Understanding Air France 447 by Bill Palmer

issue. However, the sidestick input is best flown with a gentle hand. Light fingertip pressure is often all that is needed to fly precisely. A firm grip and strong inputs are almost certain to result in over controlling the airplane. Constant displacement on the sidestick is almost never required, but that is exactly what the pilot flying aboard AF447 was doing as he was losing control of the airplane.
    Flight Control Laws
    In the past, flight controls were designed to meet two sets of criteria: they had to be “well harmonized” and had to meet the criteria for certification. With Fly-By-Wire, three possibilities have been added: improved safety by restricting maneuvers which could lead to a loss of control, reduced weight of the structure with the prohibition of some actions which may increase the loads, and finally improved comfort for the passengers. 23
    On the Airbus family, the flight control laws are similar, but not exactly the same between models A319/320/321, A330/A340, A380, and soon, the A350. Each enjoy the results of progress in design with their respective age, as well as differences due to the nature of the aircraft itself. The point being that the A320 flight control system, flight control laws and the transition between them are not the same as the A330. As a result, there are a number of posts, articles, and opinions based on A320 flight control system that do not apply correctly to the A330.
    On the A330, the flight control laws are provided by five redundant computers: 3 Primary (also called PRIMs or FCPC [Flight Control Primary Computer]), and 2 Secondary (also called SECs or FCSC [Flight Control Secondary Computer]). The flight control computers control the two ailerons and seven spoilers on each wing, the two independent elevators, trimmable horizontal stabilizer, and the rudder, using hydraulic actuators powered by 3 independent hydraulic systems. (Flaps and slats are controlled by a separate computer.)
    Each computer controls specific hydraulic actuators for the flight control surfaces. All of the flight control surfaces can be actuated by at least two hydraulic actuators (except spoilers which each have one and the rudder which has three) and each actuator on the inboard ailerons and elevators can be driven by either of two computers. Suffice it to say there is a lot of redundancy built into the system that even with multiple hydraulic and flight control computer failures, control of the airplane can be maintained, although handling may be degraded.
    There are four basic levels of flight control laws:
     
Normal Law
Alternate Law (with two versions: Alternate 1 and Alternate 2)
Direct Law
Backup Control
    Airbus refers to the flight control laws other than Normal Law as reconfiguration laws.
    Normal Law is the flight control law normally in effect unless there are multiple failures. Normal Law allows the airplane to handle consistently throughout the flight envelope with automatic trim. For each movement of the sidestick, instead of commanding a specific amount of flight control surface deflection, Normal Law commands a performance. So the same sidestick input consistently results in the same aircraft handling response. The amount of flight control surface deflection required to provide that performance will vary with airspeed and other conditions. While the control deflection can be seen on the flight control display, it is not normally monitored except for a pre-takeoff control check. Normal Law also provides the highest degree of protections to prevent the airplane from leaving the normal operating envelope. Protections are provided for high and low speed (including stall protection), bank and pitch angles, and g load. Though not usually listed as a protection, roll rate is also limited by the design.
    In the roll axis, sidestick deflection signals a rate-of-roll demand. This means that for a given amount of sidestick deflection left or right, the pilot commands a given roll rate, with a maximum