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were recorded by the PFR.
For elevator control, the active servo-controller in normal operation was PRIM 1.
The servo-controller priority order was PRIM 1, PRIM 2, SEC 1 and SEC2. If
PRIM 1 could not perform this function, then the servo-control function reverted to
PRIM 2 and so on.
Table 4 provides a sequence of events for the PRIMs and is based on a review of
the FDR and PFR data by the aircraft manufacturer and investigation team.
Table 4: PRIM sequence of events
Time (UTC):
(hh:mm:ss)
Master
PRIM:
Active
Law:
Pitch
servocontroller:
Event:
Prior to
04:42:30
PRIM 1 Normal PRIM 1 Uneventful flight (takeoff, climb and
initial cruise)
04:42:30 PRIM 3 Normal PRIM 2 F/CTL PRIM 1 Pitch Fault (during first
pitch-down event). PRIM 3 became
master PRIM.
04:42:31 PRIM 2 Normal PRIM 2 PRIM 3 Fault (duration: 120 seconds).
PRIM 2 became master PRIM.
04:43:31 PRIM 2 Normal PRIM 2 PRIM 3 status changed from Fault to
No Fault. This was consistent with it
having been reset by the crew.
04:45:10 PRIM 3 Normal SEC 1 F/CTL PRIM 2 Pitch Fault (during
second pitch-down event). PRIM 3
became master PRIM.
From 04:45:10
until the end of
flight
PRIM 1 Alternate SEC 1 PRIM 3 Fault. PRIM 1 became master
PRIM, but because it already had a
Pitch Fault it could not operate in
- 28 -
normal law and reverted to alternate
law.
In summary, the PRIM PITCH FAULTs and PRIM 3 FAULTs that occurred during
the flight were consistent with the system design. They were consequences of the
pitch-down events and not the initiators of those events.
Review of PRIM angle of attack processing
In addition to identifying the nature of the ADIRU failure which led to erroneous
data outputs, a key aspect of the investigation was to determine why the erroneous
data outputs had an undesirable and abrupt effect on the aircraft’s elevator
positions. As part of the investigation, the manufacturer conducted a detailed
review of how AOA data was processed by the PRIMs on the A330.
General ADIRU data processing algorithms
As with other modern airline aircraft, the A330 used a variety of redundancy and
error-checking mechanisms to minimise the probability of erroneous ADIRU data
having a detrimental effect on the aircraft’s flight controls.
For most of the ADIRU parameters, the PRIMs obtained three different values of
the same parameter. Each value came from a different sensor and was processed by
a different ADIRU. The PRIMs compared the value of the parameter coming from
each ADIRU. If the value of any of the parameters differed from the median
(middle) value by more than a threshold amount for more than a set period of time,
then the relevant part (that is, ADR or IR) of the associated ADIRU would no
longer be used by the PRIMs.
In addition, for ADIRU parameters except for AOA, when all three values were
valid, the median value was used for calculating the flight control commands. The
use of the median values was robust to any error from one data source.
Angle of attack data processing algorithms
There was a potential for the AOA sensors on the right side of the aircraft (AOA 2
and AOA 3) to provide different values to the AOA sensor on the left side of the
aircraft (AOA 1) in some situations due to aircraft sideslip.28 In order to minimise
the potential effect of this difference, the PRIMs used different processes for AOA
compared with other parameters when determining the value to use for calculating
flight control commands. More specifically, the processing of AOA data involved
the following:
• As with the other parameters, the PRIMs would continuously monitor the AOA
values from the three ADIRUs. AOA data was sampled about 20 times per
second.
• To confirm the validity of the AOA data, the PRIMs would compare the median
value from all three ADIRUs with the value from each ADIRU. If the difference
was greater than a set value for more than 1 second continuously, then the PRIM
28 Sideslip: a condition in which the oncoming airflow is at a sideways angle to the aircraft’s
centreline.
- 29 -
would flag the ADR part of the associated ADIRU as faulty and ignore its data
for the remainder of the flight.
• To calculate a value of AOA to use for calculating flight control commands, the
PRIMs would use the average value of AOA 1 and AOA 2. In other words,
(AOA 1 + AOA 2)/2. This value was passed through a rate limiter to prevent
rapid changes in the value of the data due to short-duration anomalies (for
example, as a result of turbulence).
• If the difference between AOA 1 (or AOA 2) and the median value from all
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