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Protocols are in place for the oversight of the testing, regular reporting of the results
to investigation team members and analysis of the results.
Completed ADIRU tests
The tests completed at the time of publication of this report were:
• Physical inspection: the three ADIRUs were inspected visually for damage with
particular emphasis on the connector pins.
• Ground integrity test: various connector pins on ADIRU 1 were electrically
tested for ground integrity.
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• Program verification: the three ADIRUs were connected to a test bench and the
operational flight program (OFP) software was downloaded from the units to
check that it was the correct version and was not corrupted.
• Recorded data download: BITE data from the three ADIRUs was downloaded
and analysed.
• Built-in test and manufacturing test procedure: the three ADIRUs were
connected to a test bench and the units’ internal test equipment was run.
Additional functional tests were also performed on the bench.
• Bus tests: ADIRU 1 was connected to a test bench and the bus traffic was
recorded while different bus load impedances were simulated. The bus output
waveforms were also recorded and analysed for comparison with the
specification.
• Internal visual inspection: the case of ADIRU 1 was opened and an internal
visual inspection was completed without removing any internal equipment.
• Environmental tests: ADIRU 1 was subjected to a range of environmental tests
including vibration and temperature. One environmental stress screening test
used a temperature range of -40 °C to +70 °C. ADIRU 1 was also subjected to
electromagnetic interference (EMI) tests in accordance with the frequencies and
field strengths specified in DO-160C.27 In addition to the frequencies specified
in the standard, ADIRU 1 was also subjected to specific conducted susceptibility
tests at the Harold E. Holt Naval Communication Station frequency of 19.8 kHz
and a field strength of 100 Volts/metre (see Electromagnetic interference).
ADIRU test results
The BITE data from ADIRUs 2 and 3 was successfully recovered and showed that
there were anomalies in the way that ADIRU 1 had been transmitting data to other
aircraft systems. The BITE data did not show any problems with the performance of
ADIRUs 2 and 3.
BITE data from ADIRU 1 was recovered and showed:
• No data had been stored for the time periods relating to the pitch-down events.
• Several routine BITE messages that were expected to have been stored were not
recorded.
• There were anomalies in the BITE elapsed time interval parameter.
Following the BITE downloads and successful completion of the standard
manufacturing test procedures, the investigation team agreed that no further testing
of ADIRUs 2 and 3 was required.
None of the testing that has been completed to date on ADIRU 1 has produced any
faults that were related to the pitch-down events. While some faults have been
detected during the extensive testing, they have been confirmed as being due to the
artificial nature of the testing or problems with the test equipment.
27 DO-160C, Environmental Conditions and Test Procedures for Airborne Equipment,
produced by the Radio Technical Commission for Aeronautics (RTCA). Issued 12 April
1989.
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Further testing of ADIRU 1 is in progress (see ONGOING INVESTIGATION
ACTIVITIES).
Review of PRIM monitoring functions
The aircraft’s flight control system included three flight control primary computers
(FCPCs, commonly known as PRIMs) and two flight control secondary computers
(FCSCs, commonly known as SECs). One PRIM functioned as the master while the
other two PRIMs could take over as master if a fault in the current master was
detected. The master PRIM processed and sent control surface deflection orders to
other computers, which executed them using servo-controls. The two other PRIMs
continuously computed control orders and monitored control surface deflections but
those orders were not actioned.
Each PRIM consisted of two independent parts, a Command (COM) part and a
Monitor (MON) part. The MON part monitored the performance of the COM part
and the position of the control surfaces. If there was a discrepancy between COM
and MON, then the PRIM would ‘fault’ itself. The fault could be for only a part of
the PRIM (for example, pitch channel) or for the whole PRIM. A PRIM could not
generate a fault for the whole PRIM unless it was the master. The PRIM Fault
parameter recorded by the FDR was active only for a fault of the whole PRIM and
not for a partial fault (for example, a pitch channel fault). However, partial faults
　
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