Incident Details
On April 11, 2011, at 1015 central daylight time, a Beech 90B (registration N37OU) sustained minor damage when two of the three right aileron hinges became disconnected while the airplane was in the traffic pattern at 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 landed uneventfully on runway 35. The CFI, commercial student pilot, and two pilot-rated passengers were not injured. The airplane was operated by the University of Oklahoma under 14 CFR Part 91 for an instructional flight. Visual meteorological conditions prevailed; the flight departed at 0915 on a local instructional flight with no flight plan filed.
Maintenance History
Maintenance records indicate the airplane had undergone a phase inspection on April 1, 2011. During that inspection, the right aileron was removed to repair light surface corrosion and was reinstalled. The incident occurred 5.3 flight hours after this maintenance.
Post-Incident Inspection
Following the incident, the operator's mechanic inspected the airplane and found that the two inboard aileron hinges had separated from the airframe. The mechanic determined that the eight screws (four upper, four lower) attaching the aileron to the hinge points were seated in the aileron skin but did not engage the corresponding nut plates on the hinge points. The aileron itself was undamaged. The mechanic properly attached the aileron to the hinge points, and the airplane was returned to service.
Manufacturer Procedures and Communications
The Beech King Air 90 Series Maintenance Manual (Chapter 27-10-00-201) provides aileron installation procedures. Step B states, "Carefully align the three hinges with the aileron and install the bolts in each hinge bracket and the aileron." Step C states, "Pull on the aileron straight away from the wing. If any movement is detected, carefully check the bolt installation."
In 2003, Raytheon Aircraft Company issued King Air Communique No. 2003-13, noting reports of MS27039-1-11 screws missing nut plates during aileron installation. The communique stated, "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."
Related Investigation
The National Transportation Safety Board (NTSB) is investigating a similar accident involving a Beech E-90 on February 15, 2011, near Des Moines, Iowa (CEN11LA192), where the aileron departed the airplane in flight after recent maintenance. The pilot in that event reported the airplane was "very controllable" and landed without incident.