1 of 7. A fast response craft from Australian Defence Vessel Ocean Shield tows Able Seaman Clearance Diver Michael Arnold as he searches the ocean for debris in the search zone in the southern Indian Ocean for the missing Malaysian Airlines flight MH370 in this picture released by the Australian Defence Force April 8, 2014. 
 
 SYDNEY/PERTH, Australia (Reuters) - An Australian ship which picked up possible "pings" from the black box recorders of a missing Malaysia Airlines jetliner has been unable to detect any further signals and time is running out to narrow the massive search, officials said on Tuesday. 
 
 Angus Houston, head of the Australian agency coordinating the search for Malaysia Airlines Flight MH370, said the month-long hunt in the Indian Ocean was at a critical stage given the batteries in the black box beacons had already reached the end of their 30-day expected life. 
 
 A U.S. Navy "towed pinger locator" onboard Australia's Ocean Shield picked up two signals consistent with black box locator beacons over the weekend - the first for more than two hours and the second for about 13 minutes. 
 
 Houston said the signals represented the best lead in the search yet, but efforts to pick up the pings again had so far been unsuccessful. 
 
 "If we don't get any further transmissions, we have a reasonably large search area of the bottom of the ocean to prosecute and that will take a long, long time. It's very slow, painstaking work," said Houston. 
 
 The black boxes record cockpit data and may provide answers about what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8 and flew thousands of kilometers off its Kuala Lumpur-to-Beijing route. 
 
 Authorities say evidence, including the loss of communications, suggests the plane was deliberately diverted by someone familiar with the aircraft, but have not ruled out mechanical problems. 
 
 Analysis of satellite data led investigators to conclude the Boeing 777 came down in an area some 1,680 km (1,040 miles) northwest of Perth, near where possible pings were picked up and the search is now focused. 
 
 BLUEFIN ON HOLD 
 
 An autonomous underwater vehicle (AUV) named Bluefin-21 is onboard the Ocean Shield and could be sent to look for wreckage on the sea floor, but narrowing the search zone first was critical, Houston said. 
 
 "It is a large area for a small submersible that has a very narrow field of search, and of course, it is literally crawling along the bottom of the ocean," he said. 
 
 "That's why it's so important to get another transmission and we need to continue until there's absolutely no chance the (black box) device is still transmitting." 
 
 The Bluefin will scour the ocean floor in 20-hour missions using sonar in an attempt to find the aircraft, before its findings are downloaded and analyzed on board the Ocean Shield. 
 
 If anything unusual is spotted, the sonar on board the robotic vehicle will be replaced with a camera to take a closer look. The potential search area is about 4.5 km (2.8 miles) deep, the outer reach of the Bluefin's range. 
 
 Some 133 missions have been completed so far in the multinational aerial hunt for debris in the southern Indian Ocean and would continue, officials said. 
 
 Up to 11 military planes, three civilian planes and 14 ships planned to take part in the search on Tuesday, with good weather in the search area. But so far, searchers have only turned up fishing gear and other detritus. 
 
 "Even after the black box is found, the surface search will continue because they're looking for evidence to investigate why the aircraft went down," Lieutenant Commander Adam Schantz, the officer in charge of U.S. Navy P-8 Poseidon search unit, told Reuters. 
 
 "So, we'll continue for a while trying to find anything on the surface that might give a reasoning behind the accident." 
 
 (Additional reporting by Jane Wardell in Sydney and Anuradha Raghu in Kuala Lumpur; Editing by Dean Yates and Michael Perry) ||||| Media caption Richard Westcott reports on the use of a pinger locator to find a black box 
 
 The hunt for missing Malaysia Airlines flight MH370 moved into a new phase on Friday with the deployment of a towed pinger locator to find the black box. 
 
 Two ships with locator capabilities are searching a 240km (150 mile) path in a bid to retrieve the data recorder. 
 
 But Australia's search chief said it was a race against time as the battery-powered signal fades after 30 days. 
 
 The plane disappeared on 8 March en route from Kuala Lumpur to Beijing. It was carrying 239 people. 
 
 It is believed to have crashed in the southern Indian Ocean, although no confirmed debris has been found from the plane. 
 
 The search is being co-ordinated from the city of Perth in Western Australia. 
 
 Fourteen aircraft and 11 ships were involved in Friday's search activities. 
 
 Ships sighted a number of objects in the area but none were associated with the missing plane, the coordination agency said. 
 
 'Highest probability' 
 
 Angus Houston, head of the Joint Agencies Coordination Centre (JACC) leading the search, said that two ships had "commenced the sub-surface search for emissions from [the] black box pinger". 
 
 Australia naval vessel Ocean Shield was using a towed pinger locator from the US Navy, while HMS Echo, which had similar capabilities, was also searching. 
 
 But the former military chief now coordinating the eight-nation search said it was "getting pretty close to the time when it might expire." 
 
 Beacons in the black box emit "pings" so they can be more easily found but the batteries only last for about a month. 
 
 Image copyright Reuters Image caption Officials said there was "some hope" the locators would be able to find the black box 
 
 "The two ships will search a single 240km track converging on each other," Air Chief Marshal Houston, who is retired, said. 
 
 ACM Houston said that the area had been picked on the basis of analysis of the satellite data. 
 
 MH370 - Facts at a glance 8 March: Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears 
 
 Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears Plane's transponder, which communicates with ground radar, was switched off as it left Malaysian airspace 
 
 Satellite 'pings' indicate plane was still flying seven hours after satellite contact was lost 
 
 24 March: Based on new calculations, Malaysian PM says "beyond reasonable doubt" that plane crashed in southern Indian Ocean with no survivors What we know The search for flight MH370 
 
 It was based on work regarding "how the aircraft might have performed and how it might have been flown", to choose the "area of highest probability as to where it might have entered the water". 
 
 He pointed out that this data was continuing to be refined, but the current search was based on the "best data that is available". 
 
 Given the progress in data evaluation and calculation, "there is some hope we will find the aircraft in the area we are searching", he added. 
 
 The two ships will be moving at reduced speeds, of around three knots, in their attempt to detect any signal from the pinger. 
 
 Commodore Peter Leavy, Commander of Joint Task Force 658, said that search operations generally preferred to use "physical evidence" and "drift modelling" to locate a plane. 
 
 However, "no hard evidence has been found to date so we have made the decision to search a sub-surface area on which the analysis has predicted MH370 is likely to have flown," he said. 
 
 In a statement, JACC said the focus was now on a search area of about 217,000 sq km (84,000 sq miles), 1,700 km (1,000 miles) north west of Perth. 
 
 Media caption Malaysia Airlines ex-steward Patrick Chow says the cabin crew he knew could not be responsible for the plane's disappearance 
 
 Meeting staff involved in the search on Friday, Australian Prime Minister Tony Abbott said: "It is probably the most difficult search that's ever been mounted." 
 
 "A large aircraft seems like something that would be easy enough to locate - but a large aircraft that all but disappeared and disappeared into inaccessible oceans is an extraordinary, extraordinary challenge that you're faced with." 
 
 ACM Houston said there was still a "great possibility of finding something on the surface [of the ocean]". 
 
 "There's lots of things in aircraft that float," he said, citing previous searches where life jackets from planes were found.
– Two ships have begun a race against time to try to find the black boxes of Malaysia Airlines Flight 370 before the data recorder's battery-powered pinger ceases transmission. An Australian navy ship towing a pinger locator and a British survey vessel are searching a 150-mile track where the plane could have gone down, reports Reuters. "The area of highest probability as to where the aircraft might have entered the water is the area where the underwater search will commence," search chief Angus Houston told reporters. "On best advice the locator beacon will last about a month before it ceases its transmissions so we're now getting pretty close to the time when it might expire." The Boeing 777 disappeared on March 8, so the search teams may now have just a few days to locate the signal, the BBC notes. Some 14 planes and nine ships are still searching an area of the southern Indian Ocean for floating wreckage, and Houston says there is still a strong possibility of finding debris like lifejackets. "This is a vast area, an area that’s quite remote, and we’ll continue the surface search for a good deal more time," he says. "If we find a piece of wreckage on the surface ... that gives us a much better datum to start the underwater search than we’ve currently got."