Developing standards for time-based sequencing & separation of aircraft

Abstract

For decades, safe Air Traffic Management services have been provided by maintaining sufficient distance between aircraft to ensure adequate reaction time for collision avoidance. With RADAR and voice communication as fundamental tools, separation requirements have evolved that recognize the relationship between speed and time, and convert these requirements to distance-based standards. This paper reviews physical relationships between altitude, airspeed, and environmental variables that determine the collision avoidance time between aircraft separated by existing distance-based standards to expose the inherent weaknesses of the conversion and establishes an alternative: time-based separation standards. Using data gathered through flight tests and simulations over the past two years, the performance of a modern aircraft Flight Management System (FMS) has been characterized by a distribution that expresses the probability of deviations from an assigned Required Time of Arrival (RTA). Using this distribution, a mathematical relationship between the probability of a collision and the time interval between aircraft has been developed. The equations allow the evaluation of collision probability for any given interval, or alternatively, allow calculation of a required time interval between aircraft to achieve any desired level of collision probability. These calculations then serve as the basis for development of a precise performance metric that may be used as a certification standard for RTA capable systems. The paper concludes with a number of suggestions for possible refinements to current air traffic management systems. Graphical depictions of symbology adaptations designed to provide an intuitive display of separation time between aircraft are presented along with possible innovations in implementation strategy for time-based sequencing and separation methods. Additionally, several procedural changes are described that provide modest immediate benefits and may satisfy pre-requisites to implementation of time-based operations.