Search teams hunting for the Malaysia Airlines plane believed to have plunged into the ocean have about 30 days to the find the cockpit recorders before a tracking signal they emit goes dead.
That is the battery life of the two recorders. One provides the last two hours of audio from the cockpit, and the other contains all of the flight's data.
An international fleet of vessels has expanded its search for the missing airliner, which disappeared from radar at 2:40 a.m. Saturday with 239 people on board as it flew at 35,000 feet (10,668 metres) between Malaysia's east coast and the southern tip of Vietnam.
With its underwater ping audible to sonar detectors only from a distance of a few miles, and the possibility that the recorders may be hundreds of metres below the water's surface, tracking down a black box on an ocean bottom is a daunting challenge.
"It certainly depends on the location," Sarah McComb, chief of the recorders division at the National Transportation Safety Board, said Tuesday. "I don't think the range is quite five to 10 miles, but there are a lot of different factors involved. It also can be compromised if it's buried in silt or sand."
The recorders are required to function at depths of 20,000 feet (6096m) and are built to withstand temperatures up to 1000 degrees Celsius and an impact of equal to a g-force of 34.
"They are required to emit a signal for a minimum of 30 days," McComb said. "That can actually be increased depending on the temperature and depth of the water and how new the battery is."
She said that since the challenging search for flight recorders in an Air France crash in 2009, there has been an international effort to increase beacon battery life to 90 days.
Steve Marks, a Miami aviation lawyer who represented families in two instances in which an airliner plummeted from cruising altitude, said he expected the flight recorder to be recovered intact from the Malaysia Airlines flight.
"The likelihood is extremely high that most of the bodies will be recovered. A large portion of the wreckage will be recovered," Marks said. "And most certainly the flight data in the cockpit voice recorder will be recovered intact. In Air France it took many, many months — more than a year — to locate it, and the navy was able to pull it out. And after all that time, the information was pristine."
Marks has handled cases from dozens of airplane crashes, including the 2009 Air France crash over the Atlantic, which killed 228 people. That plane was cruising at altitude just before it went down.
"You're not going to get everything in the ocean," Marks said. "A lot of the flight debris won't be very meaningful. The most important part, which is the black box, which is orange, in fact, will be recovered. It will answer almost everything. It will identify what was going on with flight controls, with the engine instruments, with the altitude, air pressures, air speed, everything."
Within five days after the Air France Airbus 330 plunged into the Atlantic en route from Brazil to Paris in June 2009, search teams found the first major wreckage from the airplane — a seat, a barrel, an orange buoy and what were described as "white pieces".
Within two weeks, 50 bodies had been recovered in two groups, separated by more than 80kms. Twenty-five days after the crash, Brazilian officials ended the search after collecting 640 pieces of debris.
But no black box. It took almost two years, until May 2011, before the box was located on the ocean floor.
The hunt for that box was exhaustive.
It was begun by a French nuclear submarine five days after the plane went down. The sub, the Emeraude, worked with a mini-sub named Nautile, using sonar to listen for the black box's ping. Their crews were helped by US underwater devices that could pick up signals almost four miles below the surface.
Though the ocean depths in that part of the Atlantic ranged far deeper and the underwater terrain was quite jagged, the searchers found hope in the 1988 recovery of a black box that was 4907m deep.
The two French submarines were able to cover 33 square kilometres of the Atlantic each day. Two other French surface vessels towed "pinger locator hydrophones" borrowed from the US Navy.
By late July, it was calculated that the 30-day battery life powering the pingers had expired.
With the black boxes located in the tail section of the plane, searchers hoped they might be lodged in a large piece of debris that could be picked up by sonar. A research vessel towing sonar worked a 75-km radius from the plane's last position without success, until late August.
Oceanographers from several nations were brought in for the third phase of the search in 2010. They covered 6215 sq km of ocean and came up empty-handed.
The following year, a team from the Woods Hole Oceanographic Institution in Massachusetts, using autonomous underwater search vessels, discovered a large debris field from the flight on a relatively flat section of ocean bottom at depths between 3810 and 3992m.
On May 2, 2011, a remotely operated vehicle found the flight recorder and carried it to the surface.
By June of that year, 154 bodies of the 228 people on board had been recovered. The search ended with 74 bodies unrecovered.
- Washington Post