On February 15, 2011, at 0930 central standard time, a Beech E-90 airplane, N51DN, sustained substantial damage when the right aileron departed in-flight near Des Moines, Iowa. The airline transport pilot and pilot-rated maintenance crew member were not injured. The airplane was registered to and operated by D-W Corporation, Des Moines, Iowa. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 test flight. The local flight departed the Des Moines International Airport (DSM), Des Moines, Iowa, at 0900. The airplane had recently underwent maintenance, which included a 800-hour inspection of the ailerons requiring removal, reinstallation, and a functional check. Prior to the test flight, the pilot performed a pre-flight inspection of the airplane, and a flight control free and correct check with no anomalies noted. The airplane departed runway 13L, and the pilot made a right turn to 160 degrees. Shortly thereafter, the pilot made a right turn to 270 degrees and was cleared to flight level (FL) 180. While level at FL180 with the autopilot engaged, the pilot and maintenance crew member performed various checks with the engines and flight instruments. After completing the checks, the pilot requested a left 180-degree turn back to DSM. Air traffic control approved the turn and the pilot selected the autopilot heading switch for a left turn to DSM. Approximately 140 degrees into the turn, the autopilot jerked, stabilized, and jerked again during the turn to level off. The pilot then noticed the right aileron had departed the airplane. The airplane landed at DSM without further incident. Examination of the airplane revealed the inboard and middle airframe aileron hinge brackets were attached to the aft spar, intact, and showed no visible damage. The outboard airframe aileron hinge bracket was pulled and separated from the aft spar. The aileron was not located. The aileron is attached to each of the three hinge brackets with two upper and two lower bolts (4 total) that engage into nut plates that are mounted on the aileron. The King Air 90 Series Maintenance Manual aileron installation procedure states, in part, "Carefully align the three hinges with the aileron and install the bolts in each hinge bracket and the aileron. Pull on the aileron straight away from the wing. If any movement is detected, carefully check the bolt installation."