By Tim Hepher
PARIS (Reuters) - French crash investigators are preparing to set out in detail what happened just before an Air France airliner flying from Rio de Janeiro to Paris plunged into the Atlantic in 2009, killing all 228 on board.
The latest update from France's BEA air accident authority comes days before the second anniversary of one of the world's most haunting and unexplained plane disasters, which led to a $50 million search operation to recover the black boxes.
The BEA said it would publish details of the chain of events leading up to the crash in a note to be published on Friday.
It is the first update since the recorders were hauled up from the seabed in May, but will not try to explain the cause. A more complete interim report will be issued around mid-July.
Investigators are confident of finding out what caused the Airbus A330 airliner to crash on June 1, 2009, now that the aircraft's recordings have been found intact.
But they need more time to analyze data gathered from the black boxes and have dismissed a series of news reports about either pilot or technical problems.
"The technical investigation, which involves comparing a large number of elements, is only just beginning," French Prime Minister Francois Fillon said in a letter to victims' families, released on Thursday.
He backed their protests over media speculation on the cause of the disaster, which he described as incomplete or wrong.
Airbus told airlines last week no new safety recommendations had arisen from preliminary readings of the data recorder.
The bulletin was widely read as tipping the focus of the investigation toward either crew actions or a combination of human factors, including training, and technical problems.
"Clearly if there is no major malfunction, you generally pay close attention to the human factor," a French aviation source said, asking not to be identified.
Preliminary investigations last year suggested that speed sensors supplied by France's Thales gave inconsistent readings, based on automated messages sent from the aircraft.
Investigators have worked on the theory that the sensors, known as pitot probes, became iced at high altitude in a storm.
That could have set off an unpredictable chain of events.
In the worst previous accident linked to such sensors, in 1996, pilots of a Boeing 757 flown by Dominican airline Alas Nacionales were confused by poor speed data and lost control, according to records kept by the Flight Safety Foundation.
The jet struck the sea, killing all 189 people on board.
French unions have dismissed reports of pilot error. The new note could however shed light on reports that the captain of Flight 447 was not in the cockpit when things first went wrong.
The BEA said in a preliminary report last year that the aircraft had three pilots on duty. This was normal for the long overnight flight from Rio de Janeiro to Paris, allowing pilots to rest in rotation and leave two at the controls.
The 37-year-old co-pilot had the correct license allowing him to take over when the captain was resting.
The 32-year-old third pilot, who would have acted as co-pilot when the second pilot temporarily took command, was less experienced on the A330, having received his "type rating" six months before the crash and flown the Rio-Paris route once.
Citing what it described as an early glimpse of the report, France Info radio reported that after the loss of reliable speed readings, the autopilot disengaged and calculators went offline. The resulting confusion went beyond crew training, it said.
The BEA dismissed the possibility of a leak.
"The BEA has not supplied any details of the information note, which is still being finalized," a spokeswoman said.
The Wall Street Journal reported this week that the pilots might have become distracted by a flurry of cockpit warnings.
Spanish lawyer Ricardo Martin-Chico, putting forward a different hypothesis on behalf of an unidentified number of French and Brazilian families, said the plane could have been depressurized after losing its rudder because of flying at excessive speed when the pilots were unable to know how fast it was flying.
The lawyer said the group was suing Air France and Airbus over an alleged design flaw in the plane -- the latest in a series of claims to be lodged, including several in the United States.
(Editing by Tim Pearce; additional reporting by Gerard Bon)