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The ‘weak’ but ‘stable’ signals were are believed to have come from the plane’s black box whose batteries are three days past their 30-day expiry date.
The search zone for the missing Malaysia Airlines plane has been limited to a narrow strip of the Indian Ocean, with the authorities confident that they will find the aircraft “in a matter of days” after two new sets of underwater signals were detected.
The latest signals, believed to be from the plane’s black box locator beacon, follow two previous sets of pulses detected over the weekend.
They have all been heard within an area about 20 miles long and 10 miles wide, about 650 miles off the north-west coast of Australia, although a final search zone is yet to be demarcated.
The authorities said an Australian ship finally regained contact with the pulses on Tuesday, with one set of signals at 4.27pm, local time, lasting five minutes and 32 seconds, and another at 10.17pm, lasting seven minutes.
The latest signals were “very weak” but “very stable, distinct and clear”. It is believed that the beacon’s batteries, which are three days past their 30-day expiry date, are running out of life.
All the signals have been detected along a narrow strip of the Indian Ocean in an area known as Wharton Basin, a mostly flat underwater region that has not been mapped in 50 years.
The basin’s ocean floor has thick layers of silt, but experts said an object with a large surface area – such as the fuselage of a plane – would not be deeply buried.
Angus Houston, the search co-ordinator, said authorities will soon send an unmanned submarine to try to sight wreckage and finally confirm the location of the Boeing 777, which disappeared on March 8 with 239 passengers and crew on board.
Mr Houston, a retired air chief marshal in the Australian air force, said that he believed the plane would be found “in the not too distant future”, adding: “I’m now optimistic that we will find the aircraft, or what is left of the aircraft.
“Hopefully with lots of transmissions we’ll have a tight, small area and hopefully in a matter of days we will be able to find something on the bottom that might confirm that this is the last resting place of MH370.”
The latest signals did not feature two pulses, which were heard on Saturday and believed to be from the separate “pingers” on the black box’s flight data recorder and cockpit voice recorder. Authorities believe it is possible that one of the “pingers” has already died.
“It is important that we gather as much information to fix the possible location of the aircraft while the pingers are still transmitting,” Mr Houston said.
Prof Charitha Pattiaratchi, from the University of Western Australia, said the waters in Wharton Basin were not particularly rough but little was known about the ocean floor, which had not been charted since an international expedition in the 1960s. “It is a big basin and it is not mapped properly,” he said. “It is pretty flat. There are some mounds.”
Despite the authorities expressing concern that the silt may cause the plane to “disappear”, Prof Pattiaratchi said the wreckage would have sunk – at most – to a depth of six feet. “A pole would sink right in but if I put a doormat on the end of the pole, it would only go 10 centimetres [four inches],” he said. “Absolutely, you will see the plane.”
Authorities will continue to try to detect more signals with the towed pinger locator and further refine the search area before using an unmanned submarine, which can only cover about 60 miles a day. The four sets of signals are believed to be unrelated to those heard on Friday by a Chinese vessel; ships are staying away from the new zone to prevent disruption of any signals.
An Australian P-3 Orion aircraft has been despatched to deploy dozens of sensor buoys in the area, which include microphones placed 1,000 feet underwater to try to detect further signals.