A copy of a post by Veruka Salt over on D & G.
Our fleet management has just published a memo to Airbus crew, containing an extract from the Airbus Operator Information Telex.
It's a detailed account of "what" actually happened, if not "why".
'As the incident is subject to a formal ICAO Annex 13 investigation led by the Australian Transport Safety Bureau (ATSB), the updated data about the incident included in this OIT have been approved for release by the ATSB.
The A330 aircraft was flying from Singapore to Perth. The aircraft has then been diverted to Learmonth (Australia).
The preliminary analysis of the DFDR, Post Flight Report (PFR) and BITE (Built-In Test Equipment) data allows to establish the following preliminary sequence of events:
The A/C was flying at FL 370 with Autopilot and Auto thrust system engaged without any reported or recorded anomaly, when the IRS 1 Fault has been triggered and the Autopilot automatically disconnected. From this moment, the crew flew manually the aircraft to the end of the flight except for a short duration of few seconds.
From the time the IRS 1 Fault has been triggered, the recorded parameters of the ADR part of ADIRU 1 include erroneous and temporary wrong values in a random manner. These values are spike values and not sustained values. ADIRUs 2 and 3 seemed to have operated normally.
This abnormal behaviour of the ADIRU 1 led to several consequences as follows:
* unjustified stall & overspeed warning
* loss of attitude information on Captain Primary Flight Display (PFD).
* several ECAM system warnings.
About 2 minutes after the initial IRS Fault, the ADIRU spikes generated very high, random and temporary values of the angle of attack leading to:
1/ the flight control laws commanding nose-down aircraft movements (A/C pitch attitude decreased from 2° nose-up to 8° nose-down and vertical load factor changed from 1g to -0,8g.
2/ the Flight Control Primary Computer (FCPC) "F/CTL PRIM 1 PITCH FAULT" ECAM WARNING was triggered
The crew timely response led to recover the A/C trajectory within seconds. During the recovery, the vertical load factor did not exceed 1,6g and the maximum altitude loss was 650 ft.
The DFDR data show that the ADR 1 continued to generate random spikes. A second nose-down aircraft movement was encountered later on, but with less important effects in terms of aircraft trajectory. It also led to generate the "F/CTL PRIM 2 PITCH FAULT" ECAM WARNING. This, combined with the previous "F/CTL PRIM 1 PITCH FAULT" ECAM WARNING led to switch from NORMAL to ALTERNATE law.
The BITE message of the ADIRU 1 does not include failure or maintenance message. However the PFR also includes other system failure messages which have been demonstrated as spurious but generated by the ADIRU 1.
Tests performed on the A/C following the incident did not reveal any abnormal results that would allow explaining the reason for the event.
At this stage of the investigation, the analysis of available data indicates ADIRU 1 abnormal behaviour is likely at the origin of the event.
The type of ADIRU, which is involved, is NORTHROP GRUMMAN (previously LITTON), PN 465020-0303-0316.'