CONF iture
The following excerpt from the Australian interrim report has already been posted in this thread. However, I will bring your attention to the statement which I have printed in bold.
"From AO2008070_interim, page 40-41
Angle of attack data processing algorithms
There was a potential for the AOA sensors on the right side of the aircraft (AOA 2 and AOA 3) to provide different values to the AOA sensor on the left side of the aircraft (AOA 1) in some situations due to aircraft sideslip. In order to minimise the potential effect of this difference, the PRIMs used different processes for AOA compared with other parameters when determining the value to use for calculating flight control commands. More specifically, the processing of AOA data involved the following:
• As with the other parameters, the PRIMs would continuously monitor the AOA values from the three ADIRUs. AOA data was sampled about 20 times per second.
• To confirm the validity of the AOA data, the PRIMs would compare the median value from all three ADIRUs with the value from each ADIRU.
If the difference was greater than a set value for more than 1 second continuously, then the PRIM would flag the ADR part of the associated ADIRU as faulty and ignore its data for the remainder of the flight.
• To calculate a value of AOA to use for calculating flight control commands, the PRIMs would use the average value of AOA 1 and AOA 2. In other words, (AOA 1 + AOA 2)/2. This value was passed through a rate limiter to prevent rapid changes in the value of the data due to short-duration anomalies (for example, as a result of turbulence).
• If the difference between AOA 1 (or AOA 2) and the median value from all three ADIRUs was higher than a set value, the PRIMs memorised the last valid average value and used that value for a period of 1.2 seconds. After 1.2 seconds, the current average value would be used.
In summary, in contrast to other parameters, only two values of AOA were used by the PRIMs when determining flight control commands. However, several risk controls were in place to minimise the potential for data inaccuracies to affect the flight control system."
I have two points that may need clarification.
1. How big should the difference from the median AoA be in order to reject ADR1?
2. Could this be the cause of a) autopilot disconnect, (b) false interpretation as due to pitot problem if ADR1 was permanently rejected due to AoA error.
regards