This paper presents the result of a 6 month research project conducted at the Maintenance Control Center of KLM (Royal Dutch Airlines) Engineering & Maintenance (E&M). The research focuses on the development of a monitoring/forecast tool to anticipate days with a high risk of network disruption. A theoretical framework is formulated based on the combination of Lean and the effectiveness-efficiency trade-off. A model is developed to evaluate operational robustness: the ability of the airline operation to absorb disruptive events and ensure a punctual operation. For each of the parameters of the model an analysis is performed to determine its relation with operational robustness. Flexibility is shown to be a key determinant. The parameters are integrated into a monitoring/forecast tool with a 2 week time horizon. Through a monitoring phase the tool is shown to successfully forecast days with high and low risk. The monitoring phase points out that further calibration of the critical values of the tool is required. Based on the output of the tool and the findings of the analyses, ways to increase operational robustness are suggested.