Wake Vortex Avoidance Concept

Domain: Terminal
Principle Investigator: NASA Langley Research Center

(see larger image)

(see larger image)

One aspect of national airspace system operations that holds potential for improvement is the separation criteria applied to aircraft for wake vortex avoidance. Wake vortices are generated during all phases of flight, as a by-product of a wing that is generating lift. Wake vortex encounters have caused injuries as well as loss-of-control accidents, particularly for smaller aircraft following larger aircraft, and must be avoided. The problem is most critical in terminal areas due to a combination of low altitude and high-density operations. Decades of research on wingtip vortices have shown that although the initial position and strength of a wake is dependent on physical parameters of the generating aircraft (weight, speed, wingspan, etc.), the wake position, movement, and strength persistence over time depend on local meteorological parameters (winds, turbulence, and temperature).

The Federal Aviation Administration (FAA) aircraft separation criteria for wake vortex avoidance were empirically developed and provide adequate spacing in worst-case weather conditions. There is no provision to reduce spacing when the weather conditions are conducive to early wake dissipation or advection from the following aircraft's flight path. Under many ambient conditions, such as moderate crosswinds or turbulence, wake hazard durations are substantially reduced. Field data has indicated that for the conditions at one particular airport, spacing is not constrained by wake hazard 2/3 of the time. In addition, current procedures at airports with parallel and intersecting runways could be made more efficient if uncertainties in wake behavior are quantified. To realize the potential gain in efficiency, NASA Langley has developed a proof-of concept Aircraft Vortex Spacing System (AVOSS). Successfully operated in a real-time field demonstration during July 2000 at the Dallas/Fort Worth International Airport, AVOSS is a novel integration of weather sensors, wake sensors, and analytical wake prediction algorithms. Predicted gains in airport throughput using AVOSS spacing as compared to the current criteria averaged 6%, with peak values approaching the theoretical maximum of 16%. The average throughput gain translates to 15-40% reductions in delay when applied to realistic capacity ratios at major airports.

The core idea of a Wake Vortex Avoidance Solution (WakeVAS) is a modification to the current aircraft separation standards for wake avoidance. The modification may be static or dynamic. A means of sensing the local weather in a terminal area and deriving expected wake behavior from these conditions is required. The wake behavior is derived from wake prediction models, which take into account the characteristics of the generating aircraft and the ambient weather conditions. Field sensors such as LIDAR and windlines are used to make wake observations. Procedures for wake avoidance may be derived from the wake observations and predictions, and may be static or dynamic depending on whether they are valid over the entire predicted and observed range of wake behavior or change with changes in the terminal weather conditions for optimum efficiency.

Independent of the technological or procedural content of a wake vortex avoidance solution, the basic functions are summarized in the flowchart above. The region of protected airspace depends on the terminal operation to be addressed. Various terminal configurations are shown in the graphic above. Ensuring a region is free of wake hazard could be realized procedurally based on historic weather or wake characterization or could be as complicated as a real-time prediction and monitoring system. Some type of safety monitor, either an active wake sensor or procedurally a set of circumstances, will be used to decide if the reduced spacing procedure can be continued or a more conservative procedure be applied.

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