A linear actuator includes a screw shaft comprising a screw thread and having a longitudinal axis A, a nut movable along the screw shaft from a retracted position to an extended position, and a plurality of rollers movable with the nut. Each roller includes a cylindrical surface configured to roll along one or more flanks of the screw thread, such that rotation of the screw shaft causes the rollers to roll along the flank(s) so that the nut translates in an axial direction along the screw shaft. The screw thread has a variable lead angle. The actuator can be part of a power door opening system of an aircraft.