Malaysia Airlines MH370: Underwater search for missing plane delayed until another signal received

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Sorry, this video has expired Video: Black box signals elude searchers in hunt for MH370 (ABC News)

Plans to send a submarine to probe the deep ocean for the remains of Malaysia Airlines flight MH370 have been put on hold until another signal is received, or it is certain the batteries in the black box recorder have expired.

At the weekend Australian Navy ship Ocean Shield picked up two acoustic signals "consistent" with those emitted by a black box recorder.

What is the Bluefin-21?

An autonomous underwater vehicle (AUV) designed for deep-sea surveying.

It has a "swappable payload". It will first use sonar in the search and will be refitted with cameras if something is detected.

It's 5m long and weighs 750kg. Has an endurance of 25 hours underwater at a speed of 3 knots, with a top speed of 4 knots.

It has a depth rating of 4,500m, meaning it will be at its limit in the Indian Ocean search zone.

Bluefin Robotics says its AUV can also be used for archaeology, oceanography, mine countermeasures, and unexploded ordnance.

Searchers were on the verge of sending an unmanned submarine to look for the wreckage today, but the man in charge of the search says another signal needs to be picked up before the search progresses.

"We need another transmission to better refine the area, then we need to go down, have a look and find confirming evidence that that's where the aircraft is," retired Air Chief Marshal Angus Houston said.

With the search for the ill-fated flight in its 32nd day, the international search team is focusing on a 600-kilometre arc in the southern Indian Ocean, about 1,600km off the West Australian coast.

But the area is still too large to launch a small submersible to scan the ocean floor.

"We will not deploy the submersibles ... unless we get another transmission, in which case we will probably have a better idea of what's down there and we will go down and have a look," Air Chief Marshal Houston said.

On Monday, Air Chief Marshal Houston confirmed a pinger locator deployed by the Ocean Shield vessel had detected two sets of signals - the first lasting two hours and 20 minutes, and the second 13 minutes.

The signals were in addition to the "acoustic noise" the Ocean Shield picked up a few days ago, and the two electronic pulse signals identified by the Chinese ship Haixun 01 at the weekend, about 600 kilometres away.

Air Chief Marshal Houston said it was the "most promising lead" so far in the hunt for the plane, which went missing one month ago with 239 people on board, including six Australians.

"What we need now is more confirmation in terms of finding something visually. Some wreckage, perhaps on the ocean floor, or some wreckage on the surface," he told ABC Radio National Breakfast this morning.

But he admits the chances of finding something on the surface are "diminishing with time".

It has now been more than a month since the plane disappeared, meaning the 30-day life for the black box batteries has passed.

If the pinger locator does pick up another signal and the searchers get a better fix on the aircraft's emergency beacons, an underwater drone called the Bluefin-21 will be dropped into the ocean.

Bluefin-21 would be operating at absolute limit

Air Chief Marshal Houston warned the 4,500 metre depth of the ocean floor was the same as the Bluefin-21 submarine's operating limit.

"It can't go deeper than that, so it's quite incredible how finely balanced all of this is," he said.

"I will stress what I said yesterday - nothing happens fast when you're working at depths of 4,500m. It's a long, painstaking process, particularly when you start searching the depths of the ocean floor."

The Bluefin-21 works primarily as a sonar device in 20-hour stints.

If it detects something unusual on the ocean floor, it will be brought back to the surface and equipped with a video camera, and then sent back to film the area.

"You can't have the side sonar and the camera down there together, it's one or the other," Air Chief Marshal Houston said.

"We will continue sortie after sortie until such time as we pick up evidence that there's something unusual on the ocean floor. We would then send down the camera.

"What we're after is wreckage, a debris field as people would say."

Deep sea explorer and engineer Ron Allum said there was "a very good chance" the wreckage would be found if the pings were confirmed as coming from MH370.

However, he says the search will not be without its difficulties.

"It (the Bluefin-21) will be relying on identifying parts of the fuselage where the flight data recorder may be and then just going in there with other vehicles that are going to be remotely operated," he told ABC News 24.

"You have a remotely operated vehicle pilot who has a joystick and he will have control of that vehicle. He can move it around the wreckage site.

"He has feedback from high-definition cameras and he will have a pretty good view, but it might take a fair bit of manipulation of that vehicle to find the actual spot."

Mr Allum, who went 5,000m below the surface in a Russian submersible for James Cameron's 2002 documentary on the Bismarck, says sending a manned vessel down in the Indian Ocean is an option.

He says "there is nothing that can replace human eyes" in a search vehicle.

"Working remotely, some of the ROVs (remotely operated vehicles) now have 3D camera systems so the operator has some sort of feel of distance from an object, so he knows how far to move a manipulator out to grab something or twist or move it to get the flight data recorder," Mr Allum said.

Topics: air-and-space, accidents, disasters-and-accidents, australia, wa, malaysia, china

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