A stability and command augmentation system for controlling an aircraft, comprising: a first member moveable by a pilot input device to a first position defining a first input; a second member moveable to a second position associated with a second input; and an adder device configured to add the first and second inputs and supply an output signal defining a command for an element to be controlled of the aircraft; the stability and command augmentation system comprises: a casing, a first and a second piston integrally movable with one another inside the casing and operatively connected to the second member; and control means configured to exert a first force on the first piston and a second force on the second piston; the second force is independent of the first force.