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air speeds.
(6)-40 °C is the
commonly
accepted value
below which
no more supercooled
water
exists and thus
the risk of ice
accretion on the
aircraft airframe.
(7)See appendix 4.
(8)Some crews
had deviated
from the planned
fl ight path for
meteorological
reasons before the
events occurred.
F-GZCP - 1st June 2009
51
All the events occurred in IMC;
The recordings of total or static temperatures(9) show increases of ten to
twenty degrees during the event, which sometimes began before noticing
any speed anomalies, except in a case where the increase was lower;
Turbulence was always recorded and reported. The levels felt by the crew
varied form slight to strong. The recorded amplitude in recorded normal
went from [0.75/1.2g] to [0.2/1.9g].
As regards the automated systems and the other systems, the following points
can be noted:
The autopilot disengaged in all the cases without any intervention by the
crew;
In all cases, the crew were able to use autopilot and auto-thrust again;
In twelve cases, the airplane switched to alternate law until the end of the
flight. In one case, this switch was temporary(10);
The autopilot disengagement was accompanied by the disappearance of
the associated flight director and sometimes of the other for a variable
duration. All the cases studied show the reappearance of the flight directors
during the event. In certain cases(11), this reappearance was recorded
simultaneously with a return to two close speed values returning;
In seven cases, the autopilot was reconnected during the event. In two of
them, the re-connection occurred when the two speeds were consistent
with each other but were erroneous;
The autothrust disconnected in ten cases, leading to the activation of the
Thrust Lock function. In five of them, this function remained connected for
over one minute;
In one case, the crew had disconnected the autothrust and displayed the
thrust corresponding to the speed recommended in turbulent atmosphere
before the event;
In two cases, the autothrust did not disconnect and the flight directors
did not disappear. The recording of the engine RPM parameters shows
fluctuations in thrust with N1 values of between 48% and 100% ;
The speed anomalies can be characterised by two distinct signatures:
Intermittent falls (peaks);
Fall followed by levelling off (continuous period).
They were accompanied by an instant increase in displayed static
temperature (and total when it was recorded), and with a “drop“ in
altitude indicated on the airplanes equipped with altimetric correction
(A330-200). In both cases, the lower speed limits recorded were below
100 knots;
The maximum continuous duration of invalid recorded speeds was three
minutes and twenty seconds;
(9)The total
temperature is not
always recorded.
(10)See the
analysis of the
« Pitot probe »
message §
1.16.2.4 in the
Interim Report n°1.
(11)Case observable
only on airplanes
equipped with
ISIS for which
both CAS values
are recorded.
F-GZCP - 1st June 2009
52
When the values of the speeds calculated by the ISIS system were recorded,
their anomalies possessed signatures and/or durations that differed from
those observed on the speed displayed on the Captain’s side display.
With regard to crew reactions, the following points are notable:
The variations in altitude stayed within a range of more or less one thousand
feet. Five cases of a voluntary descent were observed, of which one was of
3,500 feet. These descents followed a stall warning;
Four crews did not identify an unreliable airspeed(12) situation: in two cases,
the crew concluded that there was an inconsistency between the AOA sensors;
in the two other cases, the crew considered that the speeds were erroneous and
not and doubtful.
For the cases studied, the recording of the flight parameters and the crew
testimony do not suggest application of the memory items(13) in the unreliable
airspeed procedure:
The reappearance of the flight directors suggests that there were no
disconnection actions on the FCU;
The duration of the engagement of the Thrust Lock function indicates
that there was no rapid autothrust disconnection actions then manual
adjustment on the thrust to the recommended thrust;
There was no search for display of an attitude of 5°.
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