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Directive (Consigne de Navigabilite) ref. 34/98, No. 98-262-
120(B).
Following the accomplishment of the Bulletin, the weekly check on
the Alternate braking system was no longer required.
The Bulletin acknowledged that drilling a hole in the BDDV cover
could allow the ingress of water, dust and other contaminants into the
valve components, and that this was therefore also and only an interim
measure. The final solution was by way of Service Bulletin No. A320-
32-1203, which introduced a new design of seal between the cover and
the body of the valve and a clear plastic drain and stopper from the
BDDV cover. This was issued by Airbus on 4 June 1999 and make
mandatory by AD – 2000-258-146 (B) although it was initially installed
on new-build aircraft.
1.18.2. Subsequent Actions by Manufactures on the BSCU.
The BSCU has had a number of software upgrades, with Standard
7 being installed in the unit fitted to the aircraft at the time of the
accident. Software Standard 8 was introduced including a revision to
address the problem caused by short button-pushes of the Autobrake.
However as a result of in service evaluation, Standard 8 was supersedes
by Standard 9, which became available from beginning of June 2001,
reference OIT 999.0078/01/BB as a recommended customer option
upgrade.
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1.19. Investigation techniques.
The first phase of the investigation was conducted at the accident site,
there being no need to move the aircraft since it did not interfere with the Airport
operations and this offered the added facilities of being in an enclosed area, easily
accessible for investigators and with the airport facilities nearby. However,
neither in the Airport nor in the Island there were hangars or other aeronautical
facilities in which to conduct subsequent tests on the aircraft systems.
Some days later, the aircraft was moved to the apron beside the terminal
buildings, where the basic repair of hydraulic leaks was continued in order to be
able to conduct tests on the Normal and Alternate Braking systems. The
successful completion of these tests faced innumerable limitations as regards
equipment and facilities.
When the possible factors that had caused the event had been found to be
confined to the hydraulic braking system and its electric/electronic control —
although with the specific elements or units that caused the malfunction still not
identified — it became apparent that this system, complete and fitted to the
aircraft, required a comprehensive test, since in the A-320 aircraft family there is
a very close relationship among various electronic-computing pieces of
equipment and the electro-hydraulic mechanisms. For this reason, the testing of
individual units or mechanisms was thought to be less likely to reveal possible
discrepancies or abnormalities.
The aircraft had sustained relatively little damage and it could be
recovered for flying after a short time. For these and the above referred reasons, it
was suggested and accepted to ferry it after making the essential repairs to the
Manufacturer’s facilities in Toulouse, where the best possible infrastructure for
conducting the necessary tests was present.
In spite of the difficulties that the transportation of the aircraft involved,
this was successfully completed within 30 days from the accident. During the
ferry flight, taking care to cause the least possible interference with the aircraft
39
systems in order to avoid to masking or deleting the faults that caused the
accident, the general parameters were recorded and some functional tests of the
braking system were conducted in order to go ahead with the detection of the
causes of the accident.
The availability at the manufacturer’s premises of a nearly complete
aircraft with the same systems and components as those fitted to the aircraft at the
moment of the event, greatly and effectively helped to quickly clarify the causes,
since there were provided a complete braking system fitted to aircraft itself and
the resources required to conduct the tests and to obtain the maximum
information thereof.
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2. ANALYSIS.
2.1. Behaviour of the aircraft.
In this accident two independent failures occurred, within Normal and
Alternate braking systems. The initial failure occurred at autobrake selection, the
other failure was a dormant condition within the BDDV.
2.1.1. Loss of Normal braking system.
The initial failure took place during the cruise phase (phase 06) at
23.55 hours, when the handling pilot selected AUTO/BRK LO. This quick
button push and the subsequent input to the BSCU caused an internal logic
“disagreement” between the monitor function and the command function
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