The thrust to propel the aircraft forward comes from its propellers or jet engines. Reverse thrust for backing up can be generated by thrust reversers such as on the Boeing C-17 Globemaster III, or reversible pitch propellers such as on the Lockheed C-130 Hercules, a procedure known as powerback. Most aircraft, however, are not designed to back up on their own and must be pushed back either by hand or by using an aircraft tug.