On April 11, 2011, at 1015 central daylight time, a Beech 90B, N37OU, sustained minor damage when two of the three right aileron hinges became disconnected while the airplane was in the traffic pattern at the Chickasha Municipal Airport (CHK), Chickasha, Oklahoma. The certificated flight instructor (CFI) took control of the airplane, made a left turn from the downwind leg, and made an uneventful landing on runway 35 (5,101 feet by 100 feet, concrete) at CHK. The CFI, the commercial student pilot, and two pilot-rated passengers were not injured. The airplane was registered to and operated by the University of Oklahoma as a 14 Code of Federal Regulations Part 91 instructional flight. Visual meteorological conditions prevailed at the time of the incident. The flight departed a 0915 on a local instructional flight, and no flight plan was filed. Maintenance records indicated that the airplane had undergone a phase inspection on April 1, 2011. During the phase inspection, the right aileron was removed to repair some light surface corrosion and was reinstalled on the airplane. The incident occurred 5.3 hours after the maintenance was performed. The post incident inspection of the airplane by the operator's mechanic revealed that the two inboard aileron hinges were separated from the airframe. The mechanic determined that the screws (8 screws, 4 upper, 4 lower) which attached the aileron to the aileron hinge points were seated in the aileron skin, but did not screw into the corresponding nut plates on the aileron hinge points. The aileron was not damaged so the mechanic attached the aileron properly to the aileron hinge points and the airplane was returned to service. The Beech King Air 90 Series Maintenance Manual, Chapter 27-10-00-201, P/N 90-590012-13, Revision B22, Nov. 2010 provides procedures for aileron installation. Step B of the procedure states, "Carefully align the three hinges with the aileron and install the bolts in each hinge bracket and the aileron." Step C of the procedure states, "Pull on the aileron straight away from the wing. If any movement is detected, carefully check the bolt installation." In 2003, the Raytheon Aircraft Company issued King Air Communique No. 2003-13. The communique stated in part the following: "Raytheon Aircraft Company Technical Support has received reports that during aileron installation the MS27039-1-11 screws missed the nut plates on the aileron hinge points. This condition can go unnoticed. Some operators have painted witness marks on the aileron hinge brackets to give technicians a visual cue that installation is not correct." The National Transportation Safety Board (NTSB) is currently investigating a similar accident that occurred on February 15, 2011, near Des Moines, Iowa (CEN11LA192). In that accident, the aileron of a Beech E-90 departed from the airplane in-flight during a "return for operation" flight. The airplane had recently undergone maintenance to the aileron. The pilot reported that the airplane was "very controllable" and the airplane was landed without incident.