Designing freedom into trajectory-based operations

Abstract

Modern Flight Management Systems (FMS) are able to predict highly precise 4D trajectories for complete flights from take-off to touchdown. The generated trajectories take into account aircraft’s performance parameters, altitude-, speed- and time constraints, given procedures, and the expected weather conditions, as forecasted. Based on these trajectories, a traffic scheduler can generate and guarantee conflict-free traffic – as long as every aircraft sticks to the plan. However, even the most accurate trajectory becomes unrealizable when significant disturbances arise that were not foreseeable at prediction time. Uncertainties like imprecise weather forecasts, inaccurate departure times, and issues with delayed passengers influence a direct implementation of pre-planned 4D schedules in practice. If such disturbing events are rare, the overall plan may be adapted. Too many disturbances might ruin the whole schedule though. This article discusses the trade-off between built-in robustness in the initial planning stage and the obtainable efficiency. Resilience is added by increasing the separation between trajectories. Assuming that the mandatory distance for guaranteeing safe operations remains constant, the additional separation provides freedom for aircraft following their trajectories.