Online ULD Build-Up Scheduling

Abstract

Air cargo terminals must be able to maintain effective cargo preparation and allocation, even when faced with external disturbances such as delays in arrivals or departures. Unit Load Device (ULD) build-up is especially vulnerable to such disturbances because it depends on gradually arriving cargo, limited workspace capacity, and strict flight deadlines. This paper develops an online ULD build-up scheduling model with gradual look-ahead, and evaluates its contribution to air cargo terminal robustness using an embedded simulation-optimization framework. A Discrete-Event Simulation (DES) model represents terminal operations and disturbance propagation, while a time-indexed mixed-integer programming model generates rolling-horizon build-up schedules using updated terminal state information and, when available, estimated knowledge on future disturbances. The online scheduling model is benchmarked against three other configurations under different disturbance scenarios. Results show that online scheduling improves the invariance of a terminal to external disturbances, with the strongest evidence coming from full system response tests. Schedule adherence within the terminal only decreased with 0.05 to 0.35 percentage points when using the online scheduler, compared to 5.28 to 8.44 points for the current representation. The main robustness driver is the periodic re-optimization using terminal state information, rather than the gradual look-ahead mechanism on its own. The results produced can be greatly improved by using a large optimization window for the online scheduling model.