Investigators in the Air France crash face a daunting but not impossible task in retrieving the black boxes that lie somewhere at the bottom of the Atlantic.
The French government has diverted a scientific expedition from underwater volcanoes off the Azores to the search, which could shed light on the disappearance of the Airbus A330 with 228 people on board. The vessels France is deploying include the Nautile, a manned craft with a crew of three that can go as deep as 6,000 metres (19,685 feet), and the Victor, a remote-operated vehicle that can reach the same depth. The two would operate from a mother ship, the Pourquoi Pas.
If the black boxes – two separate devices containing cockpit voice recordings and instrument data – lie as is suspected in water 4,000 metres deep, they would be well within reach of the submersibles.
"It will be difficult but doable; that is well within their operating range," said Dr Doug Connelly, a geochemist at the National Oceanography Centre in Southampton. "They are designed to do this kind of work so it is extremely feasible. But they will be difficult to deploy because of the rough weather."
The black boxes have an underwater beacon called a pinger that is activated for 30 days when the recorder is immersed in water. That is why investigators are in a race against the clock.
"There is no doubt about it; you will be pushing the limits of the technology. It is not a straightforward operation," said Neil Wells, a senior lecturer in oceanography and meteorology.
A US navy report based on similar disasters, released under freedom of information last year, found it was possible to recover aircraft wreckage including black boxes from depths of up to 6,000 metres. It cited advances since the 1980s in technology such as sonar for combing rugged sea floors, new software and acoustic beacons to indicate a position underwater.
Both recorders were recovered from the crash of Air India Flight 182, which was blown up off the Irish coast in 1985. They were found 2,000 metres down in a search that lasted more than two weeks. Two years later, South African Airways Flight 295 crashed into the Indian Ocean near Mauritius, triggering the deepest hunt for an airliner yet undertaken. Investigators recovered the cockpit voice recorder after a three-month search from a record depth of more than 4,200 metres.
The beacon from a black box can transmit from depths down to 4,300 metres, according to the US National Transportation Safety Board.