A stability and command augmentation system for an aircraft , comprising: - a first member moveable by a pilot input device 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 of said aircraft ; - a casing ; - a first and a second piston 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 ; 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 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 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 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 of said aircraft ; - a casing ; - a first and a second piston 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 ; - 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 . <EOS>