Linear actuator, in particular an aircraft brake actuator

A linear actuator comprising a body having a piston and a rotary electric motor mounted thereon, together with means which are interposed between the electric motor and the piston and which define a mechanical system that transforms the rotary movements of the motor into linear displacements of said piston, the electric motor being a vibration type motor comprising at least a stator and a rotor together with excitation means for deforming said stator and/or said rotor in modes of vibration that combine tangential vibration with normal vibration so as to drive the rotor in rotation, the means interposed between the electric motor and the piston including means which mechanically decouple said piston from said electric motor when the electrical power supply thereto is interrupted, said means comprising an electromagnet suitable for holding a complementary pole piece in a given position when it is powered, the actuator being characterized in that the piston is secured to a skirt suitable for sliding on the body in order to guide the piston in its axial displacement, the electromagnet and/or the pole piece acting on a part which becomes blocked by friction against the guide skirt which is secured to the piston, or on the contrary becomes disengaged, depending on whether or not the electromagnet is powered.