June 11 (Bloomberg) -- Marc Dubois, the 58-year-old captain of the Air France flight that crashed in the Atlantic Ocean, may have been let down by an airplane part that weighs about a pound.

The plane’s three sensors, known as Pitot tubes, that measure airspeed may have malfunctioned when Dubois and two co- pilots were four hours into Flight 447, according to pilot union officials who examined the data. Air France pilots have reported mid-flight failures of one or two of the tubes before, and all three showing differing data could trigger a chain of events that break down systems meant to make air travel safer, pilots said.

“We have come so far, and accidents like these should no longer happen,” said Christophe Pesenti, an Air France pilot for 10 years who has flown as Dubois’s co-pilot in the past.

The sensors may have been damaged by ice or obstruction, causing unreliable speed readings, which may have contributed to the accident, French investigators said after reviewing data transmitted by the doomed plane in its last minutes. The plane sent 24 automated breakdown messages before crashing June 1 en route from Rio de Janeiro to Paris, with 228 people on board.

“What we know is that these pilots were confronted with serious technical problems and erroneous indications of speed in the cockpit,” said Eric Derivry, a spokesman for Air France’s biggest pilots’ union who has been a pilot there for 18 years. “Speed information is an element that’s basic to piloting an airplane. Airspeed readings are crucial for pilots to keep control of the aircraft.”

‘Near Impossible’

Pilots rely on the airspeed readings because flying too fast can damage a plane’s airframe and traveling too slowly risks losing lift in a so-called aerodynamic stall.

Pitot tubes, each weighing about 500 grams, give airplane speed by measuring air pressure, said John Hansman, director of the International Center for Air Transportation at the Massachusetts Institute of Technology in Cambridge, Massachusetts. Ice-blocked tubes can give false readings that might lead the plane’s computerized systems to misfire, he said.

Discrepancies among readings from the Airbus A330’s airspeed sensors could have triggered the shutdown of the autopilot four minutes before the last message from the jet, or pilots may have taken control when they became concerned about that data or something else, the French agency investigating the crash said.

The BEA, or Bureau d’Enquetes et d’Analyses, which is leading the inquiry, said that “no definite link” has so far been proven between the Pitot tubes and the accident.

Definitive Cause

“We don’t know,” a BEA official said by telephone. “It’s a fact there was something with the speed sensors, but we cannot establish a link.”

Following the crash, Air France and Toulouse, France-based Airbus issued reminders to pilots of procedures to follow when measurements become unreliable. Airbus Chief Executive Officer Tom Enders said in an interview today that the investigation hasn’t yet yielded a definitive cause.

“Piloting becomes very difficult, near impossible,” without reliable speed data, said Bruno Sinatti, president of Alter, Air France’s third-biggest pilots’ union, representing six percent of the airline’s pilots, on TF1 television on June 9.

Alter’s call to its members to stop flying A330s and A340s without upgrades on at least two of the three sensors prompted Air France on June 9 to say it would swap all the older Thales SA-made sensors for newer models by the end of the month.

Air France said on June 6 that it began replacing the component with a more ice-resistant version in April, 18 months after Airbus advised customers to make the switch.

‘Signs of Weakness’

“The only question we have is why, if these sensors showed signs of weakness a while ago, why the modifications that were suggested by the constructor have not been adopted by Air France?” said Sinatti.

Air France said it began changing the probes on the A320 in 2007 because “water ingress had been observed” on those models, something that didn’t show up on A330s and A340s. Today, Air France CEO Pierre Henri Gourgeon said at a press briefing that he’s “not convinced” that the sensors were the cause of the crash. He said the airline received a delivery of new sensors for its Airbus SAS A330 on May 29, just three days before the crash.

An Air Safety Report cited in French weekly Le Canard Enchaine on June 10 showed at least three of the last 10 “incidents” at Air France related to pitot sensors -- an AF279 flight from Paris to Tokyo, an AF908 flight from Paris to Antananarivo, Madagascar and another Air France flight between Cayenne and Paris.

Dubois’s Craft

“The loss of measured air speed indicators is not in itself enough to cause this type of tragedy,” said Justin Dubon, an Airbus spokesman. “At this stage, further information is vital to understanding fully what happened.”

The aircraft commanded by Dubois was an Airbus A330 whose sensors had not yet been changed. The first automated system- failure message in a string of radio alerts from the crashed jet explicitly indicated that the sensors were faulty, Alter said.

“We are trained for situations like these, every possible situation is planned for, in theory,” said Pesenti, the pilot who knew Dubois. “All the training in the world, all the experience, may end up not being enough.”

Dubois, who joined Air France in 1998, clocked 11,000 hours of flying, including 1,100 on A330s, Air France said.

“He knew the plane he was flying, he knew the route, and he could rely on weather forecasts, which give an idea of what to expect,” Derivry said. “But then he was confronted with reality and the reality a pilot finds can be very different from what he had been expecting.”

Debris, Weather

The two co-pilots, Pierre-Cedric Bonin and David Robert, had together accumulated 9,600 hours of flying. Dubois was close to retirement, Le Parisien reported June 3 on its Web site.

Bonin, 32, had taken his wife Isabelle along with him to Brazil, the newspaper said. The couple left behind two boys, aged four and eight, the French daily added. Air France declined to provide more details on the flight’s crew.

Brazilian and French authorities have recovered hundreds of pieces of debris from the Atlantic Ocean that will be used by investigators to try and piece together evidence to determine the series of events that culminated in the crash.

“An airplane accident is never the result of just one element,” said Derivry. “Even the technical dysfunction doesn’t explain the accident. The Rio-Paris route isn’t a particularly difficult one. It does pass through an inter-tropical zone, close to the equator, where weather phenomena can be pretty intense.”

Investigators may learn more about the pilots’ last minutes if they find the black box that records conversation in the cockpit. The search for that box and another that records data about the aircraft’s operation now stretches from Brazil to the coast of Africa, as a French submarine joined the hunt.

One question among pilots is why there was no distress call from the cockpit. The only thing one can say on that is that “whatever happened must have been sudden and extremely violent,” Derivry said.

To contact the reporter on this story: Albertina Torsoli in Paris at atorsoli@bloomberg.net; Laurence Frost in Paris at lfrost@bloomberg.net

Last Updated: June 11, 2009 11:36 EDT