Brazil's air force discovered passenger remains from Air France Flight 447 Saturday. As they continue the search, Clive Irving explains why the black box might be a moot point—and why Airbus is worried, as French investigators uncover a shocking 24 error messages sent from airplane’s onboard monitoring system.

French investigators looking at the disappearance of Air France Flight 447 have seen something that they do not like. On Saturday, France’s chief crash investigator, Paul-Louis Arslanian, revealed that there were 24 error messages sent from airplane’s onboard monitoring system. And, tellingly, he said that the airplane had had a problem calculating its speed. “We have seen a certain number of these faults on the A330,” he admitted.

On Friday Airbus sent out an urgent notice to airlines operating not just A330s but any Airbus model “reminding” them how to handle an airplane if the instruments recording air speed seem to be in trouble. This directive could only have come as a result of what has been revealed in data received from the doomed airliner by its Aircraft Condition Monitoring System, or ACMS.

ACMS has never before been depended on for clues to a crash. But in the absence of the airplane’s black box, the data is significant but frustratingly incomplete.

What could have happened to Flight 447 to make this specific problem of gauging air speed so critical?

Autopilots don’t know when they are getting bum information.

Indications are that the A330’s nerve ends that measure air speed—two independent “collectors” from the slipstream passing over the airplane—were confused. They thought the A330 was flying faster than it actually was. That, in turn, would cause the automatic pilot, then in control, to slow down until it believed it was reaching the normal speed, of around 500 miles per hour. Consequently, if the airplane is slowing down when, in fact, it does not need to, it can fall to a speed at which it becomes unstable and even stalls. That means that although the instruments are telling the autopilot that it’s flying at 500mph, it could be several hundred miles an hour less than that. At which point it becomes very hard to fly. In the storm there were likely to be vortexes as well as powerful updrafts and downdrafts that could seriously disrupt the airplane’s monitoring of its speed.

Becoming unstable in the kind of violent weather that Flight 447 was encountering over the southern Atlantic would be a recipe for disaster.

Saturday’s statement did not make any clearer at what stage the pilots seized control from the autopilot—or even if they ever did. Chief inspector Arslanian said it was impossible to tell from the ACMS data whether the autopilot had been disengaged or was not working. Timing is crucial. He was talking about what could be divined from the final burst of data. By then, the A330 was suffering a series of terminal failures. In effect, the airplane was becoming brain dead. The pilots should have taken control much earlier than that.

Autopilots don’t know when they are getting bum information. Unlike pilots, they have no ears, eyes, or instincts. A very relevant example occurred in thick clouds over Amsterdam on February 25. A Turkish Airlines Boeing 737 stalled and fell out of the sky. Its altimeters—measures of its altitude—were giving false readings. The autopilot thought the airplane was on final approach. In fact, it was still at 2,000 feet. The two pilots were chatting to a third in a jump seat. They failed to hear the engines power down to idle, they failed to see that they were stalling. The three in the cockpit died, along with six passengers.

Now that the French authorities seem pretty sure that the autopilot on Flight 447 was getting false information on air speed, the acuity of the pilots remains a huge question.

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Clive Irving is senior consulting editor at Condé Nast Traveler, specializing in aviation.