Cockpit voice recordings from an Air Algerie jet that crashed last month in northern Mali are unintelligible, investigators said on Thursday, depriving them of vital clues on what sent it into a sudden plunge that killed all 116 passengers and crew.

The McDonnell Douglas MD-83 aircraft, en route to Algiers, smashed into the ground on July 24 south of the Malian town of Gossi, near the border with Burkina Faso.

Experts in Paris have been examining the two "black boxes" retrieved from the wreckage. They have been unable to extract information from one, Remi Jouty, president of France's BEA air accident investigator, told a news conference.

The voice recorder on the 18-year-old aircraft used magnetic audio tape, a system since replaced by digital methods. The tape was broken or crumpled in places. Even after it was repaired, the pilot conversations could not be understood.

"There is sound on the tape but it is unintelligible," said Jouty, whose agency has been asked to support Mali's own investigation.

"The device seemed to be recording, but we don't yet know why it did not work, except that this was not a result of the crash itself," he told reporters, adding that first indications were that it was a "simple technical problem".

French officials have said they believe bad weather was an important factor in the crash of flight AH5017 but have not ruled out other explanations.

"We're trying to avoid overly hasty theories," Jouty said, adding all hypotheses were still on the table.

The pilots had asked for permission to alter their route because of a storm as they flew north after taking off from the capital of Burkina Faso, Ouagadougou.

The jet made a detour to steer around the storm cell, but as it did so it gradually lost height and speed, according to data from the other black box presented at the BEA's headquarters.

After largely resuming its original course, the aircraft abruptly turned back on itself to the left and entered a corkscrew-shaped descent.

It hit the ground at high speed and the impact was "extremely violent", Jouty said.

The strong concentration of debris in one spot on the ground leads investigators to believe that the plane crashed rather than disintegrating in the air.

SPEED REDUCTION

Jouty said it was too early to explain the unusual pattern, but experts said attention was likely to focus on what happened in the crucial minutes as the MD-83 steered round the high-altitude storm.

Just two minutes before the detour, the aircraft had reached its cruise height of 31,000 feet and cruise speed of 290 knots. By the time it tipped leftwards into a sudden and spiralling descent, its speed had fallen to 160 knots, the BEA said.

"It seems for some reason the aircraft became too slow and may have stalled, after which control was lost," said Hugh Dibley, a council member of the Royal Aeronautical Society, who took part in industry work on "upset prevention and recovery".

Training to avoid loss of control and help pilots deal with unexpected events, such as exceptional winds, has been one of the aviation industry's top safety priorities in recent years.

Between 2001 and 2011, accidents resulting from a loss of control in flight were the leading cause of deaths in commercial aviation, according to a recent set of guidelines published by the International Civil Aviation Organization.

"Training and knowledge of the reduced performance of aircraft at high altitude are the most important ways of preventing such accidents," Dibley said.

Without the voice recordings, however, experts face a complex task in establishing what happened in the cockpit, including whether the pilots had noticed a technical fault.

Jouty said investigators would continue to try to extract useful recordings from the tape and meanwhile build up an alternative picture of what the pilots may have been facing by studying air traffic control recordings and other data.

Investigators are also trying to model the jet's performance with the help of U.S. planemaker Boeing, which bought the manufacturer McDonnell Douglas in 1997, as well as the U.S. National Transportation Safety Board, Jouty said.

"Boeing is providing technical assistance to the (NTSB) which is assisting authorities who are investigating the accident," a Boeing spokesman said.

A first report into the crash will be published in mid-September, said the head of the Malian investigating committee. - Reuters