A stability and command augmentation system for an aircraft, comprising: - a first member moveable by a pilot input device (10, 11, 15) to a first position defining a first input (xi); - a second member moveable to a second position associated with a second input (xd); and - an adder device configured to add said first and second inputs (xi, xd), and supply an output signal (xv) defining a command for an element to be controlled (9, 14) of said aircraft; - a casing; - a first and a second piston (31, 32) integrally movable with one another inside said casing and operatively connected to said second member; and - control means configured to exert a first force on said first piston and a second force on said second piston (31, 32); said second force being, in use, independent of said first force; said system further comprising: - a first valve configured to exert said first force on said first piston; and - at least a second valve configured to exert said second force on said second piston; said casing defining a first chamber and a second chamber inside of which said first and second pistons (31, 32) sealingly slide, respectively; said first valve being selectively controllable to create a first differential pressure value in first portions of said first chamber arranged at respective opposite ends of said first piston; said second valve being selectively controllable to create a second differential pressure value in second portions of said second chamber arranged at respective opposite 3t ends of said second piston; said system further comprising a rod sliding inside said casing, on which said first and second pistons (31, 32) are fixed and integrally movable with said second member; characterized in that it comprises an adjustable limit device for the travel of said second member between a first and a second stop position; at least one of said first and second stop positions being selectively adjustable; said adjustable limit device comprising: - an internally threaded cylinder having an axis orthogonal to the axis of said rod, through which said rod passes, and is arranged with play inside an unloaded portion of said rod; and - a threaded head screwed on said cylinder. - a first member moveable by a pilot input device (10, 11, 15) to a first position defining a first input (xi); - a second member moveable to a second position associated with a second input (xd); and - an adder device configured to add said first and second inputs (xi, xd), and supply an output signal (xv) defining a command for an element to be controlled (9, 14) of said aircraft; - a casing; - a first and a second piston (31, 32) integrally movable with one another inside said casing and operatively connected to said second member; and - control means configured to exert a first force on said first piston and a second force on said second piston (31, 32); - a first valve configured to exert said first force on said first piston; and - at least a second valve configured to exert said second force on said second piston; - an internally threaded cylinder having an axis orthogonal to the axis of said rod, through which said rod passes, and is arranged with play inside an unloaded portion of said rod; and - a threaded head screwed on said cylinder.