A British navy ship with sophisticated sound-locating equipment arrived Monday in a patch of the southern Indian Ocean to determine whether underwater sounds picked up by a Chinese ship crew using a hand-held device came from the missing Malaysia Airlines black boxes.
Britain reported the HMS Echo had arrived in the new area. It will be in a race against time to determine what the noises are, because the battery-powered pingers that emit sounds from the black boxes are on the verge of dying out.
Meanwhile, the Australian navy ship Ocean Shield, which is carrying high-tech sound detectors from the US Navy, was investigating a sound it picked up in another area about 555 kilometres away. Australian authorities said once it had finished that investigation, it would head the new area to help the HMS Echo.
Searchers on Monday were anticipating good weather, with nine military planes, three civilian planes and a total of 14 ships expected to search for Flight 370, which vanished a month ago.
Hopes of finding the plane were given a boost after a Chinese ship picked up an electronic pulsing signal on Friday and again Saturday. The Ocean Shield detected a third signal in the different area Sunday, the head of the multinational search said.
The two black boxes contain flight data and cockpit voice recordings that could solve one of the most baffling mysteries in modern aviation: who or what caused Flight 370 to veer radically off course and vanish March 8 while travelling from Kuala Lumpur, Malaysia, to Beijing with 239 people on board.
But there were questions about whether any of the sounds were the breakthrough that searchers are desperately seeking or just another dead end in a hunt seemingly full of them, with experts expressing doubt that the equipment aboard the Chinese ship was capable of picking up signals from the black boxes.
"This is an important and encouraging lead, but one which I urge you to treat carefully," said retired Australian Air Chief Marshal Angus Houston, who is coordinating the search out of Perth, Australia.
He warned that the sounds were "fleeting, fleeting acoustic events," not the more extended transmissions that would be expected.
"We are dealing with very deep water. We are dealing with an environment where sometimes you can get false indications," Houston said. "There are lots of noises in the ocean, and sometimes the acoustic equipment can rebound, echo if you like."
But time is running out to find the voice and data recorders. The devices emit "pings" so they can be more easily found, but the batteries last only about a month.
China's official Xinhua News Agency reported Saturday that the patrol vessel Haixun 01 detected a "pulse signal" Friday at 37.5 kilohertz - the same frequency used by the airliner's black boxes.
Houston confirmed the report and said the Haixun 01 detected a signal again on Saturday within 2 kilometres of the original signal, for 90 seconds. He said China also reported seeing floating white objects in the area.
The crew of the Chinese ship reportedly picked up the signals using a sonar device called a hydrophone dangled over the side of a small boat - something experts said was technically possible but extremely unlikely. The equipment aboard the British and Australian ships is dragged slowly behind each vessel over long distances and is considered far more sophisticated.
Footage on China's state-run CCTV showed crew members poking into the water a device shaped like a large soup can attached to a pole. It was connected by cords to electronic equipment in a padded suitcase.
"If the Chinese have discovered this, they have found a new way of finding a needle in a haystack," said aviation expert Geoffrey Thomas, editor in chief of AirlineRatings.com. "Because this is amazing. And if it proves to be correct, it's an extraordinarily lucky break."
There are many clicks, buzzes and other sounds in the ocean from animals, but the 37.5 kHz pulse was selected for underwater locator beacons because there is nothing else in the sea that naturally makes that sound, said William Waldock, an expert on search and rescue who teaches accident investigation at Embry-Riddle Aeronautical University in Arizona.
A senior Malaysian government official said Sunday that investigators have determined that Flight 370 skirted Indonesian airspace as it flew to the southern Indian Ocean.
The official, who spoke on condition of anonymity because he was not authorised to talk to the media, said Indonesian authorities confirmed that the plane did not show up on their military radar. The plane could have deliberately flown around Indonesian airspace to avoid detection, or may have coincidentally travelled out of radar range, he said.
Houston, the search coordinator, said there had been a correction to satellite data that investigators have been using to calculate the plane's flight path. As a result, starting on Monday, the southern section of the current search zone will be given higher priority than the northern part.
The signals detected by the Chinese ship were in the southern section, Houston said.