Following an engine shut-down event due to the loss of a fan blade, the fan rotates in an unbalanced windmill condition leading to undesirable structural excitation. The flow through a fan stage containing idealised damaged rotors is investigated numerically at the windmill condition. Specifically, predictions of rotational speed and flow are presented, which agree with undamaged engine scale test data, and the governing rotor flow features are understood. Results of an undamaged rotor show the tip and hub sections operate as a turbine and compressor respectively, with small flow deviation from the geometric exit angle. Introducing axisymmetric tip damage acts to remove blade sections that induce high positive whirl thereby reducing turbine work extraction, and in turn rotational speed. Non-axisymmetric tip damage is then considered to understand the impact of rotor-to-rotor flow interaction, between blades of different damage levels, upon the rotational speed. In all instances the non-axisymmetric rotational speed is higher than the average of the component axisymmetric speeds.