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

3.3 Examination of data-spike patterns3.3.1 BackgroundAs discussed in section 1.11, the ADIRU output data for all parameters that wererecorded by the FDR and QAR contained a large number of data spikes on bothADR and IR parameters. In addition, the magnitude of the ADR spikes did notappear to be random. As noted in section 1.11.4, the AOA spikes only had threedifferent values. Some other parameters also had spikes with repeating values.Accordingly, the investigation examined the data-spike values to determine thenature of any patterns.Prior to discussing the patterns, it is important to understand the format in which theADIRU outputted its data to other systems. This format is known as the ARINC429 130 digital information transfer system. The FDR and QAR recorded data in adifferent format.3.3.2 ARINC 429 digital information transfer systemThe ARINC 429 system used 32-bit ‘words’ to represent information, with each bitcontaining a binary value (that is, a ‘0’ or a ‘1’). The word was divided into fivefields as shown in Figure 37.Figure 37: ARINC 429 data word129130A CRC is a function that takes a data stream as an input and calculates an output value. A CRCcan be used as a ‘checksum’ to detect accidental alteration of data during transmission or storage.ARINC 429 is an aviation industry standard for the transfer of digital data between avionicssystem elements. ARINC (Aeronautical Radio Inc.) sponsors aviation industry committees thatproduce standards that allow avionics interchangeability.- 113 -