We don't know for sure what caused the ADIRU to start outputting the current altitude numbers but tagged as angle of attack values but this shouldn't have caused an in-flight upset. A spike in AOA from 2.1 degrees to 50.625 degrees and back in 3 seconds is not physically possible, and indeed, the flight computers are capable of detecting and ignoring such flagrantly erroneous data, even if the ADIRU's built-in-test-equipment hasn't announced an ADIRU failure. However, the code that detected anomalous changes in ADIRU data output was a simplistic filter that was vulnerable to a very specific timing of erroneous spikes. It could not handle spikes that were 1.2 seconds (an arbitrary parameter of the filter) apart, and, well, this specific ADIRU failure happened to send erroneous data with that exact timing. The flight computer thus thought that 50.625 degrees was a valid AOA measurement, and reacted accordingly -- by commanding a pitch down with the elevators to avoid the "stall" it thought it was experiencing.
The only issue that hasn't been addressed in the report (which is good reading, https://www.atsb.gov.au/media/3532398/ao2008070.pdf) is that the angle-of-attack value is not directly displayed to the pilots on their flight displays. AOA is measured by the 3 ADIRUs and is one of the most crucial measurements used by the flight computers and their control laws -- after all, whether the wing is stalled or not is a direct function of AOA! AOA value is also critical to upset / loss-of-control / stall recovery but without an AoA instrument, the pilot needs to infer it from other instrument values -- which is nontrivial and also depends on instruments that might be malfunctioning.
Hiding the most crucial air data parameter from the pilots (who are expected to take over when the computers or the air data sensors act up) is a bad design decision. Sullenberger agrees, "We have to infer angle of attack indirectly by referencing speed. That makes stall recognition and recovery that much more difficult. For more than half a century, we've had the capability to display Angle of Attack in the cockpits of most jet transports, one of the most critical parameters, yet we choose not to do it.": http://www.safetyinengineering.com/FileUploads/Situation%20a... . This was a recommendation in the AF447 report as well, " It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the aeroplane and take the actions that may be required."
Displaying AOA in the flight displays might have helped in this incident as well. Had the pilots seen indicated AOA values spiking immediately preceding the uncommanded pitch-downs, diagnosis and corrective action (which might have included disabling the offending ADIRU) would have been much easier.
The only issue that hasn't been addressed in the report (which is good reading, https://www.atsb.gov.au/media/3532398/ao2008070.pdf) is that the angle-of-attack value is not directly displayed to the pilots on their flight displays. AOA is measured by the 3 ADIRUs and is one of the most crucial measurements used by the flight computers and their control laws -- after all, whether the wing is stalled or not is a direct function of AOA! AOA value is also critical to upset / loss-of-control / stall recovery but without an AoA instrument, the pilot needs to infer it from other instrument values -- which is nontrivial and also depends on instruments that might be malfunctioning.
Hiding the most crucial air data parameter from the pilots (who are expected to take over when the computers or the air data sensors act up) is a bad design decision. Sullenberger agrees, "We have to infer angle of attack indirectly by referencing speed. That makes stall recognition and recovery that much more difficult. For more than half a century, we've had the capability to display Angle of Attack in the cockpits of most jet transports, one of the most critical parameters, yet we choose not to do it.": http://www.safetyinengineering.com/FileUploads/Situation%20a... . This was a recommendation in the AF447 report as well, " It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the aeroplane and take the actions that may be required."
Displaying AOA in the flight displays might have helped in this incident as well. Had the pilots seen indicated AOA values spiking immediately preceding the uncommanded pitch-downs, diagnosis and corrective action (which might have included disabling the offending ADIRU) would have been much easier.