In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

AOA computation logicThe FCPC software did not use the median as the AOA value for computing flightcontrol orders (AOA FCPC input ) because of the physical location of the AOA sensors.The AOA 1 sensor was located on the left side of the fuselage and the AOA 2 andAOA 3 sensors were located close together on the right side of the fuselage(section 1.6.4). As a result, there was a potential for the AOA 2 and AOA 3 sensorsto provide values that were significantly different to the AOA 1 sensor in somesituations such as aircraft sideslip. 79 If both AOA 2 and AOA 3 varied from thecorrect value in a consistent manner, then AOA 1 would be rejected even if it wasclosest to the correct value.In order to minimise these effects, the FCPCs used the average value of AOA 1 andAOA 2 (AOA average ) to calculate AOA FCPC input . In addition to the monitoring logicdiscussed in section 2.1.4, the FCPCs used other mechanisms to preventdiscrepancies in either AOA 1 or AOA 2 from influencing AOA FCPC input as follows(see also the lower portion of Figure 28):• AOA FCPC input was rate limited to ensure that any rapid changes did not have asignificant effect on the FCPC’s computations.• If either AOA 1 or AOA 2 deviated from the median value (of all three AOAvalues) by more than a predetermined value (or ‘monitoring theshold’), the mostrecent valid AOA FCPC input value was memorised and used for 1.2 seconds.During this memorisation period, the current values for AOA 1 and AOA 2 werenot used for determining AOA FCPC input .At the end of a 1.2-second memorisation period, the FCPCs used the average of thecurrent AOA 1 and AOA 2 values as the AOA FCPC input for one sample. No ratelimiting was applied to this value, and there was no comparison between the threeAOA values (other than the 1-second consistency monitoring). After using this onesample, the FCPCs returned to the normal operating mode (that is, they used thecurrent AOA average value with rate limiting applied).The monitoring processes discussed in section 2.1.4 occurred at the same time asthe process to calculate AOA FCPC input . Therefore, if there was a discrepancy inAOA 1 (or AOA 2) such that it was significantly different from the median value,the FCPC started the 1-second monitoring period as well as the 1.2-secondmemorisation period. If the value of AOA 1 remained above the monitoringthreshold, the FCPC rejected ADR 1 after 1 second. Then, at the end of the1.2-second period, it used the average of the two remaining AOA values(AOA 2 and AOA 3) for subsequent computations of AOA FCPC input .The FCPC algorithm for processing AOA was unique to the A330/A340 aircraft.Section 2.5.1 provides information on the development of the algorithm.2.1.6 FCPCs’ ability to manage incorrect AOA dataThe FCPC algorithm for processing AOA data was able to detect and managealmost all situations involving incorrect or inconsistent AOA data being sent fromthe ADIRUs as ‘valid’ data. Some typical examples are presented in Figure 29, anddescribed further below.79Sideslip is a condition in which an aircraft’s flightpath is displaced right or left from thelongitudinal axis.- 73 -