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Standard procedural bulletin on use of speed sensors may help explain unsolved crash of Rio-to-Paris flight AF447

Airbus 330 (flickr)

A seemingly innocuous memo could revive speculation about the cause of the June 2009 crash of an Air France flight from Rio-to-Paris. Les Echos has learned that on Monday, Airbus sent an operational engineering bulletin (OEB) to all airlines operating A330s and A340-200 and 300 to remind their pilots not to reset the autopilot after a malfunctioning of the pitot speed sensors.

Airbus regularly sends OEB's to its customers to remind them of an existing procedure in the manuals. Yet this bulletin raises questions because the A330's airspeed measurement probes are suspected of triggering the crash of flight AF447 off the coast of Brazil that killed all 228 aboard, and remains officially unsolved.

More importantly, this warning contains a new element, namely that two separate pitot probes could possibly send the same erroneous information to the on-board computer, which could cause dangerous maneuvers in autopilot mode.

So far, it was thought that in case of failure of two of the three probes present on an airplane, caused for example by freezing, the data sent to the flight computer would be sufficiently inconsistent that it decides to disconnect the autopilot and put the craft in the pilots' hands until the sensors are reset to operate normally. Only then will the computer offer to return to autopilot.

At this point, however, the procedure requires a final manual check by the pilots, who must ensure that indications of speed provided by the probes are consistent with other flight parameters before resetting the autopilot. The new Airbus bulletin is focused on this final check.

Studies by the manufacturer have indeed highlighted the possibility that two Pitot probes can issue data that is false, but still coherent enough to be considered valid by the flight computer. If the pilots are not wary and trigger the automatic pilot without further verification, then major problems can occur.

There is no specific evidence at this stage that this precise scenario is behind the crash of the AF447, and the probability is very low and would mean that the crew did not follow procedure. But it is not impossible.

Airbus says Monday's bulletin has no connection with the disaster. The manufacturer, however, considers the risk serious enough to warrant not only a bulletin, but also a change in automatic steering software of the 1,200 A330 and A340 aircraft in service. Perhaps the outcome of the fourth search for black boxes, slated for February, will finally help solve the mystery.

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