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would not have produced a pitch-down event had the aircraft been below 500 ft.
Relevance to other aircraft types
The manufacturer advised that the AOA processing algorithms used by A330
aircraft were also used by A340 aircraft. However, different algorithms were in use
on other Airbus types, which were reported to be more robust to AOA spikes. The
manufacturer advised that AOA spikes matching the above scenario would not have
caused a pitch-down event on Airbus aircraft other than an A330 or A340.
29 Pitch-up is an aerodynamic anomaly that can occur in aircraft with swept wings at high altitude
and at high speed.
- 31 -
Other ADIRU-related occurrences
ADIRU reliability
Most components on modern aircraft, including ADIRUs, are highly reliable.
Nevertheless, failures do occur. The aircraft manufacturer reported that the average
mean time between failure30 (MTBF) for ADIRUs of the model used on VH-QPA,
was about 17,500 flight hours.
ADIRU failures affecting flight controls
It is extremely rare for any ADIRU failures to have an undesirable effect on an
aircraft’s flight controls.
The ATSB investigated an in-flight upset occurrence related to an ADIRU failure
on a Boeing 777-200 aircraft, which occurred on 1 August 2005, 240 km north-west
of Perth. The ADIRU on that aircraft was made by a different manufacturer and of a
different type to that on VH-QPA. Further details of that investigation can be found
on the ATSB web site.31
Airbus has reported that it is unaware of any previous occurrences where an
ADIRU failure on one of its aircraft has resulted in undesirable elevator commands.
However, there have been two other known occasions where ADIRUs have
exhibited similar anomalous behaviour to that which occurred on the 7 October
2008 accident flight, although those problems did not result in any adverse affect on
the aircraft’s flight controls. Those events occurred on 12 September 2006 and 27
December 2008.
VH-QPA, 12 September 2006
On 12 September 2006, VH-QPA was on a scheduled passenger transport service
(QF68) between Hong Kong and Perth, Australia. At 2052 UTC (0452 local time),
while the aircraft was in cruise at 41,000 ft, there was a failure of ADIRU 1. The
ADIRU was the same unit (serial number 4167) as on the 7 October 2008 flight.
The flight crew entered the problem into the aircraft’s technical log, noting that
there had been a NAV ADR 1 FAULT and that they had received numerous ECAM
messages.
The PFR showed that there was a NAV IR1 FAULT at 2052 and, subsequently, a
NAV ADR 1 FAULT at 2122. Maintenance records stated that, in accordance with
the manufacturer’s maintenance procedures for the relevant PFR fault messages, an
ADIRU re-alignment was conducted and a system test of both the IR and ADR was
conducted. No faults were found.
Following the 7 October 2008 accident, further information was obtained regarding
the 12 September 2006 occurrence. The crew reported that the event occurred at
night and that the aircraft was in clear conditions at the time of the event. The first
30 In this context, MTBF is the average time between two failures of any type requiring the unit to be
repaired.
31 See http://www.atsb.gov.au/publications/investigation_reports/2005/AAIR/aair200503722.aspx.
- 32 -
officer was the handling pilot and autopilot 2 was engaged. The crew reported that
they received numerous ECAM warning and caution messages. The messages
changed rapidly and consequently they could not be read properly or actioned.
There were also overspeed and stall warnings present.
The crew reported that they contacted maintenance watch, but subsequent
discussions could not resolve the issue. A scan of the overhead panel identified a
very weak and intermittent ADR 1 fault light. The crew decided to turn off the
ADR 1. Following that action, the warning and caution messages ceased and the
flight continued without further incident. The crew reported that at no stage was
there any effect on the aircraft’s flight controls. The autopilot and autothrust
remained engaged throughout the event.
No FDR or QAR data was available for the 12 September 2006 flight. The location
of the aircraft at the time of the NAV IR 1 FAULT was estimated using positions
reported by ACARS32 messages transmitted before and after the fault occurred.
That technique gave a position 980 km (530 NM) north of Learmonth (Figure 11).
Figure 11: Locations for each occurrence (the point shown is where the
anomalous ADIRU behaviour commenced)
The PFR for the flight contained a series of messages associated with ADIRU 1
which were similar to the PFR for the 7 October 2008 flight. Consistent with there
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