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

The average MTBUR for LTN-101 units on all Airbus A330/A340 aircraft wasabout 8,700 hours in 2004, increasing from the end of 2006 and reaching about14,900 hours in 2008. The MTBF was about 11,500 hours in 2004 and 19,600 hoursin 2008.The ADIRU manufacturer reported that it was only aware of in-service problemsassociated with units when those units were sent to it for examination. Operatorsdid not provide it with information on ADIRU fault messages that occurred but didnot require the unit to be removed. Another ADIRU manufacturer advised that asimilar situation existed for its units.3.9.2 Operator’s in-service ADIRU performanceThe MTBUR and MTBF for the operator’s A330 fleet during the period from 2004to 2007 was generally better than the worldwide Airbus A330/A340 fleet withLTN-101 ADIRUs fitted. During 2008, the operator’s MTBUR was about11,100 hours and the MTBF was about 17,100 hours, which was slightly below therest of the worldwide fleet.The investigation reviewed the operator’s maintenance records to identify eventsinvolving a reported NAV ADR [1, 2 or 3] FAULT and/or a NAV IR [1, 2 or 3]FAULT message from 2003 to 2008. 172 In addition to the three data-spikeoccurrences, the search identified 116 other events during the 2003 to 2008 period,which involved 312,834 aircraft flight hours. More detailed results were as follows:• Most (83) of the events involved both IR and ADR fault messages, with 21involving only an IR fault message and 12 only an ADR fault message.• Eighteen events occurred during the engine start phase and the rest in cruise.• Following 24 of the events, the ADIRU was removed for examination, and anactual (hard) fault was verified by the ADIRU manufacturer on 12 occasions. 173• For the majority of events, there were no fault messages on the flight’s postflightreport (PFR) other than those directly related to the ADIRU. None of theevents involved the same PFR pattern as the three known data-spikeoccurrences. For some of the events that involved other messages on the PFR(such as a NAV GPS FAULT or an autopilot disconnection), the QAR data wasreviewed and no data spikes or other similar anomalies were observed.• Unless the BITE data was obtained and a detailed investigation performed, itwas not possible to know the reasons for the reported fault messages on anyspecific occasion. However, the review found that no other events involvingdata spikes or effects on the flight control system were identified.The aircraft manufacturer advised that it was not practicable to compare the rate ofreported IR and ADR faults between the operator and other operators. 174 However,172173The review looked at technical log entries and the AIRMAN database (discussed in Appendix D).Technical log entries were also reviewed to identify flights involving abnormal ECAM behaviour;none were identified.In addition to IR or ADR faults, units could also be removed for other reported performanceproblems that did not result in a fault message, such as inertial drift.- 158 -