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Search area too deep for robot sub: MH370

Search crews sent the U.S. Navy's Bluefin 21 into the depths Monday
to begin scouring the seabed for the missing Malaysia Airlines Boeing
777 after failing for six days to detect any new signals believed to be
coming from its black boxes.

But the 16-hour mission was cut short when the unmanned sub, which is
programmed to hover 30 metres above the seabed, entered a patch that
was deeper than its maximum depth of 4,500 metres, the search
co-ordination centre and the U.S. Navy said.

A built-in safety feature returned the Bluefin to the surface and it was not damaged, they said.

The data collected by the sub was later analyzed and no sign of the
missing plane was found, the U.S. Navy said. Crews were shifting
the Bluefin's search area away from the deepest water and were hoping to
send it back on another mission later Tuesday.

Search authorities had known the primary search area for Flight MH370
was near the limit of the Bluefin's dive capabilities. Deeper-diving
submersibles have been evaluated, but none is yet available to help.

A safety margin would have been included in the Bluefin's program to
protect the device from harm if it went a bit deeper than its
4,500-metre limit, said Stefan Williams, a professor of marine robotics
at the University of Sydney.

"Maybe some areas where they are doing the survey are a little bit
deeper than they are expecting," he said. "They may not have very
reliable prior data for the area."

Meanwhile, officials were investigating an oil slick about 5,500
metres from the area where the last underwater sounds were detected.

Crews collected an oil sample and sent it back to Perth in western
Australia for analysis, a process that will take several days, said
Angus Houston, the head of the joint agency co-ordinating the search off
Australia's west coast.

He said it does not appear to be from any of the ships in the area,
but cautioned against jumping to conclusions about its source.

The Bluefin can create a three-dimensional sonar map of any debris on
the ocean floor. But the search is more challenging in this area
because the seabed is covered in silt that could potentially cover part
of the plane.

"What they're going to have to be looking for is contrast between
hard objects, like bits of a fuselage, and that silty bottom," Williams
said. "With the types of sonars they are using, if stuff is sitting up
on top of the silt, say a wing was there, you could likely see that …
but small items might sink down into the silt and be covered and then
it's going to be a lot more challenging."

The search moved below the surface after crews picked up a series of
underwater sounds over the past two weeks that were consistent with
signals from an aircraft's black boxes, which record flight data and
cockpit conversations. The devices emit "pings" so they can be more
easily found, but their batteries last only about a month and no sounds
have been heard for seven days.

Australian Prime Minister Tony Abbott raised hopes last week when he
said authorities were "very confident" the underwater signals were from
the black boxes on Flight MH370, which disappeared March 8 during a
flight from Kuala Lumpur, Malaysia, to Beijing with 239 people on board,
mostly Chinese.

Houston said Monday that the signals were a promising lead, but that
finding aircraft wreckage in the remote, deep patch of ocean remains
extremely difficult.

The submarine is programmed to take 24 hours to complete each
mission: two hours to dive to the bottom, 16 hours to search the
seafloor, two hours to return to the surface, and four hours to upload
the data.

The black boxes could contain the key to unravelling the mystery of
what happened to Flight MH370. Investigators believe the plane went down
in the southern Indian Ocean based on a flight path calculated from its
contacts with a satellite and an analysis of its speed and fuel
capacity. But they still don't know why.

On Tuesday, Malaysia's Defence Minister, Hishamuddin Hussein, pledged
to reveal the full contents of the black boxes if they are found.

"It's about finding out the truth," he told reporters in Kuala Lumpur. "There is no question of it not being released."

Up to 11 planes and as many ships were scouring a 62,000-square
kilometre patch of ocean about 2,200 kilometres northwest of Perth on
Tuesday, hunting for any floating debris.

The weeks-long surface search is expected to end in the next two
days. Officials haven't found a single piece of debris confirmed to be
from the plane, and Houston said the chances that any would be found
have "greatly diminished."