An Air France memo to its pilots about the crash of Flight 447 says the airline is replacing flight-speed sensors in all its medium- and long-haul Airbus jets.

Air France declines to comment on the memo obtained by The Associated Press, saying it is for pilots only.

Airbus says the matter is part of the probe into the crash that killed 228 people flying from Rio de Janeiro to Paris.

The memo sent Friday says Air France has been replacing instruments known as pitot tubes and will finish in "coming weeks." It does not say when it started.

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One theory of the crash is that the tubes feeding speed sensors may have iced over, confusing plane computers and causing the plane to fly too fast or slow in rough weather.

French officials reiterated their doubts Friday about finding the remains of Air France Flight 447, a day after Brazilian officials announced that what was believed to be "chunks" of the doomed flight were really "sea trash."

France's transportation minister said no signs of the Airbus A330 airplane that vanished over the Atlantic have been found and urged "extreme prudence" about suspected debris taken from the ocean.

Dominique Bussereau said he regretted that an announcement by Brazilian teams that they had recovered plane debris turned out to be false.

The Brazilian air force announced Thursday afternoon that a helicopter plucked an airplane cargo pallet from the sea that came the Air France flight, but then said six hours later that it was not from the Airbus.

"French authorities have been saying for several days that we have to be extremely prudent," Bussereau told France's RTL radio. "Our planes and naval ships have seen nothing."

Bussereau said the search must continue and stressed that the priority was finding the flight recorders. The plane went down Sunday night with 228 people on board in the world's worst aviation disaster since 2001.

The plane's creator, Airbus, is warning airline crews to follow standard procedures if they suspect speed indicators on their crafts are faulty, suggesting that technical malfunction may have played a role in this week's Air France crash, Reuters reported.

Investigators know from the aircraft's final automated messages, which were sent over a period of three minutes, that there was an inconsistency between the different measured airspeeds shortly after the plane entered a storm zone, according to Reuters.

Such warnings from Airbus to its clients are only sent if accident investigators have established facts that they consider important enough to pass on immediately to airlines, an industry official told Reuters.

The French air accident investigation agency (BEA) has said the speed levels registered by the slew of messages from the plane showed "incoherence," Reuters reports.

France's defense minister and the Pentagon have said there were no signs that terrorism was involved. Brazil's defense minister said the possibility was never considered.

French Foreign Minister Bernard Kouchner, speaking Thursday in Rio de Janeiro where he attended a Mass honoring the crash victims, said experts had not found signs that would back up a "terrorism theory."

"But we cannot discard that for now," he told reporters. "Nothing leads us to believe that there was an explosion, but that doesn't mean there wasn't one."

"All the paths are open and we will not give priority to a single premise because that would be immoral," he added.

Investigators are looking into whether malfunctions in instruments used to determine airspeed may have led the plane to be traveling at the wrong speed when it encountered turbulence from towering thunderstorms over the Atlantic Ocean.

Two aviation industry officials told The Associated Press on Thursday that investigators were studying the possibility that an external probe that measures air pressure may have iced over. The probe feeds data used to calculate air speed and altitude to onboard computers. Another possibility is that sensors inside the aircraft reading the data malfunctioned.

If the instruments were not reporting accurate information, the jet could have been traveling too fast or too slow as it hit turbulence from violent thunderstorms, according to the officials.

Jetliners need to be flying at just the right speed when encountering violent weather, experts say — too fast and they run the risk of breaking apart. Too slow, and they could lose control.

But Gerard Feldzer, a former Air France pilot who flew A330s from 2000 to 2004, cautioned against drawing conclusions about the cause of the crash.

"We don't know whether there was depressurization. Perhaps a quarter of an hour later it was resolved and it (the cause of the crash) was something completely different."

European planemaker Airbus has sent an advisory to all operators of the A330 reminding them of how to handle the plane in conditions similar to those experienced by Flight 447, which was an Airbus A330-200 version.

Airbus spokesman Justin Dubon said the planemaker sent a reminder of A330 operating procedures to airlines late Thursday after the French agency investigating the crash said the doomed flight had faced turbulent weather and inconsistency in the speed readings by different instruments. That meant "the air speed of the aircraft was unclear," Dubon said.

In such circumstances, flight crews should maintain thrust and pitch and — if necessary — level off the plane and start troubleshooting procedures as detailed in operating manuals, Dubon said.

Meteorologists said the Air France jet entered an unusual storm with 100 mph (160 kph) updrafts that acted as a vacuum, sucking water up from the ocean. The moist air rushed up to the plane's high altitude, where it quickly froze in minus-40 degree temperatures. The updrafts also would have created dangerous turbulence.

The jetliner's computer systems ultimately failed, and the plane broke apart likely in midair as it crashed into the Atlantic on a flight from Rio de Janeiro to Paris Sunday night.

But investigators will have little to go on until they recover the plane's "black box" flight data and voice recorders, now likely on the ocean floor miles beneath the surface.

The Associated Press contributed to this report.