Signals from the black boxes of Air France Flight 447 are fading, weakening along with hopes of resolving what experts are calling one of history's most challenging plane crash investigations.

Emergency beacons attached to cockpit voice and data recorders are built to emit strong "pings" for 30 days after a crash before fading away, though experts said they could continue for as long as 45 days.

Wednesday marks Day 30 since the plane dropped out of the sky with 228 people on board in a remote area of the Atlantic far off Brazil's northeastern coast and from radar coverage. A burst of automated messages emitted by the plane before it fell gave rescuers only a vague location to begin their search.

"Without that starting point, the 'needle in the haystack' analogy would look like an easy assignment compared to this," said Peter Goelz, a former managing director of the US National Transportation Safety Board. "This is the most difficult accident in terms of recovery operations that I've ever seen."

Those hunting for the two black boxes said the search will continue. On Tuesday, Martine del Bono, spokeswoman for the French air accident agency leading the investigation, said it "is continuing the search" as long as there is a "reasonable" chance of locating the black boxes. She gave no final deadline.

US Air Force Col. Willie Berges, the Brazil-based commander of the American military forces supporting the effort, has said searchers are likely to keep looking for 12 to 15 days beyond the crash's 30-day mark. The Americans are operating two US Navy pinger locators that are being towed by French-contracted ships. A French nuclear submarine is scouring a search area with a radius of 50 miles (80 kilometers) in the area where the plane is thought to have crashed.

... contd.