On July 15, 2002, at 1457 Pacific daylight time, a Schweizer 269C-1, N20256, sustained an in-flight fracture and separation of the center aft mast support tube about 10 miles east of Concord, California. The certified flight instructor (CFI) and the dual primary student were not injured; the helicopter sustained substantial damage. The helicopter was owned and operated by Helicopter Adventures, Inc., for a local area dual instructional flight under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed and no flight plan had been filed. In a written statement, the flight instructor reported that they were in level cruise flight en route to the practice area when a slight but unusual vibration was felt through the airframe. As he was reaching for the controls, the vibration level suddenly became severe. The left door departed the helicopter and the flight instructor entered an autorotation. During the descent, he looked back and found the tail rotor was disabled. The autorotation was completed to a field. A Federal Aviation Administration (FAA) airworthiness inspector examined the helicopter. He observed that the aft center mast support tube (p/n 269A2172-5) had fractured. He noted that a dark corrosion like appearance was present over a large part of the center of the fracture face. The part was painted with an epoxy coating and the inspector opined that this would have hindered visual detection of any crack in the underlying metal. He also reported that the tail rotor drive shaft had separated just aft of the oil cooler, and stated his belief that this was a secondary event to the vibration. The main rotor mast support tube was sent to the National Transportation Safety Board Materials Laboratory Division, Washington, D.C., for examination. The complete metallurgical examination report is in the docket for this accident. The examination revealed a fracture surface, relatively smooth and flat, in a plane perpendicular to the longitudinal axis. A region of the fracture had a reddish brown corrosion layer with smooth curving boundaries; a feature consistent with fatigue. Closer examination using scanning electron microscopy (SEM) of the fracture surface origin area, revealed three crack arrest lines and further corrosion. Scratches were also observed on the left side of the aft support tube in the paint at the lower edge and near the origin of the fracture surface.