Originally Posted by A33Zab
FCPC using median AOA (or outvoted AOA#1), AoAsw using highest AOA value.
According to the ATSB interim accident report on QF72, the FCPC treats AoA differently from the other parameters (which was an important factor in the QF72 occurrence):
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.28 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
(Note 28 - Sideslip: a condition in which the oncoming airflow is at a sideways angle to the aircraft’s
centreline.)
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.