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

3.4.4 Range checkingTwenty-eight ADIRU output parameters, including altitude, AOA, groundspeed andMach, were range tested by the ADIRU’s BITE before being sent to theinput/output modules. Any values that exceeded the predefined limits resulted in theparameter’s data being set as invalid (for as long as the data exceeded the limits).The investigation examined the data for the 7 October 2008 occurrence forparameters where the range check boundaries were exceeded. The recorded data(including data spikes) for most of the parameters were within the range-checklimits. For example, the range check limits for AOA were -40 to +90°, and none ofthe AOA spikes were outside that range. However, there was relevant datainvolving computed airspeed (an ADR parameter), Mach (an ADR parameter) andgroundspeed (an IR parameter) that is summarised in Table 23.Table 23: Range check information for computed airspeed, Mach andgroundspeedParameter Relevant range check Recorded data (7 October 2008)Computedairspeed(CAS)The allowable CAS range was 0 to450 kts. If CAS was outside thisrange then it was set to 0 and theSSM was marked as invalid bysetting it to ‘failure warning’ (FW). Ifthe range check passed, then afurther check was performed to seeif CAS was less than 30 kts. If so,then it was set to 0 and the SSMwas marked as invalid by setting itto ‘no computed data’ (NCD).Mach The allowable Mach range was 0to 1. If Mach was outside this rangethen it was set to 0 and the SSMwas marked as invalid by setting itto ‘failure warning’ (FW). If therange check passed, then a furthercheck was performed to see if Machwas less than 0.1. If so, then it wasset to 0 and the SSM was markedas invalid by setting it to ‘nocomputed data’ (NCD).GroundspeedIf the calculated groundspeedexceeded 1,000 kts, then the outputvalue was set to 1,000 kts and theSSM was marked as invalid bysetting it to ‘failure warning’ (FW).Several CAS spike values wererecorded between 0 and 30 ktswithout being flagged as invalid.During landing at Learmonth, thecomputed airspeed was set to 0after the speed decreased through30 kts. This indicated that the rangecheck function was operatingcorrectly at that time.Two Mach spike values of 1.016were recorded without beingflagged as invalid.During landing at Learmonth, theMach was set to 0 after the speeddecreased below 0.1. This indicatedthat the range check function wasoperating correctly at that timeNo values of groundspeed wererecorded that were greater than1,000 kts.The underlying (and incorrect)groundspeed data showed agradual increase up to the value of1,000 kts and remained at thatvalue with intermittent spikes tolower values.Overall, the pattern of data indicated that the range-check functions were operatingnormally when the underlying data exceeded the predefined limits. However, in thecase where data spikes exceeded the limits, the range checks were not applied,indicating that the data spikes were introduced after the range checks wereconducted.- 124 -