As Battery-Powered Signal Fades, Crews Caught in Race Against Time

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Four weeks after a Malaysia jetliner headed to Beijing went missing, time is running out for the flight data recorders' pulse signaling system, as their batteries are expected to give out any day.

So-called black boxes are designed to emit sonic pings for roughly 30 days to aid searchers. Malaysia Airlines Flight 370 disappeared on March 8.

The Chinese news agency Xinhua on Saturday said a black box detector deployed by a Chinese ship, Haixun 01, heard a signal at 37.5 kilohertz, which is set aside for the black-box pinger as other underwater sounds typically aren't emitted at that frequency.

On Sunday, the Australian official coordinating the search said the Haixun 01 had detected two sets of signals and an Australian ship was investigating another "acoustic event" on Sunday.

“If we’ve found the haystack, now we’ve got to find the needle."

William Waldock, an expert in aviation accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz., said it’s possible for other sources — for example, a passing submarine — to transmit in frequencies close to 37.5 kHz. The one-second pulse from a pinger, however, is “a very distinctive pattern.”

“If it’s verified, it’s a major break in the case,” he told NBC News. “Frankly, they got lucky.”

He said the spotty nature of the transmission would make sense, because the Chinese ship was probably sailing through the edge of the reception area. Pingers are designed to be detected within a four-mile radius. Picking up the intermittent signal is expected to spark a more concentrated, more methodical search — not only by the Chinese, but by other ships from the international armada in the area.

“If it’s confirmed, this is right up there with finding Amelia Earhart."