Nowadays, commercial airplanes do not fly with metal wires moving the ailerons anymore. Now it is managed digitally, from the source (the sidestick or yoke movements) to the control surfaces movement. This system is called Fly-by-wire.

The fly-by-wire system consists of some computers (5 computers on board the Airbus’ planes, 3 primary and 2 secondary), that read the pilot movements, interpret them, and it sends a modified signal (from the original one) to the control surfaces. 

All these means that the airplane movements will not match proportionally what the pilot is doing actually. This philosophy shocked a lot of pilots, they couldn’t imagine flying something without a cable between the yoke and the surfaces. Finally, it was demonstrated that this system was much efficient, safer and much versatile.

Airbus was the first one using fully fly-by-wire (A320 were the first) and they create a set of laws that would govern this system. There are 4 laws: normal law, alternate law, direct law and mechanical back-up depending on the degradation of the airplane.

• Normal Law: It is on when normal operation is carried out. It controls the 3 axis (pitch, roll and yaw) balancing the aircraft and interpreting the sidestick movements. In addition, it offers some envelope  flight protections (on pitch attitude and roll attitude) and alleviate load factors, like the gravity when turning for a much better comfort.
• Alternate Law: If a minor failure exists (sensors disagreement, computer blackout, hydraulic system upset,etc) this law becomes the active one. Basically it provides the same control as the normal law, but without protections. Also, the roll control becomes “direct law-like” control.
• Direct Law: If a major failure exists (radio altimeter failure when approaching, 3 inertial references down, etc) this law become the active one. Then, the movements of the control surfaces are the same as the sidestick movements. There is no auto trim, and any protection system will be available (even alpha-floor for stall warning).

• Mechanical Back-up: This mechanical aid should be used when the electrical system is all down. Then, the airplane can only be controlled with the trim wheel for pitching, and rudder pedals for rolling.