Technique would ‘pack ’em in’ to increase runway capacity

 - October 8, 2007, 5:40 AM

Researchers at Raytheon Co. have proposed a novel technique to increase future runway capacity–in some cases potentially doubling an airport’s throughput–while at the same time avoiding wake turbulence.

At a recent NASA conference on integrated communications, navigation and surveillance (I-CNS), Raytheon’s Mary Ellen Miller and Steven Dougherty described a concept developed under NASA’s virtual airspace modeling and simulation (VAMS) project. The concept is based on two key assumptions:
• Passenger demand will double over the next 20 years, requiring the addition of many new runways, among other things.
• Environmental and other pressures will limit future expansion of airport boundaries to add new runways–which typically consume two square miles of land each when conventionally separated from existing runways, especially at those major airports that need them most. Therefore, most new runways will have to be built within an airport’s boundaries, usually parallel to existing runways, and often closely spaced.

However, parallel runways spaced less than 4,300 feet apart are currently subject to varying degrees of restriction in IMC. Usually this takes the form of “dependent” (staggered) approach spacing, and, for very closely spaced parallels–such as San Francisco’s 28L and 28R, separated by just 750 feet–required special equipment, such as a Precision Runway Monitor (see AIN, January 2003, page 67) or low-RNP-value avionics certification accompanied by a localizer directional aid.
And at less than 2,500 feet separation, wake turbulence becomes an issue. These limitations, when added to the standard in-trail wake-avoidance separations between small, large and heavy aircraft, mean that today’s parallel runway procedures will not be sufficient to meet tomorrow’s capacity imperatives.

The Raytheon proposal, called the Terminal Area Capacity Enhancement Concept (TACEC), would dramatically change–perhaps revolutionize is a better word–approaches to double, triple or even quadruple the capacity of parallel runways, even when they are separated by as little as 750 feet.

Through the use of next-generation flight management and flight control systems, very-high-accuracy positioning systems and advanced datalink technology, waves or groups of inbound aircraft would be automatically directed through a series of precise waypoints to arrive on their individual final approach paths in an echelon formation for almost simultaneous touchdowns.

The two, three or four landing aircraft would be progressively spaced back from each other by between five and 30 seconds, depending on their small, large or heavy category, to eliminate any wake-turbulence effects. (Approach wind conditions, which can affect wake movement, would also be factored in.) The next landing wave would follow, between 60 and 90 seconds later, again depending on category.

The echelon was selected over the line-abreast (wingtip to wingtip) formation to minimize any effects of residual flight technical error (FTE), and to allow more leeway to aircraft executing a missed approach. The procedure would be continuously monitored from the ground, and pilots would also monitor their positions relative to their formation partners on ADS-B displays on the flight deck. A similar technique would also be applied to departures.

The researchers calculate that TACEC could double airport capacity, allowing DFW, for example, to safely handle 282 arrivals and 264 departures per hour under IMC. Other airports would derive similar benefits, relative to their numbers of parallel runways.

While the concept seems rather startling at first sight, if the assumptions upon which TACEC is based are accurate, it might well be the way that instrument approaches in 2025 must be flo