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signal in system 2 was available again almost at the
same time as that in system 1. Thus the failure of both
systems is confirmed.
After the statements of the PIC had been confirmed in
the course of the investigation to a large extent, there is
no reason to doubt the statements of the PIC relating to
the duration of the interruption of the airspeed
indications on the PFDs. However, it was not possible to
come to an agreement. The aeroplane manufacturer is
of the opinion that the interruption itself lasted only 14
seconds and that the PIC due to heavy work load just
was not earlier in a position to switch thea utopilot on
again. In the Airplane Operating Manual it is stated that
if a comparison between both ADR computer systems
(ADR DISAGREE) is not possible and also the standby
system is not available, the crew should act at their
own discretion/on the basis of their experience.
Even if a time of reaction to the indications re-appearing
is accounted for, it may be considered proved that the
interruption of the airspeed indications on theP FDs
was considerably longer than the duration recorded by
the FDR. A definite assessment would be possible if
either the System Status Mode or the airspeed
information would be recorded directly by the indication
in the PFD.
Unstable airspeed indications under certain meteorological
conditions have been reported already by several A 320
operators. In July 1993, the aeroplane manufacturer
issued the Technical Information TFU no. 34.10.00.011
dealing with this problem and thus informed all operators
of A 320, A 321, A 330 and A 340. With this Information
and on the basis of experience gathered in daily flight
operations, all operators and maintenance organisations in
principle had knowledge of weather related malfunctions in
the airspeed indication systems of certain AIRBUS types.
Failure or malfunction of one system will normally be
eliminated by actions according to instructions given by the
aeroplane manufacturer (AIRCRAFT MAINTENANCE
MANUAL, SERVICE BULLETIN). With the SERVICE
INFORMATION LETTER no. 34-047, the aeroplane
manufacturer issued an instruction which took into
account the fact that normal trouble shooting according
to the TROUBLE SHOOTING MANUAL provided for
this purpose was not suitable for this special complaint.
When accomplishing the actions described, obstructed
drain holes or residuals have not been found in any of
the three pitot tubes. The Service Information Letter
provides that the aeroplane manufacturer is to be
informed by the maintenance organisation about all
results of the actions. Instructions as to further actions
had not been given to the maintenance organisation.
The aeroplane manufacturer was also kept informed
about findings made in the framework of the
investigation. The aeroplane manufacturer did not
suggest further investigations.
The hearing of the personnel charged with the technical
inspection of the aeroplane revealed that the actions of
these staff members were determined to a very large
extent by the special knowledge gained from
experience. Since for the case of a synchronous
5 X 002-0/98 Seite 3
failure of more than one system of the same kind
(loss of redundancy, total loss of all systems) the
aeroplane manufacturer has issued only flight
operational instructions but no instructions for repair,
knowledge gained from experience was very useful.
The inspection and also the release to service of the
aeroplane was performed on the basis of the normal
procedure, i.e. with all three systems functioning properly,
the aeroplane was serviceable again and thus was
returned to the operator for flight operations. The total
failure of all airspeed indication systems has not caused
the aeroplane manufacturer and the operator to charge
the maintenance organisation with special actions. The
inspection of the aeroplane had not led to definite
findings.
Failure of one system cannot be avoided. This fact is taken
into account by technical designs (REDUNDANCY, FAIL
SAFE DESIGN). Also cases of repetition cannot be
excluded by normal repair procedures, they can only be
minimized at best. This problem is being monitored in the
framework of RELIABILITY considerations and solved, if
necessary. But if the safety reserves of a flight had
considerably been reduced by the synchronous failure of
several systems of the same kind, this should be a reason
to deviate from normal procedures or to change them. In
order to avoid cases of repetition or, as in this case, to
confirm the assumption which is based on the
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