On July 20, 2002, at 1445 eastern daylight time, a Piper PA-28-180, N16319, operated by Berz Flying Service, Inc., as a rental airplane, returned from a local flight with substantial damage resulting from a crack through its vertical stabilizer spar. Visual meteorological conditions prevailed during the flight. The 14 CFR Part 91 personal flight was not operating on a flight plan. The private rated rental pilot was uninjured. The local flight originated from the Berz-Macomb Airport (UIZ), Macomb, Michigan, at 1330. The 1973 airplane, serial number 28-7305224, was used throughout its service life as a rental/flight training airplane. The airplane and vertical stabilizer accumulated a total time of 13,810 hours. According to maintenance personnel, the crack was not present during the airplane’s last annual inspection, which was performed 7 hours before the flight. The operator's mechanic had installed a doubler at the root of the rudder as a preventative measure against skin cracks. 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. Another Piper PA-28-180, N56136, serial number 28-7305538, operated by Berz Flying Service, Inc., returned from a local flight with a similar crack through its vertical stabilizer. The vertical stabilizers were sent to the National Transportation Safety Board's Material's Laboratory for examination. According to the NTSB's Materials Laboratory Factual Report, which is included in the docket of this report, the right sides of the spars and the right skin panels were locally deformed, consistent with buckling under compressive loads. The left side of the spars and skin panels were slightly deformed, also consistent with compressive loading. At the local deformation, the skin, skin web, and inner support brace were fractured in a generally horizontal plane. The fracture surfaces were on slant planes with a light gray appearance, features consistent with overstress fractures. The hardness values of the skin were typical for 2024-T4 aluminum alloy. The skin thickness, including paint, measured 0.0225 inches. The manufacturer’s skin thickness specification is 0.020 inches. A sample of the rivets attaching the skin panel to the rear spar were examined and measured. According to the manufacturer, the rivets were specified as MS20740-AD3-4 rivets. Paint was removed from the rivet heads and a small dimple was observed in the center of most of the heads, consistent with the specified marking for AD rivets. The bucked ends of the rivets appeared consistent in size and shape. The measured diameters of the bucked ends ranged from 0.154 - 0.166 inches. The Federal Aviation Administration and New Piper Aircraft, Inc., are parties to the investigation.