Time-based Spacing for 4D Approaches using Speed-Profiles

Two of the major projects in ATM development, SESAR and NextGen, both fore- cast the use of 4D trajectories as an intermediate phase in the development of full Performance Based Trajectories. Using 4D trajectories, the full positional and time coordinates of the aircraft are known throughout the planned trajectory. During approach, when reduced separation minimums are applied, the accuracy of this profile is most important to ensure a safe approach to the runway. One implementation of 4D approaches is by using Required-Time of Arrival (RTA) to separate aircraft during approach. The latest Flight Management Computers are capable of calculating a flight-path w.r.t. to a RTA. This paper describes the amount of time error that can occur during approaches where an RTA is set at the runway threshold that could still be resolved by increasing or decreasing the speed-profile. The minimum and maximum bounds are referred to as control space. Using simulations, the recoverable time error is calculated. Lateral trajectories from Amsterdam Airport Schiphol, different wind conditions and two different aircraft types were included to investigate different factors influencing the time error, such as aircraft type, speed restrictions and wind. Finally, the paper discusses a new method to control time-based spacing using a closed-loop speed controller