With Brazil and France disagreeing about whether pieces of the jet have even been found in the Atlantic, investigators are using the last messages sent by the plane to determine the cause and try to avoid future disasters.
Air France declined to comment on the memo obtained by The Associated Press, saying it was for pilots only.
Airbus said the matter was part of the investigation into the crash that killed 228 people flying from Rio de Janeiro to Paris Sunday. The Bureau of Investigation and Analysis, which is leading the French probe of the crash, said it would address all questions at a Saturday news conference.
The memo sent Friday said Air France has been replacing instruments known as Pitot tubes and will finish in "coming weeks." It does not say when the replacement process started.
The plane's "black boxes" may be miles below the surface and investigators are looking for clues in the messages sent from the plane's computers just before it disappeared. One theory: the outside probes that feed speed sensors may have iced over, giving incorrect information to the plane's computers. The autopilot may have then directed the plane to fly too fast or too slow when it met turbulence from towering thunderstorms.
Airbus sent an advisory to airlines late Thursday reminding them how to handle the A330 in similar conditions.
The memo sent by Air France Friday says that a series of actions to reduce the risks of loss of airspeed information are being reinforced by "notably, the improvement of pitot models on Airbus' fleet of medium- and long-haul flights."
"On this topic, a program of replacing pitots with new models is under way," the memo reads. "It should be completed in coming weeks."
Pitot tubes are L-shaped metal tubes - about eight inches (20 centimeters) long on their longer side - that protrude from the wing or fuselage of a plane. The pressure of the air entering the tube lets sensors measure the speed and angle of the flight, along with less vital information like outside air temperature.
They are heated to prevent icing.
A blocked or malfunctioning Pitot tube could cause an airspeed sensor to work incorrectly and cause the computer controlling the plane to accelerate or decelerate in a potentially dangerous fashion.
Airbus said the French agency investigating the crash found that the doomed flight had faced turbulent weather and inconsistency in the speed readings by different instruments.
That meant "the measured air speed of the aircraft was unclear," Justin Dubon said.
In such circumstances, flight crews should maintain thrust and pitch and - if necessary - level off the plane and start troubleshooting, 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 likely broke apart in midair.
Brazilian officials have insisted for three days that military pilots have spotted wreckage from Flight 447 scattered across the ocean's surface. Air Force Brig. Gen. Ramon Cardoso again expressed confidence Friday that at least some of the objects - an airplane seat, a slick of kerosene and other pieces - are from the plane that vanished Sunday with 228 people on board.
"This is the material that we've seen that really was part of the plane," Cardoso said.
But ships guided by planes in the search area have been hampered by extremely poor visibility, and have recovered no wreckage. "We don't have any information yet that any of the ships are near any of the objects," Cardoso said.
The only piece retrieved so far, a cargo pallet, turned out to be sea garbage. Like other suspicious objects, it had to be hauled up and checked out, said Brazilian Navy Adm. Edson Lawrence.
French officials stopped short of criticizing their Brazilian counterparts, but France's Transportation Minister Dominique Bussereau said his own country's searchers have found no signs of the Airbus A330.
"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."
A French Defense Ministry official, speaking only on condition of anonymity because he was not authorized to speak publicly on the matter, also questioned the Brazilian claims, saying French teams "cannot precisely confirm the zone where the plane went down."
Cardoso also said a large oil slick spotted by search plane pilots was not from the Airbus, but that authorities believe another slick of kerosene was probably from the downed passenger jet.
France is sending a submarine to the zone where the plane is thought to have gone down to detect signals from the two black boxes, said military spokesman Christophe Prazuck. The Emeraude, already in the Atlantic, will arrive next week, he said. The vessel, which can dive up to 980 feet (300 meters) is to be used to help try to detect the signals noises from the boxes - believed to be up to 13,100 feet (4,000 meters) underwater. It will try to capture the acoustic signals, which can last 30 days, Prazuck said.
The Pentagon has said there are no signs terrorism was involved. Brazil's defense minister said the possibility was never considered. French Foreign Minister Bernard Kouchner agreed that there is no evidence supporting a "terrorism theory," but said "we cannot discard that for now."
Brazil's Air Force was flying relatives of victims to the search command post in the northeastern city of Recife Friday to tour the operation and ask questions. Recife has a large air force base where debris and any human remains would be brought.
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Keller reported from Paris. Alan Clendenning contributed from Sao Paulo. Emma Vandore, Laurence Joan-Grange, Jamey Keaten and Philippe Sotto in Paris, Daniel Woolls in Madrid and Slobodan Lekic in Modena, Italy also contributed to this report.
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