"Black box" flight recorders should be modified to emit signals for longer, experts investigating June's Air France disaster over the Atlantic have said.

负责在大西洋上调查法国航空六月空难的专家称，飞行记录器“黑匣子”应该被改进可以更长时间地发射信号。

Only a few pieces of the aircraft have been recovered

Their inquiry调查 into why the flight from Rio de Janeiro to Paris crashed has been hampered限制，阻碍 by the failure to find the jet's recorders.

Investigators³ said underwater beacons指向标，信号灯 on recorders should emit signals for 90 days rather than 30, in a report.

All 228 on board died in the accident, the worst in the airline's history.

The report, by France's BEA air accident authority, said the cause of the crash was still not known.

The reliability⁶可靠性 of speed sensors⁷ on the Airbus A330 has been questioned and BEA experts have suggested that new safety standards might be needed.

"At this stage, despite the extensive analyses carried out by the BEA on the basis of the available information, it is still not possible to understand the causes and the circumstances of the accident," the BEA report said.

Another recommendation推荐，建议 was that passenger planes flying over the sea should be equipped with an extra beacon⁴ transmitting on a different frequency, which would increase the chances of finding the wreckage⁸残骸.

The report - the second since the crash - added that problems with speed probes on the doomed⁹ flight AF 447 were one factor in a series of events that led to the crash, but not the only cause.

A string of automated¹⁰ messages just before the crash showed there were inconsistencies不一致 in data from the sensors, known as pitot tubes皮托管.

BEA said safety tests on the equipment did not replicate¹¹复制，折叠 high-altitude conditions and it called for more studies of cloud masses and icy conditions.

Families of victims have said they are disappointed by the failure to find a cause of the disaster.

"We can't really say we are satisfied, because there has been no answer. We don't know exactly what happened," said John Clemes, vice-president of the association of victims' families.

Mr Clemes, whose brother was on the flight, said they had been told by investigators that the plane fell into the sea in less than five minutes and that the pilots had battled to save it.

"Some things were reset¹² in the last minute, so they were obviously trying to desperately¹³ get the plane back under control," he said.

A new search to find the recorders - believed to be at a depth of nearly 19,000ft feet (6,000m) - is planned for February.