We investigate some domain decomposition techniques to solve large scale aerodynamics problems on unstructured grids. Where implicit time advancing scheme are used, a large sparse linear system have to be solved at each step. To obtain good scalability and CPU times, a good preconditioner is needed for the parallel iterative solution of these systems. For the widely-used Schwarz technique this can be achieved by a coarse level operator. Since many of the current coarse operators are difficult to implement on unstructured 2D and 3D meshes, we have developed a purely algebraic procedure, that requires the entries of the matrix only