Corrective transmission system operation can help integrate more renewable energy sources and save redispatch costs by providing a higher utilization of the power grid. However, reliable and fast provision of flexibility are key to achieve corrective operation. This work develops a new method to determine if flexibility from distribution grids is available on transmission corridors when needed. An analysis of the German energy system in the year 2030 is performed to estimate the potential of different flexibility options and shows the potential flexibility distribution systems can contribute to a corrective transmission system operation.