The data recorders emit a "ping" that can be detected by special equipment towed by a ship in the immediate vicinity. But the battery-powered recorders stop transmitting the pings about 30 days after a crash. Locating the data recorders and wreckage after that is possible, but it becomes an even more daunting task.
Malaysia has been criticized by the relatives of some Chinese passengers on board, who accuse it of not providing enough information or even lying about what it knows about the final movements of the plane. In the early days of the crisis, the Chinese government itself expressed irritation at the speed of the probe and the lack of information.
On Wednesday, China's ambassador to Malaysia sought to distance the government from the more strident criticism, perhaps concerned about any lasting damage to ties between Beijing and Kuala Lumpur.
"I wish to responsibly point out that these extreme and even somewhat irresponsible views are not representative of the overall group of Chinese relatives and even more so not representative of the Chinese government's attitude," Huang Huikang told reporters.
Scores of relatives are staying in hotels in Beijing and Kuala Lumpur, courtesy of Malaysia Airlines.
Authorities organized a closed-door briefing in Malaysia for the families with officials and experts involved in the hunt, including the chief of the Malaysian air force.
It was relayed by video conferencing technologies to the relatives in Beijing. Malaysia's civil aviation chief, Azharuddin Abdul Rahman, said officials answered all the questions raised by the relatives and that they had "a very good meeting." Several relatives interviewed after the session said officials showed them more satellite and other data, but that they were still not satisfied.
"The fact is they didn't give us any convincing information," said Steve Wang, a representative of some of the Chinese families in Beijing. "They said themselves that there are many different possibilities, but they are judging on the basis of just one of them."
Malaysian officials have on occasion given conflicting accounts and contradictory information over the last three weeks. They maintain they are doing their best in what it is an unprecedented situation, and stress they want the same thing as the families, namely to locate the plane as quickly as possible.
Perry reported from Perth, Australia. Associated Press writers Gillian Wong in Kuala Lumpur, Rod McGuirk in Canberra, Australia, and Kristen Gelineau in Sydney contributed to this report.
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