"Black boxes," as these devices are known, are the key part of accident investigations, unlocking vital insights into a doomed aircraft's final moments.

They are equipped with a beacon, which sends out an underwater signal to help their location, powered by a battery designed to last for 30 days.

Recovering these recorders from great depths has been done before.

The record is the retrieval of a cockpit voice recorder (CVR) at 4500 metres from a South African Airways jumbo that crashed in the Indian Ocean in 1987, killing all 159 onboard, after a fire broke out.

Recovery teams were able to locate the CVR weeks after the pingers' batteries had expired. But they also had a touch of luck on their side.

Firstly, there was a pretty good idea as to where the 747 had gone down — and this enabled sidescan sonar to highlight a debris trail that led to clusters of wreckage, which lay on a seabed that helpfully was flat and featureless.

The CVR was eventually hauled up from one of these clusters, but the flight data recorder, which registers the plane's speed, altitude and so on, could not be retrieved.

"Deep and mountainous" submarine terrain

Specialists are unwilling to take bets, though, in the outcome of the search for the two recorders of the Air France A330 Rio-Paris flight, which went down in the mid-Atlantic on Monday with the loss of 228 lives.

The director of the French air investigation agency, Paul-Louis Arslanian, said on Wednesday the site was characterised by "deep and mountainous" submarine terrain.

"We can't rule out not finding the recorders," he told a news conference in Paris. Even if the devices were found there is no guarantee the data would explain the mystery, he added.

Pierre Cochonat of the French marine research institute Ifremer said the first task will be to narrow down the search area so that submersibles can be usefully deployed.

"This is the step which determines everything else," he said.

"If you can reduce the search to a radius of a few (nautical) miles, there is a chance," agreed Frederic Gauch, head of underwater systems at Comex, a French company that specialises in deep-sea search.

"If the zone is bigger, it's equivalent to looking for a needle in a haystack."

Surface debris from the Airbus has been spotted in water whose depths are between 3000 and 5000 metres. According to Ifremer, the recorders' "pinger" can be detected at a distance of 1500 metres.

To help narrow the gap, searchers can use a "towed pinger locator," essentially a microphone hauled at a depth of 2000 to 3000 metres. But this is a very slow business, because travelling at speed causes the array to pop up to the surface.

Once a signal or telltale trail of debris is spotted, the next step in the operation belongs to a specialised Ifremer ship, called the Pourquoi Pas, which is expected to be in the area by early next week.

'Titanic sub' to be used

The Pourquoi Pas has a robot submersible called Victor, equipped with a camera, and a three-person mini-submarine, the Nautile, that has a 12-hour range.

The Nautile — once used on a mission to the Titanic — has powerful lights, sonar and robot arms that can pick up nearby objects and place them in a basket to haul them to the surface.

But all this technology could be of little use if the recorders are entangled in the smashed hull of the aircraft, or if the wreckage is in one of the ravines that pit the area.

"It is very disturbed topography, very chaotic," said Cochonat. "You're close to the mid-Atlantic ridge, a submarine mountain range which runs from the North to the South Atlantic. It's a landscape of hollows, crags and volcanic relief.

He added though: "It's going to be tough, but that's not the question. We have to do everything we can can to find the black boxes."

AFP