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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
knowledge gained from experience, the
accomplishment of special actions should be normal or
be prescribed.
Conclusions
The investigation has revealed thata ll airspeed
indication systems had failed for a short time due to
an occlusion of the pressure ports as a result of ice
formation on the pitot tubes. The result of the
investigation confirms the assumption based on past
reports by several operators on varying/failed airspeed
indications that the design of the pitot tubes does not
allow unrestricted flight operations with the aeroplane
type in heavy rain and under severe icing
conditions.Since the AOM and other documents for the
aeroplane type A 320 do not define restrictions for
flights under severe icing conditions, the incident is
finally due to a type design problem.
Safety Recommendations
The result of the investigation has prompted the BFU to
issue the following safety recommendations:
01/99 The specification for the pitot tubes should
be changed so as to allow unrestricted
flight operations in heavy rain and under
severe icing conditions. The installation of
the improved pitot tubes already designed
should subsequently be prescribed for all
types concerned by the SIL no. 34-0147 (A
320, A 321, A 330, A 340).
02/99 In case of a synchronous failure or
malfunction of systems of the same kind
(loss of redundancy, total failure) the
accomplishment of special actions (e.g. an
　
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