A prerequisite for the prediction of ice accretion on an aircraft flying through clouds of supercooled liquid water is the accurate determination of the water impingement rate on various components of the aircraft. For this purpose, a droplet impingement module has been developed using the datastructure of the unstructured Navier-Stokes solver TAU. Since nowadays large computational grids are common practice, an efficient algorithm for determination of the droplet trajectories on such grids had to be implemented. This paper describes the physics and details of the implemented numerical algorithm. It summarizes lessons learned during development. The paper concludes with the presentation of code validation results and examples of applications.