Skyway Airlines Flight 1233

The flight crew of the incident aircraft reported lateral control problems shortly after takeoff, noting that about 45 degrees of right control wheel deflection was required to maintain straight flight. They noted no other anomalies and seemed to have full flight control authority. However, as a precaution, the flight crew elected to return and land at the departure airport. The crew reported that the landing was normal with the exception of the required control wheel offset. A post incident examination of the aileron control system revealed a 360º fracture in the silver-brazed joint between the drive sprocket and its mating shaft of the left-hand aileron control sprocket assembly. The failure of the brazed joint allowed both control wheels to be rotated approximately 45 degrees from neutral without a corresponding aileron control surface response. NTSB metallurgical examination of the left-hand sprocket assembly revealed that it was not brazed according to the referenced specifications. Specifically, the examination noted that the braze filler material had been introduced from both ends of the joint, resulting in a lack of penetration of about 20 percent of the total joint area. The applicable specification states, "the filler metal shall be introduced at one edge...and shall flow by capillary action to fill the interstice." The NTSB's review of the incident aircraft's maintenance records showed that a flight data recorder (FDR) functional check was completed the night before the incident flight. The roll control parameter, which is designed to measure the aileron surface position, was one of the parameters checked. To verify the accuracy of this parameter, the left control wheel is rotated either clockwise or counterclockwise until the ailerons contact their respective up or down stops. When the stops are contacted, the control wheel is held in position and the FDR roll control parameter value is checked against an approved maintenance manual limit. The control wheel is then rotated in the opposite direction and the FDR parameter is verified. The mechanic who conducted the functional check reported that the FDR functional check was routine. NTSB analysis of available recorded FDR data, incident flight and over 83 flights before the incident flight, showed that the maximum values recorded for the FDR roll control parameter prior to the functional check were 14.6 degrees aileron trailing edge down (TED) and 23.4 degrees aileron trailing edge up (TEU). The recorded aileron TED value was not in the range required by the approved operators B1900D maintenance inspection procedures document. The analysis also showed that, during the FDR functional check, the maximum values recorded for the roll parameter exceeded the maximum values recorded prior to the incident by 1.9 degrees in the aileron TED direction and 1.0 degree in the aileron TEU direction. This was the only time that both FDR values for the roll control parameter were in the TED range required by the functional check. The aircraft manufacturer and the NTSB tested the aileron control system on an exemplar Beechcraft 1900D aircraft in Wichita. The purpose of the test was to determine the amount of tangential force, applied to the control wheel, required to change the FDR roll control parameter value by 1.9 degrees in the aileron TED direction. Results of the testing showed that an additional tangential force of 80 pounds applied to the control wheel, after it was rotated such that an aileron was resting against its control surface stop, increased the roll parameter value by 0.7 degrees in the aileron TED direction and 1.3 degrees aileron TEU. Additional force was not applied to the control wheel because the limit load for the aileron control system is 80.4 pounds. The airplane manufacturer provided a Beechcraft 1900D Engineering report that documented tests that were conducted on the aileron control system to demonstrate compliance with the Code of Federal Regulations. Test data show that the aileron system did not yield while applying a limit tangential load of 80.4 pounds to the control wheel or fail when an ultimate tangential load of 120.6 pounds was applied to the control wheel. The aileron control system, including the brazed joint on the sprocket assembly, met all Federal Aviation Administration airworthiness standards for commuter-category aircraft. The investigation concluded that the failed sprocket assembly significantly reduced the pilots' control of the ailerons because the failure affects pilot input from both control wheels.

Verbatim NTSB analysis and probable cause from the NTSB dataset