PARIS - Aircraft black boxes should be adapted to emit signals for longer to make it easier to find the flight recorders after crashes at sea, experts investigating an Air France disaster said on Thursday.

France's BEA air accident investigation authority has been hampered in its attempts to establish why a June 1 flight from Rio de Janeiro to Paris crashed into the Atlantic by the fact that the black boxes have not been found.

Six months after the disaster, which killed all 228 people on board, the causes remain mysterious. A new sea search will be launched in February 2010 in the hope of finding the black boxes or at least more wreckage from the plane.

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 latest BEA report said.

It recommended that the underwater locator beacons fitted on flight recorders should emit signals for 90 days rather than the current 30, to give more time for search operations.

It also said passenger planes flying over the sea should be equipped with an additional beacon transmitting with a different frequency, in order to increase the chances of finding wreckage.

BEA experts said that problems with speed probes on the doomed Air France flight AF 447 were one factor in a string of events that led to the crash, but not the sole cause.

The report said current tests of the devices did not appear to replicate the conditions of real flights because not enough was known about the composition of cloud masses at high altitude.

In this context, the tests aimed at the validation of this equipment do not appear to be well-adapted to flights at high altitude, it said.

Only small parts of the Airbus A330 were found. However, a string of automated messages just before the crash showed there were inconsistencies in data from the speed probes.
The BEA said it was difficult to know what impact the weather conditions could have had on the probes.

The certification criteria (for probes) are not representative of the conditions that are really encountered at high altitude, for example with regard to temperatures, it said.

In addition, it appears that some elements, such as the size of the ice crystals within cloud masses, are little-known.

The BEA recommended that authorities should undertake studies to determine with greater precision the composition of high-altitude cloud masses, and modify the certification criteria on the basis of the results obtained.

(Reporting by Estelle Shirbon; editing by Andrew Roche)