To assess the e↵ect of flights departing within the horizon of the extended arrival manager (socalled pop-up flights), fast-time simulations were performed using an arrival manager research model. This model is tailored for operations at Amsterdam Schiphol Airport and integrated in BlueSky, an open-source air tra c management simulator. Simulation results show a significantly negative impact of pop-up flights on extended arrival management, in terms of flight crew and air tra c control task load, sequence stability and delay (cost). This impact could be mitigated by pre-planning pop-up flights prior to departure, using their take-o↵ time estimates. This will, however, only be beneficial when these estimates are su ciently accurate (better than two minutes). With currently achievable accuracies, it is better to discard these estimates in the context of extended arrival management.