Abstract: Hydraulic actuator comprises a piston-cylinder which has a fail-safe feature in the form of a lost motion coupling between the cylinder head and portions of the cylinder. The coupling comprises a constraining rod which is secured to the cylinder at the opposite end thereof from the cylinder head. The constraining rod extends into a bore in the piston rod and has a shoulder which is opposed to a shoulder on the piston. The amplitude of the lost motion is such that the coupling does not interfere with normal operation of the actuator but is effective in the event of failure of the cylinder or cylinder head. The lost motion coupling would prevent movement of any parts broken away by the fracture any significant distance from the remaining parts of the cylinder. An energy absorbing means is provided in the lost motion coupling. A second lost motion coupling is provided between the end of the cylinder and the external surface of the cylinder head.