The black boxes aboard the Air France jet that vanished over the Atlantic can survive as deep as 6000 metres underwater and emit a signal for one month, according to French air disaster investigators.

But none has ever been found this deep in the ocean, Martine Del Bono, spokesperson for the Investigations and Analysis Bureau (BEA), told AFP.

"At such depths, it becomes doubtful," Del Bono said.

Spotter planes searching for the missing Airbus A330 are scouring an area where the deepest section is around 4700 metres, said Pierre-Yves Dupuy of the French Navy's Oceanographic and Hydrographic Service (SHOM).

The ocean's depth, temperature, salt content and currents can all have an effect on how the beacon travels through the water, Dupuy said.

"If a signal is captured, you must take all of that into account before deploying submarines," he said.

The BEA has asked SHOM to examine the ocean bottom in the area where the Air France flight is being searched. The plane disappeared Monday while flying from Rio de Janeiro to Paris with 228 people on board.

In January 2004, the black boxes of an Egyptian charter flight that crashed off the coast of Sharm el-Sheikh were found after a two-week search, but just 1022 metres below water.

Actually orange in colour, an airplane's two black boxes record valuable information for investigators.

The Flight Data Recorder contains data about the speed, altitude and direction of an airliner, while the Cockpit Voice Recorder keeps track of cockpit conversations.


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