American Institute of Aeronautics and Astronautics 22

The Reduced Spacing Operations (RSO) subelement of TAP, at the Langley Research Center, is developing the Aircraft Vortex Spacing System (AVOSS) which is described by Hinton1. The purpose of the AVOSS is to integrate current and predicted weather conditions, wake vortex transport and decay knowledge, wake vortex sensor data, and operational definitions of acceptable strengths for vortex encounters to produce dynamic wake vortex separation criteria. By considering ambient weather effects on wake transport and decay, the wake separation distances can be decreased during appropriate periods of airport operation. With the appropriate interface to CTAS, spacing can be tailored to specific leader/follower aircraft types rather than just a few broad weight categories of aircraft. In a manual ATC system, a simplified form of the AVOSS concept may be used to inform ATC when a fixed alternate, reduced wake separation standard becomes safe for the ?large? and ?heavy? aircraft categories.

The AVOSS development program has as its target a field demonstration of a prototype AVOSS system in the year 2000. To support this goal, current plans include three increasingly complex AVOSS field deployments to be conducted at the Dallas-Fort Worth International Airport. This paper describes the AVOSS concept and development program, the related wake vortex modeling effort, the wake vortex hazard definition studies, and the wake vortex sensors development.

AVOSS CONCEPT

AVOSS is envisioned as an automated process which combines meteorological data, rules describing the atmospheric modification of wakes, and aircraft and airspace operational procedures to provide dynamic wake vortex separation constraints to ATC. The AVOSS system concept by Hinton borrows from previous efforts conducted in the 1970's by Eberle2 et al. The philosophy behind the AVOSS system is to avoid aircraft encounters with vortices above an "operationally acceptable strength." This avoidance is obtained through consideration of two factors, wake vortex motion away from the flight path of a following aircraft and wake vortex decay. Since these factors are highly dependent on ambient meteorological conditions, as well as the generating aircraft position and type, the wake vortex constraints on aircraft separation are expected to vary significantly with the weather. The AVOSS will quantify the wake separation required for generator/follower aircraft pairs during final approach, or the initial climb, and provide

this matrix to an automated ATC system such as CTAS described by Erzberger3 et al. Capacity gains can be expected due to considering both wake transport and decay, and also by providing a large matrix of aircraft pair separations to ATC automation rather than just the three category system utilized in today?s system.

The wake separation constraints will be delivered to the automated ATC system with adequate lead time and stability to be used in the process of metering and spacing. Final approach aircraft spacing may be established 5 to 10 minutes prior to landing, while the metering rate at which aircraft are accepted into the TRACON is established earlier. Since a current weather observation will frequently not reflect the wake situation 5 to 50 minutes in the future, an effective AVOSS must utilize short-term weather predictions (nowcasting) to provide the lead time required for increasing terminal area capacity. Although AVOSS/CTAS interface simulations have not yet been performed, the concept currently envisions a zero to 15 minute weather prediction being used to establish individual aircraft pair spacing for final approach and a 30 to 60 minute prediction being used to regulate the rate at which aircraft are accepted by the TRACON facility from enroute airspace. This nowcasting capability, coupled with CTAS and the AVOSS predictor capability, will ensure that adequate aircraft are available for approach when minimal spacing is possible and the arrival rate is reduced when larger spacing is required to avoid inefficient low altitude path stretching and holding. This weather predictive requirement will drive all efforts in the development areas of meteorological sensors and system architecture. The automated nowcasting element is an important difference between the AVOSS concept and earlier concepts that proposed to utilize only real-time surface weather observations to regulate final approach spacing.

A number of ground rules will be followed during the AVOSS development. The development effort will be focused on a practical system that can be approved for operational use. This will require a large degree of robustness, reliance on readily available meteorological and wake sensors, graceful system degradation when sensors or subsystems fail, and cost realism. The safety provided must be equal to or greater than the current system. The AVOSS will not require an increment in pilot skill levels or training requirements, nor any aircraft structural or on-board systems modifications. The AVOSS will not alter current pilot functions nor change airborne/ground responsibilities. ATC controllers will not be required to monitor or predict weather conditions. During peak traffic demand