Flight computers at heart of Air France crash?





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The reason for the concern is that A330 series jets have recently experienced a number of potentially disastrous computer anomalies. In one case, erratic computer commands caused an A330 to suddenly pitch nose-down until the captain could regain control, according to a report by the Australian Transport Safety Bureau [PDF]. That incident occurred in October on an A330 operated by Qantas Airways. The plane diverted to the nearest airport and landed safely, but 11 passengers and one crew member were seriously injured. A similar incident occurred on a Qantas A330 in December.

But other analysts and Airbus, which manufactures the A330, caution against such speculation, saying any parallels are superficial at best. The component responsible for the Qantas A330's erratic movements, a computer known as an Air Data Inertial Reference Unit (ADIRU), was made by a different manufacturer than the one installed in the Air France A330, says an Airbus spokesman.

But that explanation does not assuage some A330 pilots, who say they have experienced other unusual computer anomalies in their years in the A330 cockpit.

"The question isn't did the Air France plane have the same computers as Qantas, but [rather] could a computer fault bring down a modern jet?" one A330 pilot writes in an e-mail, who asked to remain anonymous because he is not authorized to speak to the press. "The answer is, yes, given the Airbus flight control system."

In the past week, speculation about what caused the crash has centered around Pitot sensors, small tubes at the front of the plane that send information about airspeed to the plane's various computers. At high altitudes, certain older Pitot sensors can ice up and produce inaccurate readings about a plane's speed. Just prior to AF447's disappearance, the jet's computers sent out a flurry of error messages that included inconsistent speed readings from the Pitot tubes. In the past week, Air France has replaced all the Pitot tubes on its Airbus 330s as a precautionary measure.

But the airline and sources within Airbus say problems with the Pitot sensors that could produce inaccurate speed readings would not, by themselves, be enough to bring down a plane.

That has led pilots and some analysts to focus on the plane's computer systems.

"The Pitot tubes are just part of an automated system, and if they're feeding wrong information to the autopilot, and the autopilot system itself has problems, you can see how this could just become a chain of cascading events," says Lee Gaillard, an aviation analyst in Saranac Lake, N.Y.

The Airbus 330 is one of the most automated planes flying today. Like many newer planes, its flight controls are operated by a "fly by wire" system, which means the flight controls are not moved by pulleys and cables but are electrically controlled. Simply put, that means that when a pilot wants to move components such as ailerons – which are on the wings and control aircraft roll along the longitudinal axis – he or she moves the controls in the cockpit, which send an electronic signal to a hydraulic mechanism near the ailerons, which then moves the ailerons in the direction indicated by the pilot. At the center of this system is a computer that directs the flow of information between the pilot and the plane's components.

Airbus has taken this system "a step further" than other manufacturers, according to the A330 pilot. That's because it also designed into the computer various "protection modes" that automatically move the plane's components if the aircraft is suddenly thrown off course by a powerful air current or some other natural event, or if a pilot simply makes a foolish error.

"It's designed to limit the ability of pilots or nature to put the plane in a stall or over-speed situation" that could jeopardize the plane, says the pilot, who has 22 years in military and commercial aviation and is certified to fly in four types of Airbuses and four types of Boeing planes. "However, the problem with these systems is that once they're activated, they are designed so the pilot cannot easily override them. So, if the computers have bad information because of an electronic anomaly or because of nature (say, the Pitot tubes are frozen and sending bad information), these built-in protections are activated immediately, and they can make it difficult for the pilot to control the plane."

In the Qantas mishap in October, the captain reported that he twice had trouble regaining control after the aircraft abruptly pitched nose-down.

"The captain reported that he applied back pressure on his sidestick to arrest the pitch-down movement. He said that initially this action seemed to have no effect, but then the aircraft responded to his control input and he commenced recovery to the assigned altitude," according to the report from the Australian Transport Safety Bureau. Less than three minutes later, the plane again suddenly pitched nose-down for no discernible reason, and the pilot again had trouble regaining control, the report found.

An Airbus spokesman says what happened to the Qantas flight has no connection to the Air France flight, because the ADIRU that was blamed for the Qantas problems and the one on the Qantas flight were made by "two totally different manufacturers with two totally different systems."

He also notes that the European Aviation Safety Agency last week sought to discourage any speculation and emphasized that Airbus planes are safe.

"I don't know what more to add to that. All Airbus aircraft and the A330s are certified by the world's air-worthiness authorities as safe," says Airbus spokesman Justin Dubon. "Our job now is to provide assistance" to the accident investigators.

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