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never re-issued to match the tasks of the TSM. This TFU was closed in August 2003,
once the Service Bulletins A340-32-4187 and A-340-32-4193 were issued by Airbus to
solve the problem of spurious indication. It is considered convenient to issue a safety
recommendation regarding the need of consistency between the different maintenance
documentation.
Additionally, it seemed that different interpretations existed of what to do after the
«Fault confirmation» of the task 32-40-00-810-927, because the maintenance people
Technical report IN-062/2002
48
seemed to be unaware of the intermittent failure concept of the TSM, and therefore
two safety recommendations on the matter are issued.
In summary, it seems that the intermittent nature of the residual pressure, together with
differences of interpretation of the maintenance documentation or lack of knowledge
regarding the meaning of «fault confirmation» for «hard faults» and «intermittent
faults», masked the solution of the problem.
The operator also stated that they used to communicate with the manufacturer in many
other instances of malfunctions reported by the flight crews.
The main factor that caused that the squawks in June 2002 were not communicated to
Airbus was the awareness of the maintenance personnel, and even of most flight crews,
of a long history of spurious cautions and indications associated to the BSCU, both in
A-340 (including EC-GUP in year 2001) and in A-320, reflected in TFU and FOT of the
manufacturer. Most of the corrections of the reported abnormalities included tests of
the channels of the BSCU. In several instances, the correct result of those tests led to
the conclusion that the caution or indication was due to a possible temporary malfunction
of that component.
It seems clear that this factor influenced both the line maintenance actions in Tenerife
and the decisions made by the flight crew. Additionally to the history of spurious indications,
there was the fact that it was a new aircraft recently delivered, and their experience
indicated that in the event of an unexplained indication, the probability of that
indication being spurious was greater in that period.
In any case, apart from any previous history of spurious indications and from the interpretation
of the available maintenance documentation, there were clues in some of the
events (like high temperature after landing) to indicate that the pressure could have
been real, although with an intermittent nature, and that more maintenance actions
were needed.
Taking into account the maintenance carried out before the incident, it was considered
necessary to issue two safety recommendations regarding enhanced analysis of reported
intermittent malfunctions and enhanced training to the maintenance personnel
focused to analysis of reported intermittent malfunctions.
After 11-6-2002, there were no further reports of residual pressure in flight until the
aircraft had a bird impact on 28-7-2002 and was removed from service for repairs.
Then, after the first flight since that repair, the incident happened on 7-9-2002.
In summary, since 21 May (start of operation of the aircraft) until 6 June there was no
report of residual pressure. Since 7 to 11 of June there were 5 reports during 5 continuous
flights, and then the pressure disappeared, or at least was no longer reported,
49
Technical report IN-062/2002
until it appeared again in the first two flights after the bird impact (Madrid-Tenerife and
Tenerife-Madrid).
No evidence has been found to explain this behaviour. One hypothesis was that the
maintenance tasks carried out during the fixing of the 5 complaints, together with the
braking during those landings, eventually «corrected» in some way the behaviour of the
left master cylinder. Then, in the repairs after the bird impact, the brake pedals were
disassembled and re-assembled as a whole assembly to the fuselage, and no part or
subcomponent of the assembly were removed in those operations. In paragraph 1.16 it
is explained that the rigging of the pedals was considered correct in the tests carried
out by Airbus after the incident.
In any case, it can be concluded that the tolerances and rigging of the master cylinder
are so tight that, under certain conditions there may be susceptibility to environmental
conditions simply by means of small deviations of those tolerances.
It seems clear that it would be positive that the design of the master cylinder was modified
to make it more resistant to the influence of the mentioned factors. Airbus has already
initiated this and other safety actions (see 1.18.3). It is considered convenient to
　
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