PARIS/LONDON - The first sighting off Brazil's coast of possible wreckage from a missing Air France jet signals the start of what could be one of the most challenging operations ever mounted to retrieve the tell-tale black box.

The box, which is in fact two separate devices containing cockpit voice recordings and instrument data, offers the best chance of finding out why the Airbus jetliner vanished in an Atlantic storm en route to Paris with 228 people on board.

The devices are designed to send homing signals when they hit water, but merely locating them presents one of the most daunting recovery tasks since the exploration of the Titanic and barring good fortune, could take months, experts said.

If they are in waters as deep as some people fear, 4,000 meters (13,100 ft) or more, unmanned submersibles would be tested to their limits. Yet past disasters have led to advances in equipment which do give hope for finding out what happened.

There is a good chance that the recorder would survive but the main problem would be finding it, said Derek Clarke, joint managing director of Aberdeen-based Divex, which designs and builds military and commercial diving equipment.

If you think how long it took to find the Titanic and that the debris would be smaller, you are looking for a needle in haystack. You are very quickly looking at a large area to survey and could spend months running sonars down to a deep depth.

Black boxes have an underwater beacon called a pinger which is activated when the recorder is immersed in water. The beacon can transmit from depths down to 14,000 feet, according to the U.S. National Transportation Safety Board.

RECORD DEPTHS

Clarke spends time preparing for the unthinkable as part of an industry network on stand-by to help rescue submarines.

But the depths in this stretch of ocean far exceed the 600 meter maximum at which any navy could attempt a useful submarine rescue, a senior diving expert at Britain's Royal Navy said.

Brazil said on Tuesday its military planes had spotted wreckage 400 miles off its northern coast. [nN02494080]

Speaking beforehand, based on reports of the plane's probable location, Neil Wells, senior lecturer in oceanography and meteorology at Britain's National Oceanography Center, said the black box could be more than 4,000 metres below the surface.

There is no doubt about it; you will be pushing the limits of the technology. It is not a straightforward operation.

The oil industry has significant unmanned deep-sea capability but only operates down to 3,000 metres, Clarke said.

Such depths are well below the reach of manned craft.

A handful of deep-sea prowlers such as the U.S. Navy's Alvin, which surveyed the wreck of the Titanic at 4,000 metres below the Atlantic in 1986, could be equipped for such depths.

A U.S. Navy report based on similar disasters, released under the Freedom of Information Act late last year, found it was possible to recover aircraft wreckage including the 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 or pingers which indicate a position under water.

Both recorders were recovered from the crash of Air India Flight 182, which was blown up off the Irish coast in 1985.

They were recovered from some 2,000 metres in a search which 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, with investigators recovering the cockpit voice recorder after a three-month search from a record depth of more than 4,200 metres.

Whatever the challenges, industry experts say the stakes are too high to give up on the search. Not knowing would be totally unacceptable to Airbus and to aviation in general, said David Learmount, safety and operations editor of British-based aerospace magazine Flight International.

(Additional reporting by Jonathan Saul, Helen Massy-Beresford, editing by Janet McBride)

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