Abstract: A hydraulic jack, for lifting a heavy-weight structure such as a wing of an airplane, comprises a vertical primary cylinder, a ram vertically movably inserted in the primary cylinder, a lock nut threadedly mounted on an external thread of the ram, a first gear provided on the lock nut, a nut rotatably supported in a housing, a second gear provided on the nut and releasably meshing with the first gear of the lock nut, and a screw rod extending parallel to the ram and connected to the ram via a bracket. The screw rod has a second external thread and threadedly extends through the nut for vertical movement.