capable of wrenching the recorders free from the wreckage and returning them to the surface.

With costs for the ongoing search for floating debris soaring into the millions of dollars – and no trace yet found despite more than a dozen military and civilian aircraft from several nations flying long-duration missions from an air base near Perth – officials are now considering whether to call off the surface search.

“The chances of any floating material being recovered have greatly diminished and it will be appropriate to consult with Australia’s partners to decide the way ahead later this week,” Mr. Houston added. “It’s very expensive and all of the countries that are contributing to this are running up big costs.”

He also said samples from a small oil slick, found on the weekend a few kilometres away from the area where the “pings” were heard, have been sent ashore for analysis. Remnants of jet fuel – even if the aircraft exhausted its tanks – or hydraulic fluids used in the Boeing 777 could be leaking from the wreck up to the surface.

Like the scores of debris sightings – all of which have proved unrelated to the missing aircraft – Mr. Houston said the oil slick will be investigated. “We will look at it; we have discounted all of [the other surface sightings] thus far.

Follow on Twitter: @PaulKoring

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