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three ADIRUs was higher than a set value, the PRIMs memorised the last valid
average value and used that value for a period of 1.2 seconds. After 1.2 seconds,
the current average value would be used.
In summary, in contrast to other parameters, only two values of AOA were used by
the PRIMs when determining flight control commands. However, several risk
controls were in place to minimise the potential for data inaccuracies to affect the
flight control system.
Scenario where AOA spikes could influence flight controls
The aircraft manufacturer advised that the AOA processing algorithms would
prevent most types of erroneous AOA inputs provided by the ADIRUs having an
influence on flight control commands. This included situations such as an AOA
‘runaway’ (or a continuous divergence from the correct value), single AOA spikes
and most situations where there were multiple AOA spikes. However, the
manufacturer identified that, in a very specific situation, the PRIMs could generate
an undesired nose-down elevator command. This specific situation involved
multiple AOA data spikes with the following properties:
• there were at least two short duration, high amplitude spikes
• the first spike was shorter than 1 second
• the second spike occurred and was still present 1.2 seconds after the detection of
the first spike.
Recorded flight data from the accident flight showed that there were 42 recorded
spikes in AOA 1 data. Due to recorder sampling rate limitations, it is likely that
there were additional AOA 1 spikes that were not evident in the recorded data and it
is not possible to reconstruct the exact duration and timing of any of the spikes.
Although a large number of AOA 1 spikes occurred on the accident flight, on all
but two of those occasions, the processing algorithm filtered them out and they had
no influence on the flight controls.
The aircraft manufacturer advised that AOA spikes may occur on many flights, but
in its experience, there were usually only a very small number of spikes on any
particular flight. It was not aware of any previous event where AOA spikes had met
the above conditions and resulted in an in-flight upset.
Simulation studies
As part of the investigation, the manufacturer reported that it had performed
simulation studies concerning the filtering of AOA spikes by a PRIM. The
simulation studies confirmed that the input of two AOA spikes which met the
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conditions listed above, were not effectively filtered by the PRIM, and could lead to
undesired nose-down elevator commands.
Flight envelope mechanisms influenced by AOA spikes
The aircraft manufacturer reported that, based on its analysis of the available data
and its review of system design, two of the flight envelope mechanisms were
influenced by the AOA spikes during the accident flight: high angle of attack
protection (alpha prot) and anti pitch-up compensation.
Alpha prot was designed to protect the aircraft from high AOAs which could lead to
a stall and loss of control. If the PRIMs detected that the aircraft’s AOA exceeded a
predefined threshold, the computers would command a nose-down elevator
movement to reduce the AOA. Alpha prot was only available when the aircraft was
in normal law. When the aircraft was above 500 ft above ground level, alpha prot
was effective immediately, while below 500 ft it was only active after the AOA
exceeded the threshold for 2 seconds or more.
Anti pitch-up was a pre-command included in the control laws to compensate for a
pitch-up at high Mach due to aerodynamic effect.29 The compensation was available
above Mach 0.65 and when the aircraft was in a ‘clean’ configuration (that is, with
the landing gear and flaps retracted). The maximum authority of the anti pitch-up
compensation was 6 degrees of elevator movement.
The aircraft manufacturer advised that the 10-degree elevator command associated
with the first in-flight upset, was the result of 4 degrees of alpha prot and the 6
degree authority of the anti pitch-up compensation. The 10-degree command was
close to the worst possible scenario that could arise from the design limitation in the
AOA processing algorithm.
For the accident flight, there was only a limited potential for additional upsets to
occur. After the second upset, alpha prot was no longer operative as the flight
control law had reverted from normal law to alternate law. From approximately 18
minutes after the second upset, the aircraft was below Mach 0.65 and anti pitch-up
compensation was no longer active.
The manufacturer advised that a simulation performed with the AOA profile
identified during the first pitch-down event, showed that such an AOA profile
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