Open-loop planning has been a popular approach for developing aircraft collision avoidance systems. Open-loop planning computes a future plan to follow without anticipation of how future observations can affect the future course of action. Closed-loop planning, in contrast, takes into account the ability to react to future information. This paper explores trade-offs that exist between the two strategies as they apply to aircraft collision avoidance. It demonstrates some of the performance gains that con be realized by adopting a closed-loop planning strategy.