The airplane returned from a local flight with substantial damage resulting from a crack through its vertical stabilizer spar. The airplane was used throughout its service life as a rental/flight training airplane. The airplane and vertical stabilizer accumulated a total time of 14,155 hours. According to operator's maintenance personnel, the crack was not present during the airplane’s last annual inspection, which was performed 10 hours before the flight. The crack is located approximately 6 inches above the base of the vertical stabilizer and extends horizontally approximately 1 1/2 inches from the right side of the stabilizer into the 3 1/2 inch spar. The maintenance personnel had installed a doubler at the root of the rudder as a preventative measure against skin cracks. Examination of the fracture surfaces were consistent with overstress fractures.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

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