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• electrical power generation system
• elevator hydraulic actuation and pitch control.
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A fault was identified with the elevator hydraulic control, but this was considered
by the aircraft manufacturer to be unrelated to the circumstances of the occurrence.
The fault had a known cause that was only triggered under a very specific set of
circumstances, different to those seen during the occurrence. The aircraft systems
passed all other tests.
Flight control primary computers (PRIMs)
The three PRIMs were removed from the aircraft and examined by an authorised
agency. It was confirmed that each PRIM was loaded with identical operational
software (version P7/M16). The PRIMs were tested and the BITE data was
downloaded from each unit. The results were:
• FCPC 1 (serial number 7270): During testing, no fault was found. No faults
were stored in BITE data.
• FCPC 2 (serial number 6165): During testing, no fault was found. The BITE
data did not contain any faults relevant to the pitch-down events.
• FCPC 3 (serial number 6170): During testing, the unit failed a lightning
protection test. The aircraft manufacturer advised that this result was unrelated
to the pitch-down events. The BITE data did not contain any faults relevant to
the pitch-down event.
Based on a review of the recorded data and system functionality, the PRIM faults
recorded by the FDR and PFR were found to be consequences of the pitch-down
events (see Review of PRIM monitoring functions).
Probe heat computer
Some of the PFR messages indicated a potential fault with the number-1 probe heat
computer (PHC). Those messages could be generated by either a PHC fault or by an
ADIRU fault. The PHC (serial number 2083) was tested by an authorised agency
and no fault was found. Based on a review of available information, the messages
related to the PHC were considered to be spurious.
Angle of attack sensor
The AOA 1 sensor (serial number 0861ED-972) was tested by an authorised
agency. No fault was found with the sensor and all test parameters were within
limits.
ADIRU testing
The ATSB took custody of ADIRU 1 (serial number 4167) in Learmonth on 10
October 2008 while ADIRUs 2 and 3 (serial numbers 4687 and 4663 respectively)
remained installed in the aircraft. On 15 October 2008, after the aircraft had been
ferried back to Sydney, ADIRUs 2 and 3 were removed from the aircraft and
quarantined by the operator. Also on 15 October 2008, custody of ADIRU 1 was
transferred from the ATSB to the operator.
The three ADIRUs were despatched to the ADIRU manufacturer’s facility in Los
Angeles. ADIRU 1 was received on 17 November 2008 while ADIRUs 2 and 3
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were received on 18 November 2008. All three ADIRUs were quarantined on
arrival and locked in a secure storage room awaiting the arrival of the investigation
team.
ADIRU test plan
To make the testing process efficient, it was necessary to have an agreed test plan in
place before the investigation teams arrived at the manufacturer’s facility. The
testing priorities were to:
• minimise the chance of losing perishable data
• use standard test procedures before testing the ADIRUs with novel procedures
• review current test results before proceeding with the next test
• order the testing so that ‘whole box’ testing was completed before an ADIRU
was disassembled
To minimise the chance of losing perishable data or the chance that test
equipment/procedures might damage an ADIRU, an exemplar ADIRU was
included in the testing. The exemplar unit was provided by the ADIRU
manufacturer and was functionally identical to ADIRU 1, and had the same
hardware/software modification status. ADIRU testing was performed on the
exemplar unit before being performed on ADIRU 1.
Participants
The following organisations attended the ADIRU testing at the manufacturer’s
facilities in the US: the ATSB; the French Bureau d’Enquêtes et d’Analyses pour la
sécurité de l’aviation civile (BEA); the US National Transportation Safety Board
(NTSB); the aircraft manufacturer; the ADIRU manufacturer; the operator; and the
US Federal Aviation Administration (FAA).
ADIRU test schedule
Testing commenced on 17 November 2008 with all the participating organisations
present. Daily reviews and discussions of the test results were held. Once it was
realised that an obvious fault with ADIRU 1 was not going to be found, an ongoing
test program was developed and agreed. The witnessed testing period concluded on
25 November 2008 and the test program is continuing at the manufacturer’s facility.
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