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in both channels simultaneously, ie in the active one (Channel 2) and in the
passive one (Channel 1). One of the functions, either the monitor function
or the command function, registered the button push, but the other one did
not.
This disagreement prompted an ECAM warning message,
BRAKES BSCU CH 2 FAULT, which required no action by the crew
in accordance with the Aircraft´s Flight Crew Operating Manual section
3.02.32 (FCOM). However, 35 seconds after AUTO/BRK LO selection,
the pilot deselected it in an attempt to correct the malfunction and
selected it again 1m35s later.
The Brakes and Steering Control Unit (BSCU) was left with a
malfunction in both channels. Channel 2, the currently active channel,
transferred control to Channel 1, which remained as active channel in
spite of having a fault. This malfunction resulted in the Autobrake fault
and the failure of the Normal braking system.
41
During landing, as it was to be expected after the occurrence of the
failure, the malfunction became apparent in the autobrake function and
the Normal braking system. However, it did not affect the nose wheel
steering, since this was operative as evidenced by the swerves
performed at the end of the path followed by the aircraft on the runway.
2.1.2. Loss of Alternate braking system
The second failure was detected during the landing roll when the
handling pilot applied the brake pedals, at some seconds after 00.49
hours. At this moment, on application of the brake pedals, the Alternate
braking system should have been available. However, the BDDV failed
to function because of the presence of a frozen aqueous solution in its
lower cover that caused the internal rocker assembly to be locked.
This second failure completely prevented the brake pedal
application from sending pressure to the main gear wheels through the
Alternate servo valves via the Yellow hydraulic system. During the
landing roll the brakes did not receive pressure from the Green (Normal
braking system) or the Yelow (Alternate braking system) hydraulic
circuits, in spite of there being hydraulic pressure available on both
braking systems.
The only braking system working after these two failures was the
Parking Brake, which uses unmodulated hydraulic pressure from the
Yelow hydraulic circuit. In this case the Parking Brake was not used by
the crew.
2.1.3. Detailed analysis of failures.
Post accident tests indicated that no significant failures had
occurred in the BSCU, with the result that attention was focused on the
consequences of the “short button-push” during Autobrake selection.
The independent processors within the command and monitor areas of
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each BSCU channels spend only limited time, around 20 milliseconds,
“looking” for a change of state of the Autobrake push-button signal
wires, ie a voltage drop from 14v to 0v.
A short button-push of around 20 to 50 milliseconds can result in
either the command or the monitor function not registering the 0v signal,
thus producing the command/monitor disagreement. It is probable that
such a condition had occurred previously on Airbus aircraft, since
Airbus Industrie were aware of the possibility and there are precedents
of similar faults concerning at least the use of the AUTO/BRK MAX
button, although this was probably the first known occasion that the
disagreement had occurred simultaneously on both the active and the
passive channels.
The design logic decreed that, following the failure of a BSCU
channel, the remaining one could not quit. However the
command/monitor disagree condition still existed in the channel that was
now active, thus as soon as the brake servo valves started to open (as
commanded by the command function), the monitor failed the channel.
Since this was always going to happen, it could be argued that the
condition was detectable and therefore capable of being transmitted to
the crew, via an ECAM message, well before landing. Such a warning
would at least have permitted the option of resetting the BSCU, which
would have rectified the problem in this case. Thus the logic fault not
only failed the Autobrake facility, but also failed to allow manual ‘brake
by wire’ using the normal, and otherwise perfectly serviceable, hydraulic
system. There must be a concern that other circumstances could conspire
to cause the software to similarly hazard the aircraft by closing down a
serviceable system.
In this case, computer control of the brakes ended when the BSCU
closed the normal selector valve, thus causing the spring-loaded
automatic selector valve to bring in the alternate braking system. The
　
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