Search ships on Tuesday failed to detect any more underwater pulses after signals possibly from the missing Malaysian jet were recorded over the weekend, even as investigators were racing against time to locate the black box of the ill-fated aircraft before its beacons fall silent.
"There have been no further contacts with any transmission and we need to continue (searching) for several days right up to the point at which there's absolutely no doubt that the batteries will have expired," Air Chief Marshal (retd) Angus Houston, the head of the Joint Agency Coordination Centre which is leading the search, said.
If, till then, the US navy listening equipment being towed behind Australia's Ocean Shield fails to pick up any signals, an unmanned sub on board the ship will be deployed to scour the southern Indian Ocean seabed.
Investigators are hoping that the signals recorded by the Ocean Shield could be locater beacons from the Flight MH370 data recorders, but they are not sure yet. However, buoyed by the hope that they are closing in, the size of the search area was reduced on Tuesday, focusing on 77,580 square kilometres area in the Indian Ocean about 2,270 kilometres northwest of Perth. This is about a third of the size of the previous search zone.
Houston said the towed pinger was still trying to re-locate the signals and will do so for "several days". On deploying an underwater vessel 'Blue Fin 21', Houston said, "until we stop the pinger search we will not deploy the submersible."
He said Ocean Shield required to be allowed to continue its work trying to find another signal. "If we can get more transmissions we can get a better fix on the ocean floor which will enable a much more narrowly focused visual search for wreckage," Houston said.
Finding the black box is crucial to know what happened on March 8 before the Beijing-bound Malaysia Airlines Boeing 777 with 239 people, including five Indians, on board disappeared under mysterious circumstances.
Houston said the chances of finding something on the surface were "diminishing with time". Referring to the beacons on aircraft black boxes which had a nominal 30-day operating period, Houston said, "there is a chance that the locater beacon is about to cease transmission or has ceased transmission."
Australian Defence Minister David Johnston said that the search could take more time than expected. He said planes and ships were "flat out" trying to progress the latest "lead". "I want to confirm that we have at least several days of intense action ahead of us," he said, adding that "it's very challenging," he said.
"We deployed at least 20 sonar buoys, data buoys that indicate the flow of the water. We have a good understanding of where the debris, if there is debris will have gone to. "We are currently actively and aggressively pursuing where we think that debris field might be so as to give us further information to calculate back where the point of entry might have been," Johnston said, adding 133 missions have been completed so far.
Houston stressed that the search area also required silence and cannot be crowded with other ships dropping things into the water. "The calculations of the search area are right where we're picking up these transmissions," he said. Houston said a discussion about when to send the vessel down were yet to happen.
"I imagine we would be getting very close to that point (of sending the vessel down). 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 said.
"It is absolutely imperative to find something else. Hopefully when we put the autonomous vehicle down its capability is such that we will be able to find wreckage," Houston said.
"We might proceed for several days more and then if we don't get any further transmissions we have reasonably large search area on the bottom of the ocean to prosecute and with device we have got that would take a long long time. A very slow painstaking work," he added. "We will have a better idea whats down there and we will go down there and have a look," he said, adding that they were now waiting for another transmission.
The Joint Agency Coordination Centre on Tuesday said up to 11 military planes, three civil planes and 14 ships were part of the search operations for the plane.
he mystery of the missing plane continued to baffle aviation and security authorities who have so far failed to trace the aircraft despite deploying hi-tech radar and other gadgets.