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Search crews have failed to relocate faint pulses heard deep in the Indian Ocean, possibly from the missing Malaysian jetliner's black boxes, which will soon fall silent as their batteries die.
Angus Houston, the retired Australian air chief marshal who is heading the search far off Australia's west coast, said equipment on board the vessel Ocean Shield has picked up no trace of the signals since they were first heard late Saturday and early Sunday. The signals sparked hopes of a breakthrough in the search for Flight MH370 — which is on track to becoming the most expensive search in aviation history, with 26 countries contributing planes, ships, submarines and satellites to the effort.
Estimates compiled by Reuters show that at least $44 million has been spent on the deployment of military ships and aircraft in the Indian Ocean and South China Sea by Australia, China, the United States and Vietnam. The figure is based on defense force statistics on available hourly costs of various assets, estimates by defense analysts and costs reported by the Pentagon.
Finding the black boxes quickly is critical because their locator beacons have a battery life of only about a month — and Tuesday marks exactly one month since the plane vanished. Once the beacons blink off, locating the black boxes in such deep water will be immensely difficult, if not impossible.
"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," Houston said.
If at that point, the U.S. Navy listening equipment being towed behind the Ocean Shield has failed to pick up any signals, a submersible on the ship will be deployed to try chart any debris on the seafloor. If the sub maps out a debris field, the crew will replace its sonar system with a camera unit to photograph it.
A remote-controlled autonomous underwater vehicle, the Bluefin-21, is on board the Ocean Shield and could be sent to look for wreckage on the seafloor. But narrowing the search zone first is 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 device is still transmitting."
Houston said Monday that the Ocean Shield detected late Saturday and early Sunday two distinct, long-lasting sounds underwater that are consistent with the pings from an aircraft's black boxes — the flight data and cockpit voice recorders.
He described the find as "a most promising lead" but warned it could take days to determine whether the sounds were connected to Flight MH370.
Finding the black boxes is key to unraveling what happened to Flight MH370 because they contain flight data and cockpit voice recordings that could explain why the plane veered so far off course during its flight from Kuala Lumpur, Malaysia, to Beijing on March 8.
"Everyone's anxious about the life of the batteries on the black box flight recorders," Truss said. "Sometimes they go on for many, many weeks longer than they're mandated to operate for. We hope that'll be the case in this instance. But clearly there is an aura of urgency about the investigation."
Equipment on board the Ocean Shield detected the first signal for two hours and 20 minutes before losing it, Houston said. The ship then turned around and picked up a signal again, this time recording two distinct pinger returns for 13 minutes.
"Significantly, this would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder," Houston said.
The black boxes normally transmit at a frequency of 37.5 kilohertz, and the signals picked up by the Ocean Shield were both 33.3 kilohertz, U.S. Navy Capt. Mark Matthews said. But the manufacturer indicated the frequency of black boxes can drift in older equipment.
The frequency used by aircraft flight recorders was chosen because no other devices use it and nothing in the natural world mimics it, said William Waldock, a search-and-rescue expert who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.
"They picked that so there wouldn't be false alarms from other things in the ocean," he said.
But the signals are being detected by computer sweeps and "not so much a guy with headphones on, listening to pings," said U.S. Navy spokesman Chris Johnson. So until the signals are fully analyzed, it's too early to say what they are, he said.
"We'll hear lots of signals at different frequencies," he said. "Marine mammals. Our own ship systems. Scientific equipment, fishing equipment, things like that. And then of course there are lots of ships operating in the area that are all radiating certain signals into the ocean."
Geoff Dell, discipline leader of accident investigation at Central Queensland University in Australia, said it would be "coincidental in the extreme" for the sounds to have come from anything other than an aircraft's flight recorder.
"If they have a got a legitimate signal and it's not from one of the other vessels or something, you would have to say they are within a bull's roar," he said, using an Australian colloquialism to mean quite close. "There's still a chance that it's a spurious signal that's coming from somewhere else and they are chasing a ghost, but it certainly is encouraging that they've found something to suggest they are in the right spot."
The Ocean Shield is dragging a ping locater at a depth of 1.9 miles. It is designed to detect signals at a range of 1.12 miles, meaning it would need to be almost on top of the recorders to detect them if they were on the ocean floor, which is about 2.8 miles deep.
Houston said the signals picked up by the Ocean Shield were stronger and lasted longer than faint signals that a Chinese ship, Haixun 01, reported hearing about 345 miles south in the remote search zone.
The Haixun detected a brief pulse signal on Friday and a second signal Saturday. The British ship HMS Echo is using sophisticated sound-locating equipment to determine whether the two signals are related to Flight MH370.
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