On May 01, 2001, at 1437 central daylight time, a Douglas DC-9-31, N9333, operated by Northwest Airlines, Inc. as flight 682, was substantially damaged when it was struck by an aircraft tug during passenger boarding. The 14 CFR Part 121 flight was parked at gate D3 at the Minneapolis-St. Paul International/Wold-Chamberlain Airport, Minneapolis, Minnesota and was bound for the Philadelphia International Airport, Philadelphia, Pennsylvania. No injuries were reported by the 4 crewmembers or 38 passengers that had boarded the aircraft. The driver of the tug and a food service worker received minor injuries. The aircraft was parked and was being prepared for departure by various ground workers including the tug driver and workers restocking the aircraft food supply. The driver of the tug said that he was moving the vehicle into position in order to connect the aircraft tow bar in preparation for pushback. He said that when he placed the tug into gear, it lurched forward into the parked aircraft. He said that pieces of the aircraft protruded through the windshield of the vehicle and pinned him into his seat. He said that he was unable to shift the vehicle into reverse and his leg was pinned on the accelerator pedal. Other ground workers in the area attempted to shut off the tug's engine. The driver said that the vehicle continued to drive forward until the engine was finally shut off. During the event, the pilot of the aircraft had applied the brakes when he noticed the unplanned movement of the airplane. The aircraft was pushed backward about 30 feet causing damage to the nose section of the fuselage. A food service worker was injured when he jumped from his vehicle which was parked next to the aircraft. Subsequent to the accident, the tug was placed on jacks and a check performed. During the check it was found that the normal engine shutoff switch would not shut the engine off if the engine was operated at high throttle settings. It was further discovered that at high throttle settings, the brake system was not able to stop the rotation of the drive wheels. No anomalies were found during this test that would explain the lurching described by the tug driver. Subsequent to the testing, the throttle system of the tug was replaced as a precautionary measure by the airline. During the course of the investigation, it was found that the tug had been involved in a previous incident where an aircraft was damaged. A report of the previous incident was obtained. The driver of the tug during the previous incident reported that the tug lurched when he was attempting to move the tug into position to connect to the aircraft. The manufacturer of the tug said, during a telephone interview, that they do not have records of operational problems associated with that model tug. He also stated that he was not aware of a history of lurching problems concerning the model tug in question. It was found that the distance from the cab of the tug to the nose of a DC-9 aircraft when the tow bar is attached is 4 feet 2 inches.