On November 4, 2005, about 0830 mountain standard time, an experimental Lee Kolb Mark III Classic, N515AB, suffered an in-flight right horizontal stabilizer attachment failure at Ryan Airfield Airport (RYN), Tucson, Arizona. The pilot operated the borrowed airplane under the provisions of 14 CFR Part 91. The airline transport certified flight instructor (CFI) pilot, and the student pilot undergoing instruction (PUI) were not injured; the airplane sustained substantial damage. The instructional flight departed RYN about 0830. Visual meteorological conditions prevailed, and no flight plan had been filed. In a written statement to the National Transportation Safety Board, the pilot stated that the accident occurred on the downwind to landing phase of the traffic pattern. The instructional flight departed with the intention of practicing takeoffs and landings. On the right downwind leg they heard a "loud bang." The airplane lurched nose up, then nose down. The flight instructor took control of the aircraft and informed the tower he had a "serious" control problem. The pilot was able to land the aircraft using cross control inputs. After landing, the CFI was able to taxi safely to parking. Upon inspection of the airplane by the CFI, it was determined that the right horizontal stabilizer had separated from the fuselage due to a stainless steel "L" bracket failure. In the CFI's Description of Damage to Aircraft and Other Property section of his written report, he indicated that a bolt in the "L" bracket was found slightly loose, "which allowed a slight vibration and this caused metal fatigue over its 545 [hours] of flight [time]." ADDITIONAL INFORMATION As a result of the accident, the pilot recommended to other Kolb owners (via the Kolb Kolb owner website) to replace the "L" brackets at the 250-hour inspection and to install a second set of stabilizer cables on the horizontal stabilizer.