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

The information presented on the flight displays was generated by two displaymanagement computers (DMCs), based on inputs from the ADIRUs, FWS andother sources.1.6.8 Overview of avionics fault-detection processesTypes of faultsModern aircraft systems are composed of line-replaceable units (LRUs) 31 and otheritems of equipment, each of which includes many components. These units willoccasionally develop physical faults or failures. 32 The faults can be permanent,transient (appear for a short time then disappear), or intermittent (occur from timeto time). Transient and intermittent faults are often known as ‘soft’ faults.Hard faults can be reliably reproduced during operation or testing, whereas softfaults are more difficult to reproduce unless the circumstances that induced themare well known. With soft faults, the equipment behaviour will generally return tonormal after the equipment is rebooted or power is cycled.Self-detected faultsAircraft systems are designed so that the probability of an equipment fault is belowa specified level. 33 In addition, systems are designed so that they can manage theoccurrence of a fault using methods such as fault detection and equipmentredundancy.The fault-detection processes on the A330 are summarised in Figure 16. In generalterms, the aircraft’s systems were designed so that the primary means of detectingfaults was self-detection (that is, internal detection by the unit or system itself). If afault was self-detected, then it was easier to manage. For example, the system couldflag incorrect output data as invalid and other aircraft systems could then ignore it.Alternatively, some systems did not provide outputs to other systems, and could notrely on external fault detection.313233An LRU is a unit that, due to its size, weight and connections, could be easily removed during linemaintenance. It would be replaced with a serviceable unit, while the original was dispatched forrepair. The ADIRUs and FCPCs are examples of LRUs.In some situations, the term ‘fault’ is used to refer to an anomaly in the system or unit, whereas theterm ‘failure’ is used to refer to when the system or unit is unable to perform its functions (due toa malfunction or loss of function). In practice, manufacturers and operators use the termsinterchangeably, and they are used interchangeably in this report.Regulatory requirements for aircraft systems such as the EFCS are discussed in section 2.3, andthe aircraft manufacturer’s equipment specification for the ADIRUs is discussed in section 3.1.1.- 22 -