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

3.3.4 Data-spike patterns for the 7 October 2008 flightMach versus altitudeA review of the data-spike values for Mach indicated that many of them appeared tovary with altitude. A more detailed examination identified the followingrelationship (see also Figure 41 and Figure 42):• While the aircraft was at 37,000 ft, the value of most of the Mach spikes was0.128. The Mach binary values corresponded to the altitude binary values for37,000 ft (see also Figure 39). 131• During the aircraft’s descent to Learmonth, the binary values for most of theMach spikes corresponded to the altitude value at the same time.• Overall, 88% of the ARINC 429 binary values for the Mach spikes matchedexactly with the corresponding altitude values at the corresponding time.Computed airspeed versus altitudeSome binary values for computed airspeed corresponded to the binary values foraltitude during the aircraft’s descent. However, the proportion of the binary valuesfor the computed airspeed spikes that matched altitude values (28%) was lower thanthat for Mach spikes (88%).AOA versus altitudeThere was also some evidence of matching between the corrected AOA data spikesand the altitude values. This evidence included the following (see also Figure 43and Figure 44):• The binary value of the first AOA spike of 50.625° corresponded to the binaryvalue for altitude at that time (37,000 ft). Although most of the 50.625° spikesoccurred when the aircraft was at 37,000 ft, there was one 50.625° spike afterthe aircraft had started descending.• The binary value of the first recorded AOA spike of 16.875° corresponded to thebinary altitude value at that time (12,492 ft). Subsequent 16.875° spike valuesdid not correspond to altitude values.131Strictly speaking, a Mach value of 0.128 could correspond to any altitude in the range 36,864 ft to36,991 ft.- 117 -