Wake turbulence—the disturbed air immediately behind an aircraft—is potentially hazardous to following and nearby aircraft if not operationally accounted for. To ensure safety, wake separation standards are devised to alleviate the adverse effect of wake turbulence. This has increasingly become a significant issue at the crowded space on and above runways, as it negatively affects capacity.

For more than 30 years, primarily via Federal Aviation Administration sponsorship, Volpe has focused on understanding the location and extent of wake turbulence to ensure safe operations for both takeoffs and landings. The accumulated evidence revealed that the existing wake separation standards, although operationally proven to be safe when procedures were followed, are overly conservative for specific meteorological conditions as well as for certain combinations of aircraft involved in a sequence.

"In recent years, R&D has taken a turn away from pure 'R' towards much more 'D'," according to Steven Lang, director of the Center for Air Traffic Systems and Operations at Volpe, The National Transportation Systems Center.

"The program was redirected from 'solely a science effort' towards finding operationally feasible solutions for individual airports," said Lang, who presented highlights of ongoing applied research during a recent Transportation Trajectories conversation at Volpe.

Until recently, airports with "closely spaced" parallel runways have not been able to make the most of the dual runway capacity, due to the long-maintained "2,500 foot rule." This rule mandated separation distances between aircraft that essentially reduce a two-runway airport's capacity to that of a single runway. This directly impacted the ability of the FAA to realize NextGen capacity goals.

"We found that using parallel runways actually reduces the risk of a wake encounter. We proved you can have runways less than 2,500 feet apart if you assign specific types of aircraft a specific sequence on approaches," explained Lang. Currently eight U.S. airports have been authorized for the new spacing—codified in FAA Order 7110.38—with two more in process.

The team has also developed wake turbulence mitigation for departures (WTMD), a crosswind-based system that enables closely spaced parallel runway departures to take place without wake turbulence constraints under specific wind conditions.

"This was a big interagency cooperation between FAA and NASA," said Lang. "This is the first automation driven wake separation change that allows dynamic separation based on meteorology and aircraft category." A simple automation system, the WTMD monitors wind data and when the wind meets certain parameters, the WTMD system provides a red light or a green light to the air traffic controllers.

A third effort, recategorization (RECAT), is revising single runway wake turbulence separation standards. It achieved operational implementation at Memphis International Airport in November 2012. "They've realized a double digit capacity benefit," said Lang, "so all the carriers are happy. The lessons learned will be rolled out to other sites," as each airport is unique in its geometry and flight tracking system.

Volpe's wake turbulence team is busy with data collection and analysis supporting additional solutions, all aimed at safely increasing the nation's aircraft capacity without requiring new construction.

For more information see the "Federal Aviation Administration Wake Turbulence Program – Recent Highlights," co-authored by Lang, Tittsworth, Barnes, Johnson and presented at the 57th Air Traffic Control Association Annual Conference & Exposition, October 2012.