Then Air Chief Marshall Angus Houston revealed that investigators had heard what they believed were undersea pings from the plane’s black-box recorders, detected by a little-known stingray-shaped device called the TPL-25, setting the room abuzz.
“The audible signal sounds to me just like an emergency locator,” he said, briefly breaking from his deadpan delivery with an unmistakable smile. A few days later, Australia’s prime minister declared sailors were likely within “some kilometers” of the plane wreck.
They were wrong.
For searchers and the families of those lost on the flight, few developments in the months-long hunt for MH370 were as deflating as subsequent reports that the pings instead might have come from electronics on the search vessel Ocean Shield, or an equipment glitch in the TPL-25.
For the aviation industry, meanwhile, it was an embarrassing case of history repeating itself, raising difficult questions about the usefulness of locator pings and the risks of relying too heavily on them.
The fickle ping signals, sent out by emergency beacons strapped to black boxes, had also proven a major distraction for searchers looking for Air France Flight 447, which went down in the mid-Atlantic in 2009. In that case, a belief that the pings could be easily detected in the deep ocean led to a series of mistakes that ultimately delayed recovery of the plane for two years, people involved in the search said.
The emergency beacons, about the size of a small energy drink can, are included on planes to help searchers locate black boxes after a disaster. Yet they produce a weak signal that can only travel a few miles through sea water. The signal itself can be deflected by wreckage, and batteries generating the noise only last 30 days.
The U.S. Federal Aviation Administration has ordered that new pingers need to have a 90-day battery life, but the rule doesn’t come into effect until next year. The United Nations’ International Civil Aviation Organization is also moving to make pingers have much lower frequencies so that the sounds they create can travel farther through the water.
The shipping industry, for instance, uses much-stronger pingers at a lower frequency of around 3.5kHz, said Christopher Nicholson, general manager of Deep Sea Systems, a Massachusetts-based ocean-exploration business. That allows them to send out signals that can travel up to 50 miles through the water, making it much simpler to locate a wreck, he said.
In the case of Air France Flight 447, investigators used a towed pinger locator to search for beacon signals near the plane’s last communicated location. When no signals were detected, investigators ruled out the possibility that the plane would be found nearby and began widening their search to include other areas.
They consulted drift-modeling experts to calculate how far the currents might have carried surface-borne debris from the original crash. A mathematical modeling group called Metron Inc. was hired to derive areas of high probability for the search from the drift-modeling data.
Woods Hole Oceanographic Institution then helped lead a massive undersea search of a 2,400-square-mile area which experts said was 95% likely to contain debris from the aircraft.
“We spent two months in that box, and they were 100% wrong about their 95%,” said David Gallo, director of special projects at Woods Hole who carried out the search with three Remus 6000 autonomous underwater vehicles. It was “incredibly disappointing,” he recalled later.
Mr. Gallo and another member of the committee charged with finding the plane, David Mearns of Blue Water Recoveries, a private firm, grew increasingly skeptical of the modeling and began to wonder if the plane might in fact be located close to where they had searched for beacon signals in the beginning. Both men said the failure to detect any pings should never have been considered conclusive proof the plane wasn’t there.
Ultimately, the plane was found by Mr. Gallo within a few miles of its last known location, near where they had earlier failed to detect beacon pings—but not until 2011.
Colleen Keller, a senior analyst at Metron, didn’t deny there was tension in the search committee. But she said Metron ultimately reached the same conclusion through its own research as Messrs. Gallo and Mearns—that the plane wreckage may have been missed by initial sweeps of a towed pinger locator.
“It is true that the find was just miles away from the last known point—of course that’s a place to search, but it’s easy to say in hindsight,” she said.
The president of France’s air-crash agency, Jean-Paul Troadec, also cited Metron’s analysis as a critical factor in the discovery of Flight 447.
The head of the Australian Transport Safety Authority, Martin Dolan, which is managing the current search for Flight 370, said the agency hopes to do further analysis of the underwater signals it picked up four times on April 5 and 8, once they have restarted the undersea search through the current tender process.
The signals were analyzed extensively by a military agency in the Australian coastal town of Nowra known as the Australian Joint Acoustic Analysis Centre. It found that the strongest of the detections, at 33.3kHz and with a regular pulse about 1 second apart, was close enough to the 37.5kHz expected from black-box locator beacons to be considered a likely match.
Different salinity and temperatures bands with varying acoustic conductivity could have warped the noises as they traveled miles to the surface from the ocean floor, explaining away the difference in frequency, experts said.
Yet an exhaustive six-week search of the ocean floor around the four pings with the Bluefin-21 AUV turned up nothing.
It’s possible no one will ever know what the sounds were.
By Daniel Stacey of WSJ
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