To sink underwater by the height of Britain’s tallest tower would still only cover 7 per cent of the descent to the floor of the Indian Ocean, 4,500 metres down, where the black boxes from Malaysia Airlines flight MH370 are presumed to lie.

Even at those 300 metre comparative shallows, all is dark. The furthest anyone has dived in a protective suit is a little over twice that. And only a handful of manned submarines have gone more than half the depth of the ocean being searched, underlining the scale of the challenge facing search teams.

Australia’s Prime Minister, Tony Abbott, said on April 11 that naval teams leading the international hunt were confident that they now knew the position of the black box flight recorder to within some kilometres.

But Angus Houston, the search co-ordinator, issued a statement saying there had been no major breakthrough. A possible signal detected on Thursday by Australian air force planes was ruled not to have been a fifth ping — diminishing hopes that the black boxes from the missing Boeing 777 were still signalling, beyond the cusp of their expected 30-day battery life. The joint agency co-ordination centre said a 46,713 square kilometre area of the Indian Ocean was still being searched for the plane, which went missing with 239 people aboard on March 8.

When transmissions from the underwater locator beacons cease, that search will become harder, but not impossible — Air France flight 447’s black boxes were found two years after signals stopped. But every ping detected allows search teams to triangulate further, rapidly narrowing the possible location.

The next likely stage, according to Prof Ian Wright, director of science and technology at the National Oceanography Centre, would be a coarse mapping of the sea floor using ships’ sonar. This would provide a base map for underwater robots. The same autonomous underwater vehicles found the AF447 wreckage at 3,900m. The ocean floor now being searched is 600m deeper still — a depth at which, in an apt analogy supplied by the US National Oceanic and Atmospheric Administration, the crushing weight of water is the equivalent of one person carrying 50 jumbo jets. This is at the limits of the Bluefin-21 robot, a 5.3 metre yellow torpedo-shaped machine which can create a high-resolution map of the seabed through sonar pulses.

While the manufacturers are confident it can work in such an extreme environment, William O’Halloran, of Bluefin Robotics, told the BBC: “To conduct surveys and searches in the subsea world, things move at about three nautical miles an hour — not very fast.” At that rate, it would take the Bluefin six years to map the search area.

Despite the greater depth, the comparatively flat Indian Ocean floor could be easier to search than the area around the mid-Atlantic ridge where AF447 was eventually found. Wright says: “That was a very young sea floor — it was very difficult to image that wreckage using sonar systems because of the rough topography and lack of sediment cover.” In the current search, the echo sounding systems should be much more effective in detecting anything unusual and providing a high resolution image. Wright says: “You would see wreckage or not. Then you would deploy an ROV.” The ROV is a remotely operated underwater vehicle that can film, scan and, crucially, pick up things from the seabed. It is tethered to a ship by an 8,000 metre cable, which not only holds the weight of the vehicle but powers it and contains fibre optics to transmit instructions and receive data.

The Remora, which can function at 6,000 metres, was used in the salvage of AF447 and other crashed planes, as well as the inspection of the Titanic. Its two hydraulic arms could either tie debris to a winch or place things in a sample basket, ideally including the flight data recorder and cockpit voice recorder.

While the robot takes care of business on the ocean floor, work remains hard on the surface. “It can run for about 24 hours. It’s a bit like being on a space mission — you’ve got a wall of TV cameras and controls. You’re talking about 50 tons of equipment and a team of six to eight people to run and maintain the ROV.” The impending southern hemisphere winter may further delay operations. The extent of the recovery will also be affected by just what damage was done on impact. While sections of fuselage have remained after some crashes — and the tailfin of AF447 was retrieved almost intact — other planes have been utterly destroyed. The investigation into Swissair flight 111, that crashed in 1998 off the coast of Nova Scotia, recovered around 2m fragments from relatively shallow waters. Decisions on recovery will be shaped by the human tragedy of MH370. The authorities are now working with the assumption that somewhere, possibly trapped within the wreckage or elsewhere in the ocean, are the remains of the 239 passengers and crew who took off for Kuala Lumpur to Beijing five weeks ago. In the wake of AF447, one of the questions that divided grieving families was whether the deceased should be left in the ocean. Just over half of the bodies were eventually brought to the surface. In the Swissair crash, most could only be identified by dental records or DNA traces.

The black boxes of a plane are made of superstrength components, designed to withstand extreme temperature, shock and the high pressure of the ocean depths — but Swissair investigators needed to recover wreckage to establish how fire had caused that crash. A cockpit voice recorder only stores the last two hours of conversation — long after the crucial manoeuvres apparently took place on MH370.

It may be that only the flight data recorder sheds any light — or indeed that an even more laborious search for clues in the depths of the ocean is needed to understand MH370’s fate. — © Guardian Newspapers Limited, 2014

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