The engineering formulas recommended by the Chinese aviation reference materials used for estimating the natural frequency of a bar in the aircraft control system designing are similar in form. However, due to the difference in the dimensional units of the quoted parameters in each formula, the estimated results of the natural frequency of a bar are obviously distinct from each other. Based on the estimated results, the description about how the bar in the control system resonates with the engine speed is not appropriate for the aircraft design in my viewpoint. Based on the mechanical vibration theory, this paper has derived the engineering formula for computing the natural frequency of a bar using the metric dimension parameters. What's more, this paper has analyzed the natural frequency of a non-uniform cross section bar by the method of Nastran software.