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one side and/or the other.
1.6.11.4 Control laws
The Airbus A330 has fly-by-wire flight controls. The aircraft is controlled by
means of two side-sticks whose movements are transmitted in the form of
electrical signals to flight control computers. This aircraft has three flight
control primary computers, called FCPC or PRIM, and two flight control
secondary computers, called FCSC or SEC. Their role is to calculate control
orders in manual control law, and to control the various control surfaces.
The laws governing this transformation are called control laws. On the A330
in nominal operation, the control law is called the normal law. In the case
where monitoring is triggered in the flight control system, it may be replaced
by degraded laws, known as the alternate (alternate 1 or 2) law or direct law.
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Normal law offers complete protection of the flight envelope: in terms of
attitude (the pitch and bank angles values are limited), load factor, at high
speed and with a high angle of attack. Outside the protections, the longitudinal
orders from the sidesticks command a load factor according to the aircraft’s
normal axis and the lateral orders command a rate of roll.
In alternate law, the longitudinal orders from the side-sticks command a
load factor according to the aircraft’s normal axis, like with normal law but
with fewer protections. In alternate 1, the lateral orders from the sidesticks
still command a rate of roll with the same protections as with normal law. In
alternate 2, they command the ailerons and lift dumpers directly.
With direct law, the orders from the sidesticks control the position of the
various control surfaces directly.
Another law, called the abnormal attitudes law, is triggered in certain cases
where the aircraft’s attitude is outside certain ranges, for example when the
bank angle exceeds 125 degrees. This is an alternate 2 law with maximum
lateral authority.
Like the FMGECs, the PRIMs consolidate the parameters that they use by
means of monitoring mechanisms. Concerning the airspeed, it is the voted
value that is used. In normal operation, this is the median value. When one
of the three speeds deviates too much from the other two, it is automatically
rejected by the PRIMs and the polled value then becomes the average of the
two remaining values. But if the difference between these two remaining
values becomes too great the PRIMs reject them and the control law switches
to alternate 2. Furthermore, another monitoring procedure is applied to the
value of the voted airspeed and triggers switching to alternate 2 law when it
falls by more than 30 kt in one second.
In alternate or direct law, the angle-of-attack protections are no longer available
but a stall warning is triggered when the greatest of the valid angle-of-attack
values exceeds a certain threshold. In clean confi guration, this threshold depends,
in particular, on the Mach value in such a way that it decreases when the Mach
increases. It is the highest of the valid Mach values that is used to determine the
threshold. If none of the three Mach values is valid, a Mach value close to zero is
used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8.
1.6.11.5 Presentation of information on the PFD
A nominal PFD and a PFD in alternate 2 are shown hereafter. The displays presented
on these PFD’s are not exact representations of those that could have been displayed
of the fl ight AF 447 crew’s PFD’s.
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Nominal PFD
PFD in alternate 2 law
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1.6.11.6 Consequences of a drop in the measured total pressure
The static pressure (Ps), total pressure (Pt) and total air temperature (TAT)
allow the ADR to calculate the following parameters in particular:
 Standard altitude
 Mach
 Calibrated Air Speed (CAS)
 True Air Speed (TAS)
The order in which these different parameters are calculated is not immaterial
because the value of the measured static pressure must be corrected to take
into account the measurement error due to the air flow disturbances in the
vicinity of the sensor. This correction depends in particular on the Mach and
has a direct influence on the standard altitude which only depends on the
static pressure. On an A330-200 in cruise flight, the measured static pressure
overestimates the real static pressure.
If Pt and Ps are known, it is possible to calculate a Mach value that provides
access to the correction of Ps. The Ps thus corrected is then used to calculate
the CAS and the altitude. When the Mach value is known, the TAT measurement
　
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