As a time reversal protected insulator ($Z_2$ insulator), we can argue that edge states are stable when there exists disorder because time revesal symmetry makes some dynamical matrix elements vanish. But when refer to a time reversal symmetry breaking Chern insulator, how can we show the robustness of its edge states? Someone just state that those modes are stable for there is no other back mode in the edge. But there can be several edge states in one edge and with velocity in different directions. How can you then just say that there is no mode for backscattering. And how can you guarantee that no edge state can scatter into some bulk states when disorder is strong. Or is there some other argument more clear and percise?