Air France A330 flight AF447 from Rio de Janeiro to Paris disappeared in the Atlantic on 1 June 2009 in stormy weather, killing in the crash all those on board (228 people).
The flight recorders from AF447 have been located to within a 5km zone, a French official has said. But Gen Christian Baptiste told AFP that retrieving the boxes from the ocean floor might be impossible.
The cause remains unknown. The plane's airspeed probes had given false readings, but officials believe other factors must also have contributed.
Finding the boxes, which record flight data and cockpit conversations, should allow investigators to finally explain the mystery of why the plane came down.
It appears the location zone has been traced using images obtained during the first phase of searching for the wreckage, when the flight recorders were still emitting a signal. A French submarine, the Emeraude, picked up signals at the time that have since been re-analyzed and that researchers determined were pings from the black boxes, the official said.
New software improvements from Thales allowed by a better sounds analysis to define a more precise wreck location.
French government and military officials have urged caution, saying there is no guarantee the flight recorders will be found.
"It's like trying to find a shoe box in an area the size of Paris, at a depth of 3,000m (9,800ft) and in a terrain as rugged as the Alps," French navy spokesman Hugues du Plessis d'Argentre told AFP.
The two recorders would be in a new area located at 20 NM (S-SW) from the last known position and could be distant one from the other by 3 to 8 km.
A fresh search was launched earlier this year involving US and Norwegian ships with sonar probes and robots (2x Remus 6000 AUV from WHOI +1x Triton ROV + 1x IFM-Geomar glider), in what officials described as "one of the most complex undersea operations ever".
The area covered by the high-tech vessels ("Seabed Worker" & "Anne Candies") will now be reduced from 1,500 sq km (580 sq miles) to just 3-5 sq km (1-2 sq miles), in a remote area far off the coast of Brazil, NW of Arquipelago Sao Pedro e Sao Paulo (carta 10/int.216).
Location on the Marine GeoGarage (DHN carta 10 'Costa Nordeste da America do Sul')
Paul-Henry Nargeolet, BEA's maritime coordinator is the deep-dive explorer (previously Ifremer Nautile bathyscaph pilot) who has led several expeditions to the Titanic Wreck.
"They are currently aimed at covering the peripheral areas of the initial search zone and at clearing up remaining doubts," says the BEA.
These areas include a region adjacent to the initial zone, to the northwest of AF447's last known position, as well as a region within the initial zone which consists of rough terrain. The search will be completed around 25 May.
See Radio Nav Warning #0102/10 issued by Brazilian Navy with area interdicted to navigation
This continuation of phase 3 will be done with fewer ships and equipment.
The U.S Navy’s ROV’s and sonar, will now be unavailable because they are being mobilized for a military operation. Also, one of the AUV (belonging to Geomar), will not be available for this continuation phase.
The search will be continued with the Norwegian ship the Seabed Worker, and two AUVs.