Analysis of Top of Descent (TOD) uncertainty

Abstract

As the National Airspace System (NAS) transitions to Trajectory-Based Operations (TBO), it is becoming increasingly important to understand the factors that contribute to differences in aircraft trajectories. Mid-term concepts include enhanced automation capabilities in the form of Decision Support Tools (DSTs) that rely on accurate predictions of aircraft trajectories to increase the efficiency and safety of the NAS by improving adherence to metering schedules and minimizing the number of interactions controllers take in streamlining flight operations. However, the benefits afforded by these concepts are only as good as the accuracy of their underlying trajectory modelers. Specifying a trajectory modeler that provides a sufficient level of accuracy is a non-trivial task, due to the vast number of factors that can affect a trajectory, many of which are accompanied by uncertainty. One area of the flight trajectory in which inaccurate trajectory predictions can be particularly problematic centers around an aircraft's Top of Descent (ToD) point, where loss of separation can occur very quickly if an aircraft begins its descent earlier or later than a controller might expect (or was led to believe by automation). As such, there is a need to identify and understand the significant factors that can affect the location of ToD for a wide range of aircraft types and configurations, as well as a better understanding of the magnitude of the differences attributed to these factors. The MITRE Corporation's Center for Advanced Aviation System Development (CAASD) conducted an analysis to begin identifying significant factors that contribute to ToD variability within an aircraft model, and the associated magnitude of that variability. It is widely understood that wind and aircraft weight are two factors that significantly contribute to ToD location. However, the contribution of other factors to ToD location is less understood. This analysis examined three factors of interest for Boeing 737 (B737) aircraft to determine their influence on the location of ToD. The factors examined include the specific series of B737, the presence of winglets, and equipped engine thrust. Wind and aircraft weight were held constant to isolate the effects of the factors of interest. The data for the analysis was generated using CAASD's General Electric (GE) Aviation Flight Management System (FMS) test bench, which simulates the actual hardware and software systems of a B737. Twenty-six configurations of B737 aircraft, which included various combinations of series (600, 700, 800, and 900 series), winglets (with and without), and equipped engine thrust values, were flown via the FMS test bench in a defined scenario. Results of the analysis indicate that two of the factors, the series of B737 and the presence (or absence) of winglets, are likely to be significant factors that can affect the location of ToD for B737 aircraft. The B737's equipped engine thrust was seen to be less significant. There are, potentially, many other factors that can affect ToD location, such as pilot-configurable settings within the FMS. Further analysis is needed to determine the impact that these additional factors may have on ToD location. This analysis also examined only a single cruising altitude of 37,000 ft. It may be beneficial to execute similar scenarios at other cruising altitudes to observe the effect that altitude may have on the magnitude, or on the relative significance of the factors, with respect to ToD location. Further analysis is also suggested for other aircraft types, beyond the B737, to determine if similar findings hold true for other aircraft types. Understanding the factors that affect ToD location, and their magnitude of effect, can be used to help to improve the accuracy and performance of trajectory modeling algorithms for current and future automation tools, and to identify areas where alternative mitigation strategies to account for that uncertainty may be required.