Planes and ships hunting for the missing Malaysian jetliner zeroed in on a targeted patch of the Indian Ocean on Thursday, after a navy ship picked up underwater signals that are consistent with a plane's black box.
Thursday's search zone was the smallest yet in the monthlong search for Flight 370 — 57,923 square kilometres (22,364 square miles) of ocean — and comes a day after the Australian official in charge of the search expressed hope that crews were closing in on the "final resting place" of the vanished jet.
Angus Houston, who is co-ordinating the search off Australia's west coast, said Wednesday that equipment on the Australian vessel Ocean Shield had picked up two sounds from deep below the surface on Tuesday, and an analysis of two other sounds detected in the same general area on Saturday showed they were consistent with a plane's flight recorders, or "black boxes."
"I'm now optimistic that we will find the aircraft, or what is left of the aircraft, in the not-too-distant future," Houston said Wednesday.
No further sounds had been picked up overnight, Houston's search co-ordinationcentre said Thursday. But the Ocean Shield was continuing its hunt, slowly dragging a U.S. navy pinger locator through the ocean's depths, hoping to find the signal again and get a more specific fix on its location.
Meanwhile, 14 planes and 13 ships were looking for floating debris across the search zone, which extends from 2,280 kilometres (1,400 miles) northwest of Perth, and China's Haixun 01 was using underwater acoustic equipment to search for signals in an area several hundred miles south of the Ocean Shield. A "large number of objects" had been spotted by searchers combing the area on Wednesday, but the few that had been retrieved by search vessels were not believed to be related to the missing plane, the co-ordinationcentre said.
Search crews have already looked in the area they were crisscrossing on Thursday, but were moving in tighter patterns, now that the search zone has been narrowed to about a quarter the size it was a few days ago, Houston said.
Finding the flight data and cockpit voice recorders soon is important because their locator beacons have a battery life of about a month, and Tuesday marked one month since Flight 370 vanished en route from Kuala Lumpur, Malaysia, to Beijing with 239 people aboard.
If the batteries fail before the recorders are located, finding them in such deep water — about 4,500 metres, or 15,000 feet — would be difficult, if not impossible.
"I believe we are searching in the right area, but we need to visually identify aircraft wreckage before we can confirm with certainty that this is the final resting place of MH370," Houston said. "For the sake of the 239 families, this is absolutely imperative."
The hope expressed by Houston on Wednesday contrasted with the frustrating monthlong search for the Boeing 777, which disappeared shortly after takeoff in one of the biggest mysteries in aviation history. The plane veered off-course for an unknown reason, with officials saying that satellite data indicates it went down in the southern Indian Ocean. The black boxes could help solve that mystery.
The signals detected 1,645 kilometres (1,020 miles) northwest of Perth by the Ocean Shield are the strongest indication yet that the plane crashed and is now at the bottom of the ocean in the area where the search is now focused.
A data analysis of the signals heard Saturday determined they were distinct, man-made and pulsed consistently, Houston said.
"They believe the signals to be consistent with the specification and description of a flight data recorder," he said.
To assist the Ocean Shield, the Australian navy dropped buoys by parachute in a pattern near where the signals were last heard.
Royal Australian Navy Commodore Peter Leavy said each buoy will dangle a hydrophone listening device about 300 metres (1,000 feet) below the surface. The hope, he said, is the buoys will help better pinpoint the signals.
Houston acknowledged searchers were running out of time, noting the last two signals were weaker and briefer than the first pair heard Saturday, suggesting the batteries are failing.
"So we need to, as we say in Australia, 'make hay while the sun shines,'" Houston said.
The weakening of the signals also could indicate the device was farther away, U.S. Navy Capt. Mark Matthews said. Temperature, water pressure or the saltiness of the sea could also be factors.
Leavy said thick silt on the ocean floor also could distort the sounds and may hide wreckage from the eventual visual search.
Houston said a decision had not yet been made on how long to use the towed ping locator while knowing the beacons' batteries will likely fail soon, saying only that a decision to deploy an unmanned submarine in the search was "not far away."
When the ping locator's use is exhausted, the sub will be sent to create a sonar map of a potential debris field on the seabed. The Bluefin 21 sub takes six times longer to cover the same area as the ping locator.
Matthews said the detections indicate the beacon is within about a 20-kilometre (12-mile) radius, equal to a 1,300-square-kilometre (500-square-mile) chunk of the ocean floor — an area the size of Los Angeles.
It would take the sub about six weeks to two months to canvass an area that big, which is why the ping locator is still being used to hone in on a more precise location, Matthews said.
The audio search was narrowed to its current position after engineers predicted a flight path by analyzing signals between the plane and a satellite and investigators used radar data to determine the plane's speed and where it may have run out of fuel.
Houston noted that all four of the pings detected since Saturday were near the site of a final, partial "handshake" signal revealed earlier in the investigation.