Commercial airports are under increasing pressure to comply with the Eurocontrol Collaborative Decision Making (CDM) initiative, to enable overall airspace improvements. An important element of a CDM system is the provision of automated decision support to aid the controllers to schedule the take-off times and the associated times at which aircraft should push back from the stands. The CDM system then aids effective operations by communicating these scheduling decisions to other relevant parties within the airport and the airspace. One of the major CDM components is aimed at predicting the target take-off times; for medium-sized airports, a common choice for this is a “PreDeparture Sequencer” (PDS). Here we describe the design and requirements challenges which arose during our development of a PDS system. Firstly, the scheduling problem is highly dynamic and event driven. For example aircraft can be delayed or runway capacity can change, and this requires a careful separation of data ownership responsibility between the system components and special attention to integrity constraints. Secondly, it is important to endusers that the system be predictable and, as far as possible, and transparent in its operation, with decisions that can be explained. These human factors, which influenced the choice of methods for solving the problem, are also explained in this abstract, along with the consequent decisions which were made.