This paper proposes a post-simulation assessment tool that aims to improve the training of air traffic controllers (ATCOs) by visualizing their performance. The tool helps the controllers to identify bottlenecks in flight traffic and find alternative solutions that might improve traffic throughput. The usefulness of the tool was evaluated in a study involving benchmark tests and interviews with five experienced ATCOs. The results from the study indicate that the tool can help ATCO students to (1) identify irregularities in their work, (2) find possible underlying causes of these irregularities, and (3) find alternative solutions preventing these irregularities. Visual feedback consisting of workflow graphs and radar replays might generate valuable insights that enable self and peer assessment during ATCO training. Our results might be interesting both for the practitioners working with ATCO training and for researcher investigating the effects of visualization in education.