Another signal has been detected in the search area for missing flight MH370, Australian authorities coordinating the operation say.
Former Defence chief, Angus Houston, who is heading the search, said the sound was picked up this afternoon by an Australian Air Force AP-3C Orion plane somewhere near the Australian Defence vessel Ocean Shield.
“The acoustic data will require further analysis overnight but shows potential of being from a man-made source,” retired Air Chief Marshal Houston said in a statement.
“I will provide a further update if, and when, further information becomes available.”
Earlier, it was announced that the surface search area for the missing Malaysia Airlines plane has been dramatically reduced, cut by more than half, signalling that authorities may be closer to dropping an underwater robot to the seafloor to visually search for wreckage.
Four signals detected by Australian Defence Vessel Ocean Shield's US towed pinger locator since Saturday has seen the search focus honed to about 58,000 square kilometres, from more than 200,000 square kilometres only days ago.
Still an enormous surface area, Thursday's target zone is slightly smaller than the size of Tasmania.
The US black box pinger locator detected a signal consistent with black box frequencies twice on Saturday; and then again it twice detected signals in the same search area, an area consistent with experts analysis of the last known "handshake" of MH370, on Tuesday.
Acoustic experts at the Australian Joint Acoustic Analysis Centre, based at HMAS Albatross in NSW, have since concluded the signals detected on Saturday were likely those of an aircraft flight data recorder; which may have been one of the two black boxes aboard the MH370.
But verification the events are linked to the missing plane would require visual confirmation through an exploration of the seafloor, which Australian authorities are reluctant to begin until they are certain the batteries on the black boxes have expired.
That could be any day now.
It's a race against the clock and the Ocean Shield can detect signals from the sea surface at six times the rate an autonomous underwater vehicle can from the seafloor, the head of Australia's Joint Agency Coordination Centre has said.
Black box transponder batteries have a manufacturer's minimum battery life of 30 days, but those aboard the MH370 were about six years old and due to be replaced when the plane vanished without a trace on March 8 — 34 days ago.
Ocean Shield and the pinger locator were again scouring an area to the northern extremity of an area about 2200 km north west of Perth on Thursday.
As many as 15 aircraft and 13 ships were scheduled to assist in today's search for the missing plane.
Search crews from the air and sea reported a large number of object sightings during Wednesday's search, but only a small number of those items were able to be recovered by ships for examination.
"None of the recovered items were believed to be associated with MH370," JACC said in a statement on Thursday.
Visibility for Thursday's search is expected to be "fair" at about 5000 metres due to moderate south easterly winds and isolated showers.
Speaking from China on Thursday Prime Minister Tony Abbott described the Australian-Asian collaboration in the MH370 search as a "powerful [antidote] to the strategic pessimism that sometimes clouds discussions of our region's future."
"The countries of our region can achieve so much more together than apart," he said.
China, Japan, Korea, Malaysian and New Zealand have are among many nations who have contributed resources and information to the month-long search off the West Australian coast.
On Wednesday, retired Air Chief Marshal Angus Houston said the deployment of a Bluefin-21 autonomous underwater vehicle would be delayed until searchers were confident the transponders' batteries had died and there was no chance of further the limiting the search zone through ping detections.
Malaysian Prime Minister Najib Razak has said after being briefed by Australian authorities he was more optimistic than ever that the signals detected by the Ocean Shield were coming from the MH370's black box transponders, Sky News reported.
But the ocean surface itself may throw up its own challenges, according to authorities and University of Western Australia Oceanographer Professor Charitha Pattiaratchi.
"Its an area that is deep, its very cold and dark," Pattiaratchi said. "Imagine going inside a refridgerator and closing the door, except its 450 times the pressure."
The search area falls within a range of sea floor known as the "abyssal plain", which is very flat, not well explored, and covered in "biological material," professor Pattiaratchi said.
Houston has previously said the black boxes could be as deep as 4.5 kilometres below the surface, which is at the extreme end of the Bluefin-21's capabilities. A thick layer of silt on the ocean floor also had the capacity to reduce visibility and mask black box beacon signals, he said.
If the area is found to be the final resting place of MH370, Pattiaratchi said the pieces of plane may have maintained their shape, but other debris could have been dramatically shrunken by the extreme pressure.
A polystyrene cup would likely have been shrunken to the size of a thimble, he said.
- FFX Aus