HONG KONG — The metronome-like pings that may signal the flight data recorders from Malaysia Airlines Flight 370 emanate from the stygian depths in the eastern Indian Ocean, 2.8 miles below the waves, where the water pressure is more than three tons per square inch. The bottom is a quarter-mile deeper than even the sandy plain near the Mid-Atlantic Ridge where the black boxes of Air France Flight 447 were found off the coast of Brazil nearly three years ago.
These are just some of the formidable obstacles that remain in finding and recovering whatever is left of the Boeing 777 that vanished a month ago with its 239 passengers and crew.
While an Australian official described the pings as the “most promising lead” in the search for the plane so far, experts described the task ahead as monumental.
There is no confirmation that the signals are coming from a submerged black box, and little time left before the boxes no longer have battery power to emit the pings. Even if the signals lead to the boxes, the search for wreckage at the bottom of the ocean could take weeks or months, and only then could the mystery of what happened to the plane begin to be unraveled.
An American remote-controlled submersible sonar device will be lowered as soon as Tuesday to scout for wreckage if a towed “pinger” locator can obtain a reliable position for their source. If sonar identifies possible wreckage, the submersible will be pulled up, refitted with a camera for close-up images and sent back down, Australian officials said.
But the estimated depth, nearly 15,000 feet, is already at the very limit of the submersible’s certified range. “We’re right on the edge of capability, and we might be limited by the capability if, for example, the aircraft ended up in deeper water,” said Angus Houston, the retired air chief marshal who used to run Australia’s armed forces and is now overseeing the search.
The towed pinger locator, essentially a powerful set of microphones towed through the depths at the end of several miles of cable, is operating in the undersea equivalent of an echo canyon. Differences in water pressure, salinity and temperature can send noises ricocheting in unpredictable ways, unlike the fairly straight lines along which sounds are usually transmitted in the air.
“That has the effect of attenuating, bending — sometimes through 90 degrees — sound waves,” said Commodore Peter Leavy of the Royal Australian Navy, who is helping to lead the search.
Despite that complexity, search officials were more optimistic on Monday than they had been in weeks. The operators of the towed locator, aboard the Ocean Shield, an Australian vessel, heard a steady pinging for 2 hours and 20 minutes during a pass on Sunday afternoon.
After turning around, the team heard pings again for an additional 13 minutes, and from two different locations, which would be consistent with the two black boxes that the missing plane was carrying. Turning around the pinger locator is a laborious process requiring several hours because the locator must be reeled in before the ship can perform a U-turn and then lower the device once more into the depths.
The refraction of undersea noises can complicate what is an otherwise straightforward process. The towed pinger locator can identify the compass bearing from which a sound is coming. As the locator is towed, the direction to an acoustic source gradually changes.
The device’s operators then calculate a series of straight lines on a map, using the location of the locator when it heard each sound and the angle at which the sound arrived. “Where they cross and make an X, if you get enough of them, then you should be pretty certain of your fix,” said Cmdr. William J. Marks, a spokesman for the United States Navy’s Seventh Fleet.
But the team on the Ocean Shield was unable to relocate the pings during an initial pass through the same area on Monday. Searchers were hoping to re-establish contact because the sonar submersible, known as a Bluefin-21, has an even shorter horizontal range than the towed pinger locator.
“Let’s say we don’t reacquire the signal, and we don’t get a good fix on where the black box is,” Commander Marks said. “It will be extraordinarily difficult to use the Bluefin in a location where we’re really not certain.”
Time is not on the searchers’ side. The batteries in the black boxes were designed to last 30 days, and 31 days have elapsed since the plane vanished. The pings are not likely to stop all at once, experts say, but fade to a whisper and then nothing.
Yet there were signs that the black box signals were still going strong at least on Sunday. The towed pinger locator initially detected a signal when it was only 1,000 feet deep, the Seventh Fleet said in a statement, adding that it was unusual for the signal from a black box on a deep ocean floor to reach such shallow waters.
One of the biggest surprises in the announcement of the pings Monday afternoon was that the towed locator seemed to have found anything at all. When the initial acoustic contacts were made on Sunday, it was only the third day that the device had been used in the search of the Indian Ocean.
Australia’s Joint Agency Coordination Center, which is organizing the search, said on Monday morning that the effort covered an area of 234,000 square kilometers, or about 90,000 square miles.
By contrast, the towed pinger locator moves at no more than 3 knots, or 3.5 miles per hour, and covers a two-mile-wide swath as it does so, for just seven square miles in an hour.
The shallowness of the initial contact by the pinger locator raised at least the possibility that the Ocean Shield’s captain had more information about the possible location of the aircraft than has been publicly disclosed.
Britain has said that it has dispatched a nuclear submarine to help in the search. The United States has a policy of not disclosing the locations of its submarines. The United States Navy does not disclose the maximum operating depth for its attack submarines and ballistic missile submarines, as opposed to deep-sea research submarines, but says on its website that they can dive to at least 800 feet.
Commander Marks declined to comment on whether the United States had provided any additional data to Australia beyond what has been publicly disclosed.
China announced on Saturday that one of its vessels, the Haixun 01, had detected pings. But China’s method for searching, in which sailors hold a marine listening device over the side of the boat, drew skepticism from oceanographers and other search experts, especially as the Chinese vessel was 375 miles away from where the Ocean Shield twice identified frequent pings.
Mr. Houston, the Australian leading the search, said that the Chinese site would also be investigated with care.
Source: http://www.nytimes.com/2014/04/08/world/asia/missing-malaysia-flight-370.html?n=Top/Reference/Times%20Topics/People/B/Bradsher,%20Keith?ref=keithbradsher&pagewanted=print