Updated Wake Turbulence Separation Standards Increase Airport CapacityApril 2014
Topics: Air Traffic Management, Aerodynamics, Airports
Every aircraft generates a powerful wake as it flies, causing turbulence that affects other planes, particularly during takeoff and landing. The FAA put separation standards in place decades ago to ensure aircraft are spaced far enough apart that they don't encounter each other's wakes. But standards protecting aircraft from wake turbulence also can inhibit airport capacity.
To address this issue, the Federal Aviation Administration assembled a multi-organization team to study the existing standards and decide how to safely revise them to reduce their effects. Using newly available data and updated analytic techniques, the team determined that some of the current standards are overly conservative.
This finding led to the launch of several projects to revise the standards. These efforts reached major milestones in 2012 and 2013, with implementation of new procedures at several airports.
Creating New Weight Categories Allows for Reduced Separation
One of these initiatives is the Recategorization of Wake Turbulence Categories (RECAT) Phase I, which uses new aircraft weight categories to update existing separation standards.
"Separation distances are currently based on the weight of aircraft, taking into account that heavier aircraft typically generate stronger wakes and can also withstand stronger wakes than lighter aircraft," says Marshall Koch, a group leader within MITRE's FAA FFRDC. Current weight categories include ones specific to the Airbus 380 (a double-deck passenger airliner) and the Boeing 757, as well as categories for Heavy, Large, and Small aircraft.
But within some of these categories—particularly the Heavy and Large designations—there are vast weight differences. This can translate into overly stringent separation standards when heavier aircraft are following lighter aircraft from the same category.
Current separation standards require Heavy aircraft to be spaced at least 4 nautical miles behind all other Heavy aircraft, despite their disparate weights. The team's research indicated that when lighter Heavies follow each other, the standard could be reduced to 2½ or 3 miles, so the Heavy category was split into two categories. "By decreasing separation distances for the new lighter category of aircraft, the team hoped to achieve greater throughput and decreases in delay," Koch explains.
MITRE performed assessments that showed the anticipated benefits of the new standard warranted proceeding with demonstrations at airports with high proportions of the "lighter Heavies." The first operational demonstration of the new standards took place in November 2012 at Memphis International Airport, where these aircraft are common.
Our staff recently determined that implementing the RECAT standards at Memphis has improved arrival, departure, and ground operations there. Peak departure throughput increased by 12 percent, peak arrival separations were reduced by 0.7 miles in poor weather, and taxi queue delays dropped by at least 37 percent.
Closely Spaced Runways Need Special Standards
The MITRE team has also collaborated on a second, related project, the Wake Turbulence Mitigation for Departures (WTMD) initiative. The effort reassesses the wake turbulence mitigation standards for aircraft taking off from closely spaced parallel runways (CSPR). These are runways less than 2,500 feet apart.
"Current standards require air traffic controllers to separate departures from CSPRs for a specified period of time to ensure that the wake of the first departing aircraft has dissipated before the second aircraft departs," says Koch. However, research by the FAA and the Volpe Center to measure wakes and learn how they move and decay over time indicates these separation standards are unnecessary during certain conditions, particularly when crosswinds transport the wakes away from the trailing aircraft. In light of that finding, MIT Lincoln Laboratory (MIT-LL) began work on a wind forecast algorithm to help determine under what scenarios the wake-dissipation waiting time for departures from CSPRs could be safely relaxed.
Together, the FAA, MIT-LL, the Volpe Center, MITRE, and other research team members developed a concept that would allow for a relaxation of the existing standards under specified conditions. This team then embarked on a plan to assess the feasibility of the concept. NASA developed a prototype WTMD system using the MIT-LL algorithm and installed it at two airports for a year, collecting initial controller feedback on information requirements and procedure viability.
As part of the viability assessment and information requirements assessment process, MITRE staff conducted human-in-the-loop feasibility simulations in our labs. The participation of controllers from St. Louis International Airport and Houston’s George Bush Intercontinental Airport in these simulations was crucial to the FAA's decision to move ahead with operational demonstrations.
Operational demonstrations of WTMD began at San Francisco International Airport in May 2013, in Houston in July 2013, and Memphis in December 2013. Both MITRE and Human Solutions, Inc. (HSI) are now working to determine the post-implementation benefits case. MITRE is responsible for determining the quantitative benefits of the procedure, and HSI is assessing it from a human factors design perspective.
Another Round of Rule Changes for Closely Spaced Runways
Another key wake research project involves FAA Order 7110.308, 1.5-Nautical Mile Dependent Approaches to Parallel Runways Spaced Less than 2,500 Feet Apart. This project, nicknamed "dot 308" after its order number, amends a standard that restricts airports with CSPRs to use single runway separation standards for landings during reduced visibility conditions.
The team determined that for many pairs of aircraft with Large or Small leaders, it would still be possible to safely use both runways during poor visibility conditions, provided aircraft were staggered by 1.5 miles and met other safety mitigations. After the team completed the concept development process, MITRE conducted human-in-the-loop simulations that showed the concept was feasible for controllers to implement.
In September 2013, dot 308 was implemented at San Francisco International Airport, which has runways that are just 750 feet apart. Under the old protocol, San Francisco limited the use of one runway during poor visibility conditions and spaced arriving aircraft by three to six miles, depending on their weight.
"This resulted in the airport being able to land approximately 30 planes per hour in bad weather—or about half its usual capacity," Koch says. "Under the new procedure, it is estimated the airport can accommodate an additional four to six arrivals per hour. That translates to a 13 to 20 percent increase over what was possible under the old rules."
The Way Ahead: More Progress for Wake Turbulence Standards?
The team continues to analyze flight and other data to seed additional concepts for reducing the effects of wake turbulence mitigations in the National Airspace System. Building on the successes of the recent initiatives, MITRE and other aviation experts are developing additional wake-related reductions in separation for both CSPRs and same-runway operations.
"Based on their response, the user community definitely sees this work as a high priority," Koch says.
—by Marlis McCollum