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the system 1 airspeed signal had been interrupted only
for 14 seconds.
On April 14th, 1998, the BFU made an enquiry of the
PIC at the operator’s. The pilot expressly affirmed that
the duration of the interruption of the airspeed
indications on both PFDs was quite exactly identical with
that of the interruption of the autopilot function.
Concerning the weather, he stated that only light to
medium icing and turbulence had been expected,
however, the icing turned out to be relatively severe.
Bundesstelle für
Flugunfalluntersuchung
Incident Investigation
Report
5X002-0/98
January 1999
5 X 002-0/98 Seite 2
Within a few seconds 2 - 3 cm of ice had accumulated
on the ice accretion meter. The official weather
expertise by the Deutsche Wetterdienst (German
Meteorological Service) proves that in showers and
thunderstorm cells at flight level FL 100 there were
conditions for severe turbulence and icing.
The operator had immediately arranged for a thorough
inspection of the aeroplane and the evaluation of the
FDR. This order was carried out by the contractual
maintenance organisation working for the operator in
accordance with the Service Information Letter 34-047
When carrying out the a.m. instruction, the maintenance
organisation found out that the systems concerned
functioned properly. Thus, the aeroplane was released
to flight service. The maintenance organisation
informed the operator, the aeroplane manufacturer, the
supervising authority and several internal departments
of this inspection result.
After the inspection of the aeroplane had been
concluded and on the basis of the evaluation of the
FDR/ECAM recordings, the investigation team
elaborated a joint catalogue of questions on April 21st,
1998 and transmitted it to the aeroplane manufacturer.
Furthermore all pitot tubes were removed and sent to
the component manufacturer for the purpose of
inspection. For the inspection of thep itot tubes, the
manufacturer requested in addition the associated
computers. Because of the relevance to the error
search on the aeroplane, on 27.4.1998 also the staff
members of the Trouble Shooting and Maintenance
departments who were involved in this matter were
heard by the Investigation Team.
Assessment
The non-appearance and/or differing storage of
warnings is to be explained by the conception on which
the ECAM system is based. The ADR ECAM warnings
are designed only for the technical availability of the
ADR computer system. The pitot tube as the input
sensor does not belong to the monitoring circuit of the
computer system. During the system comparisons by
the computer program, varying input conditions
(pressures) may result in differing messages from the
three independent ADR systems. These messages may
thus have been stored also under different menues
(POST FLIGHT REPORT, LAST LEG REPORT).
According to the information by the aeroplane
manufacturer the total air temperature recorded is
originating with system no. 2. From the assessment of
this recorded parameter it is obvious that the airspeed
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
　
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